LinuxLists.cc - [PATCH v2 00/35] bitops: add atomic find

2023-12-03 19:24:42

Subject: [PATCH v2 00/35] bitops: add atomic find_bit() operations

2023-12-03 19:25:14

Subject: [PATCH v2 01/35] lib/find: add atomic find_bit() primitives

Add helpers around test_and_{set,clear}_bit() that allow to search for
clear or set bits and flip them atomically.

The target patterns may look like this:

for (idx = 0; idx < nbits; idx++)
if (test_and_clear_bit(idx, bitmap))
do_something(idx);

Or like this:

do {
bit = find_first_bit(bitmap, nbits);
if (bit >= nbits)
return nbits;
} while (!test_and_clear_bit(bit, bitmap));
return bit;

In both cases, the opencoded loop may be converted to a single function
or iterator call. Correspondingly:

for_each_test_and_clear_bit(idx, bitmap, nbits)
do_something(idx);

Or:
return find_and_clear_bit(bitmap, nbits);

Obviously, the less routine code people have to write themself, the
less probability to make a mistake.

Those are not only handy helpers but also resolve a non-trivial
issue of using non-atomic find_bit() together with atomic
test_and_{set,clear)_bit().

The trick is that find_bit() implies that the bitmap is a regular
non-volatile piece of memory, and compiler is allowed to use such
optimization techniques like re-fetching memory instead of caching it.

For example, find_first_bit() is implemented like this:

for (idx = 0; idx * BITS_PER_LONG < sz; idx++) {
val = addr[idx];
if (val) {
sz = min(idx * BITS_PER_LONG + __ffs(val), sz);
break;
}
}

On register-memory architectures, like x86, compiler may decide to
access memory twice - first time to compare against 0, and second time
to fetch its value to pass it to __ffs().

When running find_first_bit() on volatile memory, the memory may get
changed in-between, and for instance, it may lead to passing 0 to
__ffs(), which is undefined. This is a potentially dangerous call.

find_and_clear_bit() as a wrapper around test_and_clear_bit()
naturally treats underlying bitmap as a volatile memory and prevents
compiler from such optimizations.

Now that KCSAN is catching exactly this type of situations and warns on
undercover memory modifications. We can use it to reveal improper usage
of find_bit(), and convert it to atomic find_and_*_bit() as appropriate.

The 1st patch of the series adds the following atomic primitives:

find_and_set_bit(addr, nbits);
find_and_set_next_bit(addr, nbits, start);
...

Here find_and_{set,clear} part refers to the corresponding
test_and_{set,clear}_bit function. Suffixes like _wrap or _lock
derive their semantics from corresponding find() or test() functions.

For brevity, the naming omits the fact that we search for zero bit in
find_and_set, and correspondingly search for set bit in find_and_clear
functions.

The patch also adds iterators with atomic semantics, like
for_each_test_and_set_bit(). Here, the naming rule is to simply prefix
corresponding atomic operation with 'for_each'.

All users of find_bit() API, where heavy concurrency is expected,
are encouraged to switch to atomic find_and_bit() as appropriate.

CC: Bart Van Assche <[email protected]>
CC: Sergey Shtylyov <[email protected]>
Signed-off-by: Yury Norov <[email protected]>
---
include/linux/find.h | 293 +++++++++++++++++++++++++++++++++++++++++++
lib/find_bit.c | 85 +++++++++++++
2 files changed, 378 insertions(+)

diff --git a/include/linux/find.h b/include/linux/find.h
index 5e4f39ef2e72..79b0e2589725 100644
--- a/include/linux/find.h
+++ b/include/linux/find.h
@@ -32,6 +32,16 @@ extern unsigned long _find_first_and_bit(const unsigned long *addr1,
extern unsigned long _find_first_zero_bit(const unsigned long *addr, unsigned long size);
extern unsigned long _find_last_bit(const unsigned long *addr, unsigned long size);

+unsigned long _find_and_set_bit(volatile unsigned long *addr, unsigned long nbits);
+unsigned long _find_and_set_next_bit(volatile unsigned long *addr, unsigned long nbits,
+ unsigned long start);
+unsigned long _find_and_set_bit_lock(volatile unsigned long *addr, unsigned long nbits);
+unsigned long _find_and_set_next_bit_lock(volatile unsigned long *addr, unsigned long nbits,
+ unsigned long start);
+unsigned long _find_and_clear_bit(volatile unsigned long *addr, unsigned long nbits);
+unsigned long _find_and_clear_next_bit(volatile unsigned long *addr, unsigned long nbits,
+ unsigned long start);
+
#ifdef __BIG_ENDIAN
unsigned long _find_first_zero_bit_le(const unsigned long *addr, unsigned long size);
unsigned long _find_next_zero_bit_le(const unsigned long *addr, unsigned
@@ -460,6 +470,267 @@ unsigned long __for_each_wrap(const unsigned long *bitmap, unsigned long size,
return bit < start ? bit : size;
}

+/**
+ * find_and_set_bit - Find a zero bit and set it atomically
+ * @addr: The address to base the search on
+ * @nbits: The bitmap size in bits
+ *
+ * This function is designed to operate in concurrent access environment.
+ *
+ * Because of concurrency and volatile nature of underlying bitmap, it's not
+ * guaranteed that the found bit is the 1st bit in the bitmap. It's also not
+ * guaranteed that if @nbits is returned, the bitmap is empty.
+ *
+ * The function does guarantee that if returned value is in range [0 .. @nbits),
+ * the acquired bit belongs to the caller exclusively.
+ *
+ * Returns: found and set bit, or @nbits if no bits found
+ */
+static inline
+unsigned long find_and_set_bit(volatile unsigned long *addr, unsigned long nbits)
+{
+ if (small_const_nbits(nbits)) {
+ unsigned long val, ret;
+
+ do {
+ val = *addr | ~GENMASK(nbits - 1, 0);
+ if (val == ~0UL)
+ return nbits;
+ ret = ffz(val);
+ } while (test_and_set_bit(ret, addr));
+
+ return ret;
+ }
+
+ return _find_and_set_bit(addr, nbits);
+}
+
+
+/**
+ * find_and_set_next_bit - Find a zero bit and set it, starting from @offset
+ * @addr: The address to base the search on
+ * @nbits: The bitmap nbits in bits
+ * @offset: The bitnumber to start searching at
+ *
+ * This function is designed to operate in concurrent access environment.
+ *
+ * Because of concurrency and volatile nature of underlying bitmap, it's not
+ * guaranteed that the found bit is the 1st bit in the bitmap, starting from @offset.
+ * It's also not guaranteed that if @nbits is returned, the bitmap is empty.
+ *
+ * The function does guarantee that if returned value is in range [@offset .. @nbits),
+ * the acquired bit belongs to the caller exclusively.
+ *
+ * Returns: found and set bit, or @nbits if no bits found
+ */
+static inline
+unsigned long find_and_set_next_bit(volatile unsigned long *addr,
+ unsigned long nbits, unsigned long offset)
+{
+ if (small_const_nbits(nbits)) {
+ unsigned long val, ret;
+
+ do {
+ val = *addr | ~GENMASK(nbits - 1, offset);
+ if (val == ~0UL)
+ return nbits;
+ ret = ffz(val);
+ } while (test_and_set_bit(ret, addr));
+
+ return ret;
+ }
+
+ return _find_and_set_next_bit(addr, nbits, offset);
+}
+
+/**
+ * find_and_set_bit_wrap - find and set bit starting at @offset, wrapping around zero
+ * @addr: The first address to base the search on
+ * @nbits: The bitmap size in bits
+ * @offset: The bitnumber to start searching at
+ *
+ * Returns: the bit number for the next clear bit, or first clear bit up to @offset,
+ * while atomically setting it. If no bits are found, returns @nbits.
+ */
+static inline
+unsigned long find_and_set_bit_wrap(volatile unsigned long *addr,
+ unsigned long nbits, unsigned long offset)
+{
+ unsigned long bit = find_and_set_next_bit(addr, nbits, offset);
+
+ if (bit < nbits || offset == 0)
+ return bit;
+
+ bit = find_and_set_bit(addr, offset);
+ return bit < offset ? bit : nbits;
+}
+
+/**
+ * find_and_set_bit_lock - find a zero bit, then set it atomically with lock
+ * @addr: The address to base the search on
+ * @nbits: The bitmap nbits in bits
+ *
+ * This function is designed to operate in concurrent access environment.
+ *
+ * Because of concurrency and volatile nature of underlying bitmap, it's not
+ * guaranteed that the found bit is the 1st bit in the bitmap. It's also not
+ * guaranteed that if @nbits is returned, the bitmap is empty.
+ *
+ * The function does guarantee that if returned value is in range [0 .. @nbits),
+ * the acquired bit belongs to the caller exclusively.
+ *
+ * Returns: found and set bit, or @nbits if no bits found
+ */
+static inline
+unsigned long find_and_set_bit_lock(volatile unsigned long *addr, unsigned long nbits)
+{
+ if (small_const_nbits(nbits)) {
+ unsigned long val, ret;
+
+ do {
+ val = *addr | ~GENMASK(nbits - 1, 0);
+ if (val == ~0UL)
+ return nbits;
+ ret = ffz(val);
+ } while (test_and_set_bit_lock(ret, addr));
+
+ return ret;
+ }
+
+ return _find_and_set_bit_lock(addr, nbits);
+}
+
+/**
+ * find_and_set_next_bit_lock - find a zero bit and set it atomically with lock
+ * @addr: The address to base the search on
+ * @nbits: The bitmap size in bits
+ * @offset: The bitnumber to start searching at
+ *
+ * This function is designed to operate in concurrent access environment.
+ *
+ * Because of concurrency and volatile nature of underlying bitmap, it's not
+ * guaranteed that the found bit is the 1st bit in the range. It's also not
+ * guaranteed that if @nbits is returned, the bitmap is empty.
+ *
+ * The function does guarantee that if returned value is in range [@offset .. @nbits),
+ * the acquired bit belongs to the caller exclusively.
+ *
+ * Returns: found and set bit, or @nbits if no bits found
+ */
+static inline
+unsigned long find_and_set_next_bit_lock(volatile unsigned long *addr,
+ unsigned long nbits, unsigned long offset)
+{
+ if (small_const_nbits(nbits)) {
+ unsigned long val, ret;
+
+ do {
+ val = *addr | ~GENMASK(nbits - 1, offset);
+ if (val == ~0UL)
+ return nbits;
+ ret = ffz(val);
+ } while (test_and_set_bit_lock(ret, addr));
+
+ return ret;
+ }
+
+ return _find_and_set_next_bit_lock(addr, nbits, offset);
+}
+
+/**
+ * find_and_set_bit_wrap_lock - find zero bit starting at @ofset and set it
+ * with lock, and wrap around zero if nothing found
+ * @addr: The first address to base the search on
+ * @nbits: The bitmap size in bits
+ * @offset: The bitnumber to start searching at
+ *
+ * Returns: the bit number for the next set bit, or first set bit up to @offset
+ * If no bits are set, returns @nbits.
+ */
+static inline
+unsigned long find_and_set_bit_wrap_lock(volatile unsigned long *addr,
+ unsigned long nbits, unsigned long offset)
+{
+ unsigned long bit = find_and_set_next_bit_lock(addr, nbits, offset);
+
+ if (bit < nbits || offset == 0)
+ return bit;
+
+ bit = find_and_set_bit_lock(addr, offset);
+ return bit < offset ? bit : nbits;
+}
+
+/**
+ * find_and_clear_bit - Find a set bit and clear it atomically
+ * @addr: The address to base the search on
+ * @nbits: The bitmap nbits in bits
+ *
+ * This function is designed to operate in concurrent access environment.
+ *
+ * Because of concurrency and volatile nature of underlying bitmap, it's not
+ * guaranteed that the found bit is the 1st bit in the bitmap. It's also not
+ * guaranteed that if @nbits is returned, the bitmap is empty.
+ *
+ * The function does guarantee that if returned value is in range [0 .. @nbits),
+ * the acquired bit belongs to the caller exclusively.
+ *
+ * Returns: found and cleared bit, or @nbits if no bits found
+ */
+static inline unsigned long find_and_clear_bit(volatile unsigned long *addr, unsigned long nbits)
+{
+ if (small_const_nbits(nbits)) {
+ unsigned long val, ret;
+
+ do {
+ val = *addr & GENMASK(nbits - 1, 0);
+ if (val == 0)
+ return nbits;
+ ret = __ffs(val);
+ } while (!test_and_clear_bit(ret, addr));
+
+ return ret;
+ }
+
+ return _find_and_clear_bit(addr, nbits);
+}
+
+/**
+ * find_and_clear_next_bit - Find a set bit next after @offset, and clear it atomically
+ * @addr: The address to base the search on
+ * @nbits: The bitmap nbits in bits
+ * @offset: bit offset at which to start searching
+ *
+ * This function is designed to operate in concurrent access environment.
+ *
+ * Because of concurrency and volatile nature of underlying bitmap, it's not
+ * guaranteed that the found bit is the 1st bit in the range It's also not
+ * guaranteed that if @nbits is returned, there's no set bits after @offset.
+ *
+ * The function does guarantee that if returned value is in range [@offset .. @nbits),
+ * the acquired bit belongs to the caller exclusively.
+ *
+ * Returns: found and cleared bit, or @nbits if no bits found
+ */
+static inline
+unsigned long find_and_clear_next_bit(volatile unsigned long *addr,
+ unsigned long nbits, unsigned long offset)
+{
+ if (small_const_nbits(nbits)) {
+ unsigned long val, ret;
+
+ do {
+ val = *addr & GENMASK(nbits - 1, offset);
+ if (val == 0)
+ return nbits;
+ ret = __ffs(val);
+ } while (!test_and_clear_bit(ret, addr));
+
+ return ret;
+ }
+
+ return _find_and_clear_next_bit(addr, nbits, offset);
+}
+
/**
* find_next_clump8 - find next 8-bit clump with set bits in a memory region
* @clump: location to store copy of found clump
@@ -577,6 +848,28 @@ unsigned long find_next_bit_le(const void *addr, unsigned
#define for_each_set_bit_from(bit, addr, size) \
for (; (bit) = find_next_bit((addr), (size), (bit)), (bit) < (size); (bit)++)

+/* same as for_each_set_bit() but atomically clears each found bit */
+#define for_each_test_and_clear_bit(bit, addr, size) \
+ for ((bit) = 0; \
+ (bit) = find_and_clear_next_bit((addr), (size), (bit)), (bit) < (size); \
+ (bit)++)
+
+/* same as for_each_set_bit_from() but atomically clears each found bit */
+#define for_each_test_and_clear_bit_from(bit, addr, size) \
+ for (; (bit) = find_and_clear_next_bit((addr), (size), (bit)), (bit) < (size); (bit)++)
+
+/* same as for_each_clear_bit() but atomically sets each found bit */
+#define for_each_test_and_set_bit(bit, addr, size) \
+ for ((bit) = 0; \
+ (bit) = find_and_set_next_bit((addr), (size), (bit)), (bit) < (size); \
+ (bit)++)
+
+/* same as for_each_clear_bit_from() but atomically clears each found bit */
+#define for_each_test_and_set_bit_from(bit, addr, size) \
+ for (; \
+ (bit) = find_and_set_next_bit((addr), (size), (bit)), (bit) < (size); \
+ (bit)++)
+
#define for_each_clear_bit(bit, addr, size) \
for ((bit) = 0; \
(bit) = find_next_zero_bit((addr), (size), (bit)), (bit) < (size); \
diff --git a/lib/find_bit.c b/lib/find_bit.c
index 32f99e9a670e..c9b6b9f96610 100644
--- a/lib/find_bit.c
+++ b/lib/find_bit.c
@@ -116,6 +116,91 @@ unsigned long _find_first_and_bit(const unsigned long *addr1,
EXPORT_SYMBOL(_find_first_and_bit);
#endif

+unsigned long _find_and_set_bit(volatile unsigned long *addr, unsigned long nbits)
+{
+ unsigned long bit;
+
+ do {
+ bit = FIND_FIRST_BIT(~addr[idx], /* nop */, nbits);
+ if (bit >= nbits)
+ return nbits;
+ } while (test_and_set_bit(bit, addr));
+
+ return bit;
+}
+EXPORT_SYMBOL(_find_and_set_bit);
+
+unsigned long _find_and_set_next_bit(volatile unsigned long *addr,
+ unsigned long nbits, unsigned long start)
+{
+ unsigned long bit;
+
+ do {
+ bit = FIND_NEXT_BIT(~addr[idx], /* nop */, nbits, start);
+ if (bit >= nbits)
+ return nbits;
+ } while (test_and_set_bit(bit, addr));
+
+ return bit;
+}
+EXPORT_SYMBOL(_find_and_set_next_bit);
+
+unsigned long _find_and_set_bit_lock(volatile unsigned long *addr, unsigned long nbits)
+{
+ unsigned long bit;
+
+ do {
+ bit = FIND_FIRST_BIT(~addr[idx], /* nop */, nbits);
+ if (bit >= nbits)
+ return nbits;
+ } while (test_and_set_bit_lock(bit, addr));
+
+ return bit;
+}
+EXPORT_SYMBOL(_find_and_set_bit_lock);
+
+unsigned long _find_and_set_next_bit_lock(volatile unsigned long *addr,
+ unsigned long nbits, unsigned long start)
+{
+ unsigned long bit;
+
+ do {
+ bit = FIND_NEXT_BIT(~addr[idx], /* nop */, nbits, start);
+ if (bit >= nbits)
+ return nbits;
+ } while (test_and_set_bit_lock(bit, addr));
+
+ return bit;
+}
+EXPORT_SYMBOL(_find_and_set_next_bit_lock);
+
+unsigned long _find_and_clear_bit(volatile unsigned long *addr, unsigned long nbits)
+{
+ unsigned long bit;
+
+ do {
+ bit = FIND_FIRST_BIT(addr[idx], /* nop */, nbits);
+ if (bit >= nbits)
+ return nbits;
+ } while (!test_and_clear_bit(bit, addr));
+
+ return bit;
+}
+EXPORT_SYMBOL(_find_and_clear_bit);
+
+unsigned long _find_and_clear_next_bit(volatile unsigned long *addr,
+ unsigned long nbits, unsigned long start)
+{
+ do {
+ start = FIND_NEXT_BIT(addr[idx], /* nop */, nbits, start);
+ if (start >= nbits)
+ return nbits;
+ } while (!test_and_clear_bit(start, addr));
+
+ return start;
+}
+EXPORT_SYMBOL(_find_and_clear_next_bit);
+
#ifndef find_first_zero_bit
/*
* Find the first cleared bit in a memory region.
--
2.40.1

2023-12-03 19:33:27

by Yury Norov

[permalink] [raw]

Subject: [PATCH v2 02/35] lib/find: add test for atomic find_bit() ops

Add basic functionality test for new API.

Signed-off-by: Yury Norov <[email protected]>
---
lib/test_bitmap.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 61 insertions(+)

diff --git a/lib/test_bitmap.c b/lib/test_bitmap.c
index 65f22c2578b0..277e1ca9fd28 100644
--- a/lib/test_bitmap.c
+++ b/lib/test_bitmap.c
@@ -221,6 +221,65 @@ static void __init test_zero_clear(void)
expect_eq_pbl("", bmap, 1024);
}

+static void __init test_find_and_bit(void)
+{
+ unsigned long w, w_part, bit, cnt = 0;
+ DECLARE_BITMAP(bmap, EXP1_IN_BITS);
+
+ /*
+ * Test find_and_clear{_next}_bit() and corresponding
+ * iterators
+ */
+ bitmap_copy(bmap, exp1, EXP1_IN_BITS);
+ w = bitmap_weight(bmap, EXP1_IN_BITS);
+
+ for_each_test_and_clear_bit(bit, bmap, EXP1_IN_BITS)
+ cnt++;
+
+ expect_eq_uint(w, cnt);
+ expect_eq_uint(0, bitmap_weight(bmap, EXP1_IN_BITS));
+
+ bitmap_copy(bmap, exp1, EXP1_IN_BITS);
+ w = bitmap_weight(bmap, EXP1_IN_BITS);
+ w_part = bitmap_weight(bmap, EXP1_IN_BITS / 3);
+
+ cnt = 0;
+ bit = EXP1_IN_BITS / 3;
+ for_each_test_and_clear_bit_from(bit, bmap, EXP1_IN_BITS)
+ cnt++;
+
+ expect_eq_uint(bitmap_weight(bmap, EXP1_IN_BITS), bitmap_weight(bmap, EXP1_IN_BITS / 3));
+ expect_eq_uint(w_part, bitmap_weight(bmap, EXP1_IN_BITS));
+ expect_eq_uint(w - w_part, cnt);
+
+ /*
+ * Test find_and_set{_next}_bit() and corresponding
+ * iterators
+ */
+ bitmap_copy(bmap, exp1, EXP1_IN_BITS);
+ w = bitmap_weight(bmap, EXP1_IN_BITS);
+ cnt = 0;
+
+ for_each_test_and_set_bit(bit, bmap, EXP1_IN_BITS)
+ cnt++;
+
+ expect_eq_uint(EXP1_IN_BITS - w, cnt);
+ expect_eq_uint(EXP1_IN_BITS, bitmap_weight(bmap, EXP1_IN_BITS));
+
+ bitmap_copy(bmap, exp1, EXP1_IN_BITS);
+ w = bitmap_weight(bmap, EXP1_IN_BITS);
+ w_part = bitmap_weight(bmap, EXP1_IN_BITS / 3);
+ cnt = 0;
+
+ bit = EXP1_IN_BITS / 3;
+ for_each_test_and_set_bit_from(bit, bmap, EXP1_IN_BITS)
+ cnt++;
+
+ expect_eq_uint(EXP1_IN_BITS - bitmap_weight(bmap, EXP1_IN_BITS),
+ EXP1_IN_BITS / 3 - bitmap_weight(bmap, EXP1_IN_BITS / 3));
+ expect_eq_uint(EXP1_IN_BITS * 2 / 3 - (w - w_part), cnt);
+}
+
static void __init test_find_nth_bit(void)
{
unsigned long b, bit, cnt = 0;
@@ -1273,6 +1332,8 @@ static void __init selftest(void)
test_for_each_clear_bitrange_from();
test_for_each_set_clump8();
test_for_each_set_bit_wrap();
+
+ test_find_and_bit();
}

KSTM_MODULE_LOADERS(test_bitmap);
--
2.40.1

2023-12-03 19:33:57

by Yury Norov

[permalink] [raw]

Subject: [PATCH v2 11/35] ath10k: optimize ath10k_snoc_napi_poll() by using find_bit()

ath10k_snoc_napi_poll() traverses pending_ce_irqs bitmap bit by bit.
We can do it faster by using for_each_test_and_clear_bit() iterator.

Signed-off-by: Yury Norov <[email protected]>
---
drivers/net/wireless/ath/ath10k/snoc.c | 9 ++++-----
1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/drivers/net/wireless/ath/ath10k/snoc.c b/drivers/net/wireless/ath/ath10k/snoc.c
index 2c39bad7ebfb..a1db5a973780 100644
--- a/drivers/net/wireless/ath/ath10k/snoc.c
+++ b/drivers/net/wireless/ath/ath10k/snoc.c
@@ -1237,11 +1237,10 @@ static int ath10k_snoc_napi_poll(struct napi_struct *ctx, int budget)
return done;
}

- for (ce_id = 0; ce_id < CE_COUNT; ce_id++)
- if (test_and_clear_bit(ce_id, ar_snoc->pending_ce_irqs)) {
- ath10k_ce_per_engine_service(ar, ce_id);
- ath10k_ce_enable_interrupt(ar, ce_id);
- }
+ for_each_test_and_clear_bit(ce_id, ar_snoc->pending_ce_irqs, CE_COUNT) {
+ ath10k_ce_per_engine_service(ar, ce_id);
+ ath10k_ce_enable_interrupt(ar, ce_id);
+ }

done = ath10k_htt_txrx_compl_task(ar, budget);

--
2.40.1

2023-12-03 19:34:01

by Yury Norov

[permalink] [raw]

Subject: [PATCH v2 12/35] wifi: rtw88: optimize rtw_pci_tx_kick_off() by using find_bit()

rtw_pci_tx_kick_off() traverses tx_queued bitmap bit by bit. We can do it
faster by using atomic for_each_test_and_clear_bit() iterator.

Signed-off-by: Yury Norov <[email protected]>
---
drivers/net/wireless/realtek/rtw88/pci.c | 5 ++---
drivers/net/wireless/realtek/rtw89/pci.c | 5 +----
2 files changed, 3 insertions(+), 7 deletions(-)

diff --git a/drivers/net/wireless/realtek/rtw88/pci.c b/drivers/net/wireless/realtek/rtw88/pci.c
index 2bfc0e822b8d..a0d69c75a381 100644
--- a/drivers/net/wireless/realtek/rtw88/pci.c
+++ b/drivers/net/wireless/realtek/rtw88/pci.c
@@ -789,9 +789,8 @@ static void rtw_pci_tx_kick_off(struct rtw_dev *rtwdev)
struct rtw_pci *rtwpci = (struct rtw_pci *)rtwdev->priv;
enum rtw_tx_queue_type queue;

- for (queue = 0; queue < RTK_MAX_TX_QUEUE_NUM; queue++)
- if (test_and_clear_bit(queue, rtwpci->tx_queued))
- rtw_pci_tx_kick_off_queue(rtwdev, queue);
+ for_each_test_and_clear_bit(queue, rtwpci->tx_queued, RTK_MAX_TX_QUEUE_NUM)
+ rtw_pci_tx_kick_off_queue(rtwdev, queue);
}

static int rtw_pci_tx_write_data(struct rtw_dev *rtwdev,
diff --git a/drivers/net/wireless/realtek/rtw89/pci.c b/drivers/net/wireless/realtek/rtw89/pci.c
index 14ddb0d39e63..184d41b774d7 100644
--- a/drivers/net/wireless/realtek/rtw89/pci.c
+++ b/drivers/net/wireless/realtek/rtw89/pci.c
@@ -1077,10 +1077,7 @@ static void rtw89_pci_tx_kick_off_pending(struct rtw89_dev *rtwdev)
struct rtw89_pci_tx_ring *tx_ring;
int txch;

- for (txch = 0; txch < RTW89_TXCH_NUM; txch++) {
- if (!test_and_clear_bit(txch, rtwpci->kick_map))
- continue;
-
+ for_each_test_and_clear_bit(txch, rtwpci->kick_map, RTW89_TXCH_NUM) {
tx_ring = &rtwpci->tx_rings[txch];
__rtw89_pci_tx_kick_off(rtwdev, tx_ring);
}
--
2.40.1

2023-12-04 13:07:59

by Andy Shevchenko

[permalink] [raw]

Subject: Re: [PATCH v2 00/35] bitops: add atomic find_bit() operations

On Sun, Dec 03, 2023 at 11:23:47AM -0800, Yury Norov wrote:
> Add helpers around test_and_{set,clear}_bit() that allow to search for
> clear or set bits and flip them atomically.
>
> The target patterns may look like this:
>
> for (idx = 0; idx < nbits; idx++)
> if (test_and_clear_bit(idx, bitmap))
> do_something(idx);
>
> Or like this:
>
> do {
> bit = find_first_bit(bitmap, nbits);
> if (bit >= nbits)
> return nbits;
> } while (!test_and_clear_bit(bit, bitmap));
> return bit;
>
> In both cases, the opencoded loop may be converted to a single function
> or iterator call. Correspondingly:
>
> for_each_test_and_clear_bit(idx, bitmap, nbits)
> do_something(idx);
>
> Or:
> return find_and_clear_bit(bitmap, nbits);
>
> Obviously, the less routine code people have to write themself, the
> less probability to make a mistake.
>
> Those are not only handy helpers but also resolve a non-trivial
> issue of using non-atomic find_bit() together with atomic
> test_and_{set,clear)_bit().
>
> The trick is that find_bit() implies that the bitmap is a regular
> non-volatile piece of memory, and compiler is allowed to use such
> optimization techniques like re-fetching memory instead of caching it.
>
> For example, find_first_bit() is implemented like this:
>
> for (idx = 0; idx * BITS_PER_LONG < sz; idx++) {
> val = addr[idx];
> if (val) {
> sz = min(idx * BITS_PER_LONG + __ffs(val), sz);
> break;
> }
> }
>
> On register-memory architectures, like x86, compiler may decide to
> access memory twice - first time to compare against 0, and second time
> to fetch its value to pass it to __ffs().
>
> When running find_first_bit() on volatile memory, the memory may get
> changed in-between, and for instance, it may lead to passing 0 to
> __ffs(), which is undefined. This is a potentially dangerous call.
>
> find_and_clear_bit() as a wrapper around test_and_clear_bit()
> naturally treats underlying bitmap as a volatile memory and prevents
> compiler from such optimizations.
>
> Now that KCSAN is catching exactly this type of situations and warns on
> undercover memory modifications. We can use it to reveal improper usage
> of find_bit(), and convert it to atomic find_and_*_bit() as appropriate.
>
> The 1st patch of the series adds the following atomic primitives:
>
> find_and_set_bit(addr, nbits);
> find_and_set_next_bit(addr, nbits, start);
> ...
>
> Here find_and_{set,clear} part refers to the corresponding
> test_and_{set,clear}_bit function. Suffixes like _wrap or _lock
> derive their semantics from corresponding find() or test() functions.
>
> For brevity, the naming omits the fact that we search for zero bit in
> find_and_set, and correspondingly search for set bit in find_and_clear
> functions.
>
> The patch also adds iterators with atomic semantics, like
> for_each_test_and_set_bit(). Here, the naming rule is to simply prefix
> corresponding atomic operation with 'for_each'.
>
> This series is a result of discussion [1]. All find_bit() functions imply
> exclusive access to the bitmaps. However, KCSAN reports quite a number
> of warnings related to find_bit() API. Some of them are not pointing
> to real bugs because in many situations people intentionally allow
> concurrent bitmap operations.
>
> If so, find_bit() can be annotated such that KCSAN will ignore it:
>
> bit = data_race(find_first_bit(bitmap, nbits));
>
> This series addresses the other important case where people really need
> atomic find ops. As the following patches show, the resulting code
> looks safer and more verbose comparing to opencoded loops followed by
> atomic bit flips.
>
> In [1] Mirsad reported 2% slowdown in a single-thread search test when
> switching find_bit() function to treat bitmaps as volatile arrays. On
> the other hand, kernel robot in the same thread reported +3.7% to the
> performance of will-it-scale.per_thread_ops test.
>
> Assuming that our compilers are sane and generate better code against
> properly annotated data, the above discrepancy doesn't look weird. When
> running on non-volatile bitmaps, plain find_bit() outperforms atomic
> find_and_bit(), and vice-versa.

...

In some cases the better improvements can be achieved by switching
the (very) old code to utilise IDA framework.

--
With Best Regards,
Andy Shevchenko

2023-12-04 18:51:21

by Jan Kara

[permalink] [raw]

Subject: Re: [PATCH v2 00/35] bitops: add atomic find_bit() operations

Hello Yury!

On Sun 03-12-23 11:23:47, Yury Norov wrote:
> Add helpers around test_and_{set,clear}_bit() that allow to search for
> clear or set bits and flip them atomically.
>
> The target patterns may look like this:
>
> for (idx = 0; idx < nbits; idx++)
> if (test_and_clear_bit(idx, bitmap))
> do_something(idx);
>
> Or like this:
>
> do {
> bit = find_first_bit(bitmap, nbits);
> if (bit >= nbits)
> return nbits;
> } while (!test_and_clear_bit(bit, bitmap));
> return bit;
>
> In both cases, the opencoded loop may be converted to a single function
> or iterator call. Correspondingly:
>
> for_each_test_and_clear_bit(idx, bitmap, nbits)
> do_something(idx);
>
> Or:
> return find_and_clear_bit(bitmap, nbits);

These are fine cleanups but they actually don't address the case that has
triggered all these changes - namely the xarray use of find_next_bit() in
xas_find_chunk().

...
> This series is a result of discussion [1]. All find_bit() functions imply
> exclusive access to the bitmaps. However, KCSAN reports quite a number
> of warnings related to find_bit() API. Some of them are not pointing
> to real bugs because in many situations people intentionally allow
> concurrent bitmap operations.
>
> If so, find_bit() can be annotated such that KCSAN will ignore it:
>
> bit = data_race(find_first_bit(bitmap, nbits));

No, this is not a correct thing to do. If concurrent bitmap changes can
happen, find_first_bit() as it is currently implemented isn't ever a safe
choice because it can call __ffs(0) which is dangerous as you properly note
above. I proposed adding READ_ONCE() into find_first_bit() / find_next_bit()
implementation to fix this issue but you disliked that. So other option we
have is adding find_first_bit() and find_next_bit() variants that take
volatile 'addr' and we have to use these in code like xas_find_chunk()
which cannot be converted to your new helpers.

> This series addresses the other important case where people really need
> atomic find ops. As the following patches show, the resulting code
> looks safer and more verbose comparing to opencoded loops followed by
> atomic bit flips.
>
> In [1] Mirsad reported 2% slowdown in a single-thread search test when
> switching find_bit() function to treat bitmaps as volatile arrays. On
> the other hand, kernel robot in the same thread reported +3.7% to the
> performance of will-it-scale.per_thread_ops test.

It was actually me who reported the regression here [2] but whatever :)

[2] https://lore.kernel.org/all/[email protected]

> Assuming that our compilers are sane and generate better code against
> properly annotated data, the above discrepancy doesn't look weird. When
> running on non-volatile bitmaps, plain find_bit() outperforms atomic
> find_and_bit(), and vice-versa.
>
> So, all users of find_bit() API, where heavy concurrency is expected,
> are encouraged to switch to atomic find_and_bit() as appropriate.

Well, all users where any concurrency can happen should switch. Otherwise
they are prone to the (admittedly mostly theoretical) data race issue.

Honza
--
Jan Kara <[email protected]>
SUSE Labs, CR

2023-12-06 05:25:30

by Yury Norov

[permalink] [raw]

Subject: Re: [PATCH v2 00/35] bitops: add atomic find_bit() operations

On Mon, Dec 04, 2023 at 07:51:01PM +0100, Jan Kara wrote:
> Hello Yury!
>
> On Sun 03-12-23 11:23:47, Yury Norov wrote:
> > Add helpers around test_and_{set,clear}_bit() that allow to search for
> > clear or set bits and flip them atomically.
> >
> > The target patterns may look like this:
> >
> > for (idx = 0; idx < nbits; idx++)
> > if (test_and_clear_bit(idx, bitmap))
> > do_something(idx);
> >
> > Or like this:
> >
> > do {
> > bit = find_first_bit(bitmap, nbits);
> > if (bit >= nbits)
> > return nbits;
> > } while (!test_and_clear_bit(bit, bitmap));
> > return bit;
> >
> > In both cases, the opencoded loop may be converted to a single function
> > or iterator call. Correspondingly:
> >
> > for_each_test_and_clear_bit(idx, bitmap, nbits)
> > do_something(idx);
> >
> > Or:
> > return find_and_clear_bit(bitmap, nbits);
>
> These are fine cleanups but they actually don't address the case that has
> triggered all these changes - namely the xarray use of find_next_bit() in
> xas_find_chunk().
>
> ...
> > This series is a result of discussion [1]. All find_bit() functions imply
> > exclusive access to the bitmaps. However, KCSAN reports quite a number
> > of warnings related to find_bit() API. Some of them are not pointing
> > to real bugs because in many situations people intentionally allow
> > concurrent bitmap operations.
> >
> > If so, find_bit() can be annotated such that KCSAN will ignore it:
> >
> > bit = data_race(find_first_bit(bitmap, nbits));
>
> No, this is not a correct thing to do. If concurrent bitmap changes can
> happen, find_first_bit() as it is currently implemented isn't ever a safe
> choice because it can call __ffs(0) which is dangerous as you properly note
> above. I proposed adding READ_ONCE() into find_first_bit() / find_next_bit()
> implementation to fix this issue but you disliked that. So other option we
> have is adding find_first_bit() and find_next_bit() variants that take
> volatile 'addr' and we have to use these in code like xas_find_chunk()
> which cannot be converted to your new helpers.

Here is some examples when concurrent operations with plain find_bit()
are acceptable:

- two threads running find_*_bit(): safe wrt ffs(0) and returns correct
value, because underlying bitmap is unchanged;
- find_next_bit() in parallel with set or clear_bit(), when modifying
a bit prior to the start bit to search: safe and correct;
- find_first_bit() in parallel with set_bit(): safe, but may return wrong
bit number;
- find_first_zero_bit() in parallel with clear_bit(): same as above.

In last 2 cases find_bit() may not return a correct bit number, but
it may be OK if caller requires any (not exactly first) set or clear
bit, correspondingly.

In such cases, KCSAN may be safely silenced.

> > This series addresses the other important case where people really need
> > atomic find ops. As the following patches show, the resulting code
> > looks safer and more verbose comparing to opencoded loops followed by
> > atomic bit flips.
> >
> > In [1] Mirsad reported 2% slowdown in a single-thread search test when
> > switching find_bit() function to treat bitmaps as volatile arrays. On
> > the other hand, kernel robot in the same thread reported +3.7% to the
> > performance of will-it-scale.per_thread_ops test.
>
> It was actually me who reported the regression here [2] but whatever :)
>
> [2] https://lore.kernel.org/all/[email protected]

My apologize.

> > Assuming that our compilers are sane and generate better code against
> > properly annotated data, the above discrepancy doesn't look weird. When
> > running on non-volatile bitmaps, plain find_bit() outperforms atomic
> > find_and_bit(), and vice-versa.
> >
> > So, all users of find_bit() API, where heavy concurrency is expected,
> > are encouraged to switch to atomic find_and_bit() as appropriate.
>
> Well, all users where any concurrency can happen should switch. Otherwise
> they are prone to the (admittedly mostly theoretical) data race issue.
>
> Honza
> --
> Jan Kara <[email protected]>
> SUSE Labs, CR

2023-12-07 09:11:04

by Jan Kara

[permalink] [raw]

Subject: Re: [PATCH v2 00/35] bitops: add atomic find_bit() operations

On Tue 05-12-23 21:22:59, Yury Norov wrote:
> On Mon, Dec 04, 2023 at 07:51:01PM +0100, Jan Kara wrote:
> > > This series is a result of discussion [1]. All find_bit() functions imply
> > > exclusive access to the bitmaps. However, KCSAN reports quite a number
> > > of warnings related to find_bit() API. Some of them are not pointing
> > > to real bugs because in many situations people intentionally allow
> > > concurrent bitmap operations.
> > >
> > > If so, find_bit() can be annotated such that KCSAN will ignore it:
> > >
> > > bit = data_race(find_first_bit(bitmap, nbits));
> >
> > No, this is not a correct thing to do. If concurrent bitmap changes can
> > happen, find_first_bit() as it is currently implemented isn't ever a safe
> > choice because it can call __ffs(0) which is dangerous as you properly note
> > above. I proposed adding READ_ONCE() into find_first_bit() / find_next_bit()
> > implementation to fix this issue but you disliked that. So other option we
> > have is adding find_first_bit() and find_next_bit() variants that take
> > volatile 'addr' and we have to use these in code like xas_find_chunk()
> > which cannot be converted to your new helpers.
>
> Here is some examples when concurrent operations with plain find_bit()
> are acceptable:
>
> - two threads running find_*_bit(): safe wrt ffs(0) and returns correct
> value, because underlying bitmap is unchanged;
> - find_next_bit() in parallel with set or clear_bit(), when modifying
> a bit prior to the start bit to search: safe and correct;
> - find_first_bit() in parallel with set_bit(): safe, but may return wrong
> bit number;
> - find_first_zero_bit() in parallel with clear_bit(): same as above.
>
> In last 2 cases find_bit() may not return a correct bit number, but
> it may be OK if caller requires any (not exactly first) set or clear
> bit, correspondingly.
>
> In such cases, KCSAN may be safely silenced.

True - but these are special cases. In particular the case in xas_find_chunk()
is not any of these special cases. It is using find_next_bit() which is can
be racing with clear_bit(). So what are your plans for such usecase?

Honza
--
Jan Kara <[email protected]>
SUSE Labs, CR