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Message-ID: <fb0f5ba9-7fe3-a951-0587-640e7672efec@alu.unizg.hr>
Date:   Mon, 18 Sep 2023 14:46:02 +0200
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Subject: Re: [PATCH v1 1/1] xarray: fix the data-race in xas_find_chunk() by
 using READ_ONCE()
Content-Language: en-US
To:     Jan Kara <jack@suse.cz>
Cc:     Philipp Stanner <pstanner@redhat.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Matthew Wilcox <willy@infradead.org>, Chris Mason <clm@fb.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>, linux-btrfs@vger.kernel.org,
        linux-mm@kvack.org, Yury Norov <yury.norov@gmail.com>
References: <20230918044739.29782-1-mirsad.todorovac@alu.unizg.hr>
 <20230918094116.2mgquyxhnxcawxfu@quack3>
 <22ca3ad4-42ef-43bc-51d0-78aaf274977b@alu.unizg.hr>
 <20230918113840.h3mmnuyer44e5bc5@quack3>
From:   Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
In-Reply-To: <20230918113840.h3mmnuyer44e5bc5@quack3>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk

On 9/18/23 13:38, Jan Kara wrote:
> On Mon 18-09-23 12:20:09, Mirsad Todorovac wrote:
>> On 9/18/23 11:41, Jan Kara wrote:
>>> On Mon 18-09-23 06:47:40, Mirsad Goran Todorovac wrote:
>>>> KCSAN has discovered the following data-race:
>>>>
>>>> [  206.510010] ==================================================================
>>>> [  206.510035] BUG: KCSAN: data-race in xas_clear_mark / xas_find_marked
>>>>
>>>> [  206.510067] write to 0xffff963df6a90fe0 of 8 bytes by interrupt on cpu 22:
>>>> [  206.510081] xas_clear_mark (./arch/x86/include/asm/bitops.h:178 ./include/asm-generic/bitops/instrumented-non-atomic.h:115 lib/xarray.c:102 lib/xarray.c:914)
>>>> [  206.510097] __xa_clear_mark (lib/xarray.c:1923)
>>>> [  206.510114] __folio_end_writeback (mm/page-writeback.c:2981)
>>>> [  206.510128] folio_end_writeback (mm/filemap.c:1616)
>>>> [  206.510143] end_page_writeback (mm/folio-compat.c:28)
>>>> [  206.510155] btrfs_page_clear_writeback (fs/btrfs/subpage.c:646) btrfs
>>>> [  206.510994] end_bio_extent_writepage (./include/linux/bio.h:84 fs/btrfs/extent_io.c:542) btrfs
>>>> [  206.511817] __btrfs_bio_end_io (fs/btrfs/bio.c:117 fs/btrfs/bio.c:112) btrfs
>>>> [  206.512640] btrfs_orig_bbio_end_io (fs/btrfs/bio.c:164) btrfs
>>>> [  206.513497] btrfs_simple_end_io (fs/btrfs/bio.c:380) btrfs
>>>> [  206.514350] bio_endio (block/bio.c:1617)
>>>> [  206.514362] blk_mq_end_request_batch (block/blk-mq.c:837 block/blk-mq.c:1073)
>>>> [  206.514377] nvme_pci_complete_batch (drivers/nvme/host/pci.c:986) nvme
>>>> [  206.514437] nvme_irq (drivers/nvme/host/pci.c:1086) nvme
>>>> [  206.514500] __handle_irq_event_percpu (kernel/irq/handle.c:158)
>>>> [  206.514517] handle_irq_event (kernel/irq/handle.c:195 kernel/irq/handle.c:210)
>>>> [  206.514533] handle_edge_irq (kernel/irq/chip.c:836)
>>>> [  206.514549] __common_interrupt (./include/linux/irqdesc.h:161 arch/x86/kernel/irq.c:238 arch/x86/kernel/irq.c:257)
>>>> [  206.514563] common_interrupt (arch/x86/kernel/irq.c:247 (discriminator 14))
>>>> [  206.514583] asm_common_interrupt (./arch/x86/include/asm/idtentry.h:636)
>>>> [  206.514599] kcsan_setup_watchpoint (kernel/kcsan/core.c:705 (discriminator 1))
>>>> [  206.514612] __tsan_read8 (kernel/kcsan/core.c:1025)
>>>> [  206.514626] steal_from_bitmap.part.0 (./include/linux/find.h:186 fs/btrfs/free-space-cache.c:2557 fs/btrfs/free-space-cache.c:2613) btrfs
>>>> [  206.515491] __btrfs_add_free_space (fs/btrfs/free-space-cache.c:2689 fs/btrfs/free-space-cache.c:2667) btrfs
>>>> [  206.516361] btrfs_add_free_space_async_trimmed (fs/btrfs/free-space-cache.c:2798) btrfs
>>>> [  206.517231] add_new_free_space (fs/btrfs/block-group.c:550) btrfs
>>>> [  206.518095] load_free_space_tree (fs/btrfs/free-space-tree.c:1595 fs/btrfs/free-space-tree.c:1658) btrfs
>>>> [  206.518953] caching_thread (fs/btrfs/block-group.c:873) btrfs
>>>> [  206.519800] btrfs_work_helper (fs/btrfs/async-thread.c:314) btrfs
>>>> [  206.520643] process_one_work (kernel/workqueue.c:2600)
>>>> [  206.520658] worker_thread (./include/linux/list.h:292 kernel/workqueue.c:2752)
>>>> [  206.520672] kthread (kernel/kthread.c:389)
>>>> [  206.520684] ret_from_fork (arch/x86/kernel/process.c:145)
>>>> [  206.520701] ret_from_fork_asm (arch/x86/entry/entry_64.S:312)
>>>>
>>>> [  206.520722] read to 0xffff963df6a90fe0 of 8 bytes by task 2793 on cpu 6:
>>>> [  206.520735] xas_find_marked (./include/linux/xarray.h:1706 lib/xarray.c:1354)
>>>> [  206.520750] filemap_get_folios_tag (mm/filemap.c:1975 mm/filemap.c:2273)
>>>> [  206.520763] __filemap_fdatawait_range (mm/filemap.c:519)
>>>> [  206.520777] filemap_fdatawait_range (mm/filemap.c:556)
>>>> [  206.520790] btrfs_wait_ordered_range (fs/btrfs/ordered-data.c:839) btrfs
>>>> [  206.521641] btrfs_sync_file (fs/btrfs/file.c:1859) btrfs
>>>> [  206.522495] vfs_fsync_range (fs/sync.c:188)
>>>> [  206.522509] __x64_sys_fsync (./include/linux/file.h:45 fs/sync.c:213 fs/sync.c:220 fs/sync.c:218 fs/sync.c:218)
>>>> [  206.522522] do_syscall_64 (arch/x86/entry/common.c:50 arch/x86/entry/common.c:80)
>>>> [  206.522535] entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:120)
>>>>
>>>> [  206.522557] value changed: 0xfffffffffff80000 -> 0xfffffffffff00000
>>>>
>>>> [  206.522574] Reported by Kernel Concurrency Sanitizer on:
>>>> [  206.522585] CPU: 6 PID: 2793 Comm: tracker-extract Tainted: G             L     6.5.0-rc6+ #44
>>>> [  206.522600] Hardware name: ASRock X670E PG Lightning/X670E PG Lightning, BIOS 1.21 04/26/2023
>>>> [  206.522608] ==================================================================
>>>
>>> Thanks for working on this. I guess the full KCSAN warning isn't that
>>> useful in the changelog. Rather I'd spend more time explaining the real
>>> problem here ...
>>>
>>>> As Jan Kara explained, the problem is in the function xas_find_chuck():
>>>>
>>>> /* Private */
>>>> static inline unsigned int xas_find_chunk(struct xa_state *xas, bool advance,
>>>> 		xa_mark_t mark)
>>>> {
>>>> 	unsigned long *addr = xas->xa_node->marks[(__force unsigned)mark];
>>>> 	unsigned int offset = xas->xa_offset;
>>>>
>>>> 	if (advance)
>>>> 		offset++;
>>>> 	if (XA_CHUNK_SIZE == BITS_PER_LONG) {
>>>> 		if (offset < XA_CHUNK_SIZE) {
>>>> →			unsigned long data = *addr & (~0UL << offset);
>>>> 			if (data)
>>>> 				return __ffs(data);
>>>
>>> ... which is that xas_find_chunk() is called only under RCU protection and
>>> thus the two uses of 'data' in the above code can yield different results.
>>>
>>>> 		}
>>>> 		return XA_CHUNK_SIZE;
>>>> 	}
>>>>
>>>> 	return find_next_bit(addr, XA_CHUNK_SIZE, offset);
>>>> }
>>>>
>>>> In particular, the line
>>>>
>>>> 			unsigned long data = *addr & (~0UL << offset);
>>>>
>>>> contains a data race that is best avoided using READ_ONCE(), which eliminated the KCSAN
>>>> data-race warning completely.
>>>
>>> Yes, this improves the situation for xarray use on 64-bit architectures but
>>> doesn't fix cases on 32-bit archs or if CONFIG_BASE_SMALL is set. As I
>>> mentioned in my previous reply, I'd rather:
>>>
>>> 1) Fix find_next_bit(), find_first_bit() and related functions in
>>> lib/find_bit.c to use READ_ONCE() - such as _find_first_bit() etc. It is
>>> quite some churn but I don't see how else to make these functions safe when
>>> the underlying contents can change.
>>
>> Thank you for your review.
>>
>> I assume you have the big picture, but just a stupid question:
>>
>> 	if (XA_CHUNK_SIZE == BITS_PER_LONG) {
>> 		if (offset < XA_CHUNK_SIZE) {
>> 			unsigned long data = READ_ONCE(*addr) & (~0UL << offset);
>> 			if (data)
>> 				return __ffs(data);
>> 		}
>> 		return XA_CHUNK_SIZE;
>> 	}
>>
>> I would hate to argue, but ...
> 
> No problem, asking questions isn't argueing ;).
> 
>> Wouldn't BITS_PER_LONG simply change to 32 on 32-bit architectures?
> 
> Yes, they will. But XA_CHUNK_SIZE will still be 64 on 32-bit AFAICT so
> XA_CHUNK_SIZE != BITS_PER_LONG there.

Ah, I see. This is definitely not good. But I managed to fix and test the find_next_bit()
family, but this seems that simply

-------------------------------------------
  include/linux/xarray.h | 8 --------
  1 file changed, 8 deletions(-)

diff --git a/include/linux/xarray.h b/include/linux/xarray.h
index 1715fd322d62..89918b65b00d 100644
--- a/include/linux/xarray.h
+++ b/include/linux/xarray.h
@@ -1718,14 +1718,6 @@ static inline unsigned int xas_find_chunk(struct xa_state *xas, bool advance,
  
         if (advance)
                 offset++;
-       if (XA_CHUNK_SIZE == BITS_PER_LONG) {
-               if (offset < XA_CHUNK_SIZE) {
-                       unsigned long data = READ_ONCE(*addr) & (~0UL << offset);
-                       if (data)
-                               return __ffs(data);
-               }
-               return XA_CHUNK_SIZE;
-       }
  
         return find_next_bit(addr, XA_CHUNK_SIZE, offset);
  }

seems too good to be true.

According to what you explained, the performance impact would be negligent or non-existing,
and the CONFIG_BASE_SMALL problem would disappear?

I did not even try to run that, as I am not 100% confident in the logic.

Am I doing something very wrong?

>> Is there something I am missing?
>>
>>  From include/asm-generic/bitsperlong.h:
>> ----------------------------------------
>> #ifdef CONFIG_64BIT
>> #define BITS_PER_LONG 64
>> #else
>> #define BITS_PER_LONG 32
>> #endif /* CONFIG_64BIT */
>>
>> About the CONFIG_BASE_SMALL I cannot tell:
>> ----------------------------------------
>> #ifndef XA_CHUNK_SHIFT
>> #define XA_CHUNK_SHIFT		(CONFIG_BASE_SMALL ? 4 : 6)
>> #endif
>> #define XA_CHUNK_SIZE		(1UL << XA_CHUNK_SHIFT)
>> #define XA_CHUNK_MASK		(XA_CHUNK_SIZE - 1)
>> #define XA_MAX_MARKS		3
>> #define XA_MARK_LONGS		DIV_ROUND_UP(XA_CHUNK_SIZE, BITS_PER_LONG)
>> ----------------------------------------
> 
> Again with CONFIG_BASE_SMALL we have XA_CHUNK_SIZE == 16 so it will not be
> equal to BITS_PER_LONG.
>   
>> I see why you would want find_next_bit() and find_first_bit() fixed, but
>> I am not that deep into those bitops, so I guess I cannot make this in
>> one step ... Probably it would require a lot of homework.
>>
>> _find_*_bit() functions and/or macros cause quite a number of KCSAN BUG warnings:
>>
>>   95 _find_first_and_bit (lib/find_bit.c:114 (discriminator 10))
>>   31 _find_first_zero_bit (lib/find_bit.c:125 (discriminator 10))
>> 173 _find_next_and_bit (lib/find_bit.c:171 (discriminator 2))
>> 655 _find_next_bit (lib/find_bit.c:133 (discriminator 2))
>>    5 _find_next_zero_bit
>>
>> ... but I am simply not certain what is the right thing to do ATM about
>> those and whether they are false positives.
> 
> Well, it would require some auditing to be sure but there is at least one
> user of these functions (xarray) where the problem is real so given the fix
> has no real runtime cost the fix looks justified.
> 
>> AFAICS, READ_ONCE() here solves the case of 64 and 32 architectures which is
>> an incremental step, and it works ... I am just not ready for an
>> universal solution ATM.
>>
>>> 2) Change xas_find_chunk() to unconditionally use find_next_bit() as the
>>> special case XA_CHUNK_SIZE == BITS_PER_LONG seems pointless these days
>>> because find_next_bit() is inline and does small_const_nbits(size) check.
>>
>> I see your point. A generalised solution would of course be better. But
>> from the report about data-races in those functions it seems that they
>> need a major rethink. It isn't that obvious to me what should be
>> READ_ONCE()-ed in a bit field ...
> 
> Well, it's actually not that difficult. They all need a treatment like:
> 
> unsigned long _find_next_bit(const unsigned long *addr, unsigned long nbits, uns
> {
> -       return FIND_NEXT_BIT(addr[idx], /* nop */, nbits, start);
> +       return FIND_NEXT_BIT(READ_ONCE(addr[idx]), /* nop */, nbits, start);
> }
> 
> 
>> Those functions are extensively used throughout the kernel and I get the
>> notion it is a job for someone with more experience ...
> 
> Sure, if you don't feel like doing the general change, I can look into it
> myself.
> 
> 								Honza

Hi,

I tried this patch and the

>>   95 _find_first_and_bit (lib/find_bit.c:114 (discriminator 10))
>>   31 _find_first_zero_bit (lib/find_bit.c:125 (discriminator 10))
>> 173 _find_next_and_bit (lib/find_bit.c:171 (discriminator 2))
>> 655 _find_next_bit (lib/find_bit.c:133 (discriminator 2))
>>    5 _find_next_zero_bit

data-races do not seem to appear any longer.

--------------------------------------------------------
  lib/find_bit.c | 33 +++++++++++++++++----------------
  1 file changed, 17 insertions(+), 16 deletions(-)

diff --git a/lib/find_bit.c b/lib/find_bit.c
index 32f99e9a670e..56244e4f744e 100644
--- a/lib/find_bit.c
+++ b/lib/find_bit.c
@@ -18,6 +18,7 @@
  #include <linux/math.h>
  #include <linux/minmax.h>
  #include <linux/swab.h>
+#include <asm/rwonce.h>
  
  /*
   * Common helper for find_bit() function family
@@ -98,7 +99,7 @@ out:                                                                          \
   */
  unsigned long _find_first_bit(const unsigned long *addr, unsigned long size)
  {
-       return FIND_FIRST_BIT(addr[idx], /* nop */, size);
+       return FIND_FIRST_BIT(READ_ONCE(addr[idx]), /* nop */, size);
  }
  EXPORT_SYMBOL(_find_first_bit);
  #endif
@@ -111,7 +112,7 @@ unsigned long _find_first_and_bit(const unsigned long *addr1,
                                   const unsigned long *addr2,
                                   unsigned long size)
  {
-       return FIND_FIRST_BIT(addr1[idx] & addr2[idx], /* nop */, size);
+       return FIND_FIRST_BIT(READ_ONCE(addr1[idx]) & READ_ONCE(addr2[idx]), /* nop */, size);
  }
  EXPORT_SYMBOL(_find_first_and_bit);
  #endif
@@ -122,7 +123,7 @@ EXPORT_SYMBOL(_find_first_and_bit);
   */
  unsigned long _find_first_zero_bit(const unsigned long *addr, unsigned long size)
  {
-       return FIND_FIRST_BIT(~addr[idx], /* nop */, size);
+       return FIND_FIRST_BIT(~READ_ONCE(addr[idx]), /* nop */, size);
  }
  EXPORT_SYMBOL(_find_first_zero_bit);
  #endif
@@ -130,28 +131,28 @@ EXPORT_SYMBOL(_find_first_zero_bit);
  #ifndef find_next_bit
  unsigned long _find_next_bit(const unsigned long *addr, unsigned long nbits, unsigned long start)
  {
-       return FIND_NEXT_BIT(addr[idx], /* nop */, nbits, start);
+       return FIND_NEXT_BIT(READ_ONCE(addr[idx]), /* nop */, nbits, start);
  }
  EXPORT_SYMBOL(_find_next_bit);
  #endif
  
  unsigned long __find_nth_bit(const unsigned long *addr, unsigned long size, unsigned long n)
  {
-       return FIND_NTH_BIT(addr[idx], size, n);
+       return FIND_NTH_BIT(READ_ONCE(addr[idx]), size, n);
  }
  EXPORT_SYMBOL(__find_nth_bit);
  
  unsigned long __find_nth_and_bit(const unsigned long *addr1, const unsigned long *addr2,
                                  unsigned long size, unsigned long n)
  {
-       return FIND_NTH_BIT(addr1[idx] & addr2[idx], size, n);
+       return FIND_NTH_BIT(READ_ONCE(addr1[idx]) & READ_ONCE(addr2[idx]), size, n);
  }
  EXPORT_SYMBOL(__find_nth_and_bit);
  
  unsigned long __find_nth_andnot_bit(const unsigned long *addr1, const unsigned long *addr2,
                                  unsigned long size, unsigned long n)
  {
-       return FIND_NTH_BIT(addr1[idx] & ~addr2[idx], size, n);
+       return FIND_NTH_BIT(READ_ONCE(addr1[idx]) & ~READ_ONCE(addr2[idx]), size, n);
  }
  EXPORT_SYMBOL(__find_nth_andnot_bit);
  
@@ -160,7 +161,7 @@ unsigned long __find_nth_and_andnot_bit(const unsigned long *addr1,
                                         const unsigned long *addr3,
                                         unsigned long size, unsigned long n)
  {
-       return FIND_NTH_BIT(addr1[idx] & addr2[idx] & ~addr3[idx], size, n);
+       return FIND_NTH_BIT(READ_ONCE(addr1[idx]) & READ_ONCE(addr2[idx]) & ~READ_ONCE(addr3[idx]), size, n);
  }
  EXPORT_SYMBOL(__find_nth_and_andnot_bit);
  
@@ -168,7 +169,7 @@ EXPORT_SYMBOL(__find_nth_and_andnot_bit);
  unsigned long _find_next_and_bit(const unsigned long *addr1, const unsigned long *addr2,
                                         unsigned long nbits, unsigned long start)
  {
-       return FIND_NEXT_BIT(addr1[idx] & addr2[idx], /* nop */, nbits, start);
+       return FIND_NEXT_BIT(READ_ONCE(addr1[idx]) & READ_ONCE(addr2[idx]), /* nop */, nbits, start);
  }
  EXPORT_SYMBOL(_find_next_and_bit);
  #endif
@@ -177,7 +178,7 @@ EXPORT_SYMBOL(_find_next_and_bit);
  unsigned long _find_next_andnot_bit(const unsigned long *addr1, const unsigned long *addr2,
                                         unsigned long nbits, unsigned long start)
  {
-       return FIND_NEXT_BIT(addr1[idx] & ~addr2[idx], /* nop */, nbits, start);
+       return FIND_NEXT_BIT(READ_ONCE(addr1[idx]) & ~READ_ONCE(addr2[idx]), /* nop */, nbits, start);
  }
  EXPORT_SYMBOL(_find_next_andnot_bit);
  #endif
@@ -186,7 +187,7 @@ EXPORT_SYMBOL(_find_next_andnot_bit);
  unsigned long _find_next_or_bit(const unsigned long *addr1, const unsigned long *addr2,
                                         unsigned long nbits, unsigned long start)
  {
-       return FIND_NEXT_BIT(addr1[idx] | addr2[idx], /* nop */, nbits, start);
+       return FIND_NEXT_BIT(READ_ONCE(addr1[idx]) | READ_ONCE(addr2[idx]), /* nop */, nbits, start);
  }
  EXPORT_SYMBOL(_find_next_or_bit);
  #endif
@@ -195,7 +196,7 @@ EXPORT_SYMBOL(_find_next_or_bit);
  unsigned long _find_next_zero_bit(const unsigned long *addr, unsigned long nbits,
                                          unsigned long start)
  {
-       return FIND_NEXT_BIT(~addr[idx], /* nop */, nbits, start);
+       return FIND_NEXT_BIT(~READ_ONCE(addr[idx]), /* nop */, nbits, start);
  }
  EXPORT_SYMBOL(_find_next_zero_bit);
  #endif
@@ -208,7 +209,7 @@ unsigned long _find_last_bit(const unsigned long *addr, unsigned long size)
                 unsigned long idx = (size-1) / BITS_PER_LONG;
  
                 do {
-                       val &= addr[idx];
+                       val &= READ_ONCE(addr[idx]);
                         if (val)
                                 return idx * BITS_PER_LONG + __fls(val);
  
@@ -242,7 +243,7 @@ EXPORT_SYMBOL(find_next_clump8);
   */
  unsigned long _find_first_zero_bit_le(const unsigned long *addr, unsigned long size)
  {
-       return FIND_FIRST_BIT(~addr[idx], swab, size);
+       return FIND_FIRST_BIT(~READ_ONCE(addr[idx]), swab, size);
  }
  EXPORT_SYMBOL(_find_first_zero_bit_le);
  
@@ -252,7 +253,7 @@ EXPORT_SYMBOL(_find_first_zero_bit_le);
  unsigned long _find_next_zero_bit_le(const unsigned long *addr,
                                         unsigned long size, unsigned long offset)
  {
-       return FIND_NEXT_BIT(~addr[idx], swab, size, offset);
+       return FIND_NEXT_BIT(~READ_ONCE(addr[idx]), swab, size, offset);
  }
  EXPORT_SYMBOL(_find_next_zero_bit_le);
  #endif
@@ -261,7 +262,7 @@ EXPORT_SYMBOL(_find_next_zero_bit_le);
  unsigned long _find_next_bit_le(const unsigned long *addr,
                                 unsigned long size, unsigned long offset)
  {
-       return FIND_NEXT_BIT(addr[idx], swab, size, offset);
+       return FIND_NEXT_BIT(READ_ONCE(addr[idx]), swab, size, offset);
  }
  EXPORT_SYMBOL(_find_next_bit_le);
  
--