LinuxLists.cc - [PATCH v2] mm/page_reporting: replace rcu_access_pointer() with rcu_dereference

2022-12-28 01:39:32

Subject: [PATCH v2] mm/page_reporting: replace rcu_access_pointer() with rcu_dereference_protected()

Page reporting fetches pr_dev_info using rcu_access_pointer(), which is
for safely fetching a pointer that will not be dereferenced but could
concurrently updated. The code indeed does not dereference pr_dev_info
after fetcing it using rcu_access_pointer(), but it fetches the pointer
while concurrent updtes to the pointer is avoided by holding the update
side lock, page_reporting_mutex.

In the case, rcu_dereference_protected() is recommended because it
provides better readability and performance on some cases, as
rcu_dereference_protected() avoids use of READ_ONCE(). Replace the
rcu_access_pointer() calls with rcu_dereference_protected().

Signed-off-by: SeongJae Park <[email protected]>
---
Changes from v1
(https://lore.kernel.org/linux-mm/[email protected]/)
- Explicitly set the protection condition (Matthew Wilcox)

mm/page_reporting.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/page_reporting.c b/mm/page_reporting.c
index 79a8554f024c..5c557a3e1423 100644
--- a/mm/page_reporting.c
+++ b/mm/page_reporting.c
@@ -356,7 +356,8 @@ int page_reporting_register(struct page_reporting_dev_info *prdev)
mutex_lock(&page_reporting_mutex);

/* nothing to do if already in use */
- if (rcu_access_pointer(pr_dev_info)) {
+ if (rcu_dereference_protected(pr_dev_info,
+ lockdep_is_held(&page_reporting_order))) {
err = -EBUSY;
goto err_out;
}
@@ -401,7 +402,8 @@ void page_reporting_unregister(struct page_reporting_dev_info *prdev)
{
mutex_lock(&page_reporting_mutex);

- if (rcu_access_pointer(pr_dev_info) == prdev) {
+ if (prdev == rcu_dereference_protected(pr_dev_info,
+ lockdep_is_held(&page_reporting_mutex))) {
/* Disable page reporting notification */
RCU_INIT_POINTER(pr_dev_info, NULL);
synchronize_rcu();
--
2.25.1

2022-12-28 02:17:06

by Miaohe Lin

[permalink] [raw]

Subject: Re: [PATCH v2] mm/page_reporting: replace rcu_access_pointer() with rcu_dereference_protected()

On 2022/12/28 9:29, SeongJae Park wrote:
> Page reporting fetches pr_dev_info using rcu_access_pointer(), which is
> for safely fetching a pointer that will not be dereferenced but could
> concurrently updated. The code indeed does not dereference pr_dev_info
> after fetcing it using rcu_access_pointer(), but it fetches the pointer

Thanks for your work. Might something to improve.

s/fetcing/fetching/

> while concurrent updtes to the pointer is avoided by holding the update

s/updtes/updates/

> side lock, page_reporting_mutex.
>
> In the case, rcu_dereference_protected() is recommended because it
> provides better readability and performance on some cases, as
> rcu_dereference_protected() avoids use of READ_ONCE(). Replace the
> rcu_access_pointer() calls with rcu_dereference_protected().
>
> Signed-off-by: SeongJae Park <[email protected]>
> ---
> Changes from v1
> (https://lore.kernel.org/linux-mm/[email protected]/)
> - Explicitly set the protection condition (Matthew Wilcox)
>
> mm/page_reporting.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/mm/page_reporting.c b/mm/page_reporting.c
> index 79a8554f024c..5c557a3e1423 100644
> --- a/mm/page_reporting.c
> +++ b/mm/page_reporting.c
> @@ -356,7 +356,8 @@ int page_reporting_register(struct page_reporting_dev_info *prdev)
> mutex_lock(&page_reporting_mutex);
>
> /* nothing to do if already in use */
> - if (rcu_access_pointer(pr_dev_info)) {
> + if (rcu_dereference_protected(pr_dev_info,
> + lockdep_is_held(&page_reporting_order))) {

I think it should be lockdep_is_held(&page_reporting_mutex) instead of lockdep_is_held(&page_reporting_order) here?

Thanks,
Miaohe Lin

2022-12-28 02:19:13

by SeongJae Park

[permalink] [raw]

Subject: Re: [PATCH v2] mm/page_reporting: replace rcu_access_pointer() with rcu_dereference_protected()

On Wed, 28 Dec 2022 09:45:00 +0800 Miaohe Lin <[email protected]> wrote:

> On 2022/12/28 9:29, SeongJae Park wrote:
> > Page reporting fetches pr_dev_info using rcu_access_pointer(), which is
> > for safely fetching a pointer that will not be dereferenced but could
> > concurrently updated. The code indeed does not dereference pr_dev_info
> > after fetcing it using rcu_access_pointer(), but it fetches the pointer
>
> Thanks for your work. Might something to improve.
>
> s/fetcing/fetching/
>
> > while concurrent updtes to the pointer is avoided by holding the update
>
> s/updtes/updates/

Thank you! I shall add these to scripts/spelling.txt.

>
> > side lock, page_reporting_mutex.
> >
> > In the case, rcu_dereference_protected() is recommended because it
> > provides better readability and performance on some cases, as
> > rcu_dereference_protected() avoids use of READ_ONCE(). Replace the
> > rcu_access_pointer() calls with rcu_dereference_protected().
> >
> > Signed-off-by: SeongJae Park <[email protected]>
> > ---
> > Changes from v1
> > (https://lore.kernel.org/linux-mm/[email protected]/)
> > - Explicitly set the protection condition (Matthew Wilcox)
> >
> > mm/page_reporting.c | 6 ++++--
> > 1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/page_reporting.c b/mm/page_reporting.c
> > index 79a8554f024c..5c557a3e1423 100644
> > --- a/mm/page_reporting.c
> > +++ b/mm/page_reporting.c
> > @@ -356,7 +356,8 @@ int page_reporting_register(struct page_reporting_dev_info *prdev)
> > mutex_lock(&page_reporting_mutex);
> >
> > /* nothing to do if already in use */
> > - if (rcu_access_pointer(pr_dev_info)) {
> > + if (rcu_dereference_protected(pr_dev_info,
> > + lockdep_is_held(&page_reporting_order))) {
>
> I think it should be lockdep_is_held(&page_reporting_mutex) instead of
> lockdep_is_held(&page_reporting_order) here?

You're right, thank you for finding this.

I will fix these in the next version.

Thanks,
SJ

>
> Thanks,
> Miaohe Lin

2022-12-28 05:41:05

by kernel test robot

[permalink] [raw]

Subject: Re: [PATCH v2] mm/page_reporting: replace rcu_access_pointer() with rcu_dereference_protected()

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2022-12-28 06:56:52

by kernel test robot

[permalink] [raw]

Subject: Re: [PATCH v2] mm/page_reporting: replace rcu_access_pointer() with rcu_dereference_protected()

Hi SeongJae,

I love your patch! Yet something to improve:

[auto build test ERROR on akpm-mm/mm-everything]

url: https://github.com/intel-lab-lkp/linux/commits/SeongJae-Park/mm-page_reporting-replace-rcu_access_pointer-with-rcu_dereference_protected/20221228-093057
base: https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
patch link: https://lore.kernel.org/r/20221228012902.1510901-1-sj%40kernel.org
patch subject: [PATCH v2] mm/page_reporting: replace rcu_access_pointer() with rcu_dereference_protected()
config: riscv-randconfig-r022-20221225
compiler: clang version 16.0.0 (https://github.com/llvm/llvm-project f5700e7b69048de958172fb513b336564e7f8709)
reproduce (this is a W=1 build):
wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
chmod +x ~/bin/make.cross
# install riscv cross compiling tool for clang build
# apt-get install binutils-riscv64-linux-gnu
# https://github.com/intel-lab-lkp/linux/commit/f7744809d7f12dccff86c60831ddf3c316ada60e
git remote add linux-review https://github.com/intel-lab-lkp/linux
git fetch --no-tags linux-review SeongJae-Park/mm-page_reporting-replace-rcu_access_pointer-with-rcu_dereference_protected/20221228-093057
git checkout f7744809d7f12dccff86c60831ddf3c316ada60e
# save the config file
mkdir build_dir && cp config build_dir/.config
COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=riscv olddefconfig
COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=riscv SHELL=/bin/bash

If you fix the issue, kindly add following tag where applicable
| Reported-by: kernel test robot <[email protected]>

All errors (new ones prefixed by >>):

In file included from mm/page_reporting.c:4:
In file included from include/linux/page_reporting.h:6:
In file included from include/linux/scatterlist.h:9:
In file included from arch/riscv/include/asm/io.h:136:
include/asm-generic/io.h:547:31: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
val = __raw_readb(PCI_IOBASE + addr);
~~~~~~~~~~ ^
include/asm-generic/io.h:560:61: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
val = __le16_to_cpu((__le16 __force)__raw_readw(PCI_IOBASE + addr));
~~~~~~~~~~ ^
include/uapi/linux/byteorder/little_endian.h:37:51: note: expanded from macro '__le16_to_cpu'
#define __le16_to_cpu(x) ((__force __u16)(__le16)(x))
^
In file included from mm/page_reporting.c:4:
In file included from include/linux/page_reporting.h:6:
In file included from include/linux/scatterlist.h:9:
In file included from arch/riscv/include/asm/io.h:136:
include/asm-generic/io.h:573:61: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
val = __le32_to_cpu((__le32 __force)__raw_readl(PCI_IOBASE + addr));
~~~~~~~~~~ ^
include/uapi/linux/byteorder/little_endian.h:35:51: note: expanded from macro '__le32_to_cpu'
#define __le32_to_cpu(x) ((__force __u32)(__le32)(x))
^
In file included from mm/page_reporting.c:4:
In file included from include/linux/page_reporting.h:6:
In file included from include/linux/scatterlist.h:9:
In file included from arch/riscv/include/asm/io.h:136:
include/asm-generic/io.h:584:33: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
__raw_writeb(value, PCI_IOBASE + addr);
~~~~~~~~~~ ^
include/asm-generic/io.h:594:59: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
__raw_writew((u16 __force)cpu_to_le16(value), PCI_IOBASE + addr);
~~~~~~~~~~ ^
include/asm-generic/io.h:604:59: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
__raw_writel((u32 __force)cpu_to_le32(value), PCI_IOBASE + addr);
~~~~~~~~~~ ^
include/asm-generic/io.h:743:2: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
insb(addr, buffer, count);
^~~~~~~~~~~~~~~~~~~~~~~~~
arch/riscv/include/asm/io.h:104:53: note: expanded from macro 'insb'
#define insb(addr, buffer, count) __insb(PCI_IOBASE + (addr), buffer, count)
~~~~~~~~~~ ^
In file included from mm/page_reporting.c:4:
In file included from include/linux/page_reporting.h:6:
In file included from include/linux/scatterlist.h:9:
In file included from arch/riscv/include/asm/io.h:136:
include/asm-generic/io.h:751:2: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
insw(addr, buffer, count);
^~~~~~~~~~~~~~~~~~~~~~~~~
arch/riscv/include/asm/io.h:105:53: note: expanded from macro 'insw'
#define insw(addr, buffer, count) __insw(PCI_IOBASE + (addr), buffer, count)
~~~~~~~~~~ ^
In file included from mm/page_reporting.c:4:
In file included from include/linux/page_reporting.h:6:
In file included from include/linux/scatterlist.h:9:
In file included from arch/riscv/include/asm/io.h:136:
include/asm-generic/io.h:759:2: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
insl(addr, buffer, count);
^~~~~~~~~~~~~~~~~~~~~~~~~
arch/riscv/include/asm/io.h:106:53: note: expanded from macro 'insl'
#define insl(addr, buffer, count) __insl(PCI_IOBASE + (addr), buffer, count)
~~~~~~~~~~ ^
In file included from mm/page_reporting.c:4:
In file included from include/linux/page_reporting.h:6:
In file included from include/linux/scatterlist.h:9:
In file included from arch/riscv/include/asm/io.h:136:
include/asm-generic/io.h:768:2: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
outsb(addr, buffer, count);
^~~~~~~~~~~~~~~~~~~~~~~~~~
arch/riscv/include/asm/io.h:118:55: note: expanded from macro 'outsb'
#define outsb(addr, buffer, count) __outsb(PCI_IOBASE + (addr), buffer, count)
~~~~~~~~~~ ^
In file included from mm/page_reporting.c:4:
In file included from include/linux/page_reporting.h:6:
In file included from include/linux/scatterlist.h:9:
In file included from arch/riscv/include/asm/io.h:136:
include/asm-generic/io.h:777:2: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
outsw(addr, buffer, count);
^~~~~~~~~~~~~~~~~~~~~~~~~~
arch/riscv/include/asm/io.h:119:55: note: expanded from macro 'outsw'
#define outsw(addr, buffer, count) __outsw(PCI_IOBASE + (addr), buffer, count)
~~~~~~~~~~ ^
In file included from mm/page_reporting.c:4:
In file included from include/linux/page_reporting.h:6:
In file included from include/linux/scatterlist.h:9:
In file included from arch/riscv/include/asm/io.h:136:
include/asm-generic/io.h:786:2: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
outsl(addr, buffer, count);
^~~~~~~~~~~~~~~~~~~~~~~~~~
arch/riscv/include/asm/io.h:120:55: note: expanded from macro 'outsl'
#define outsl(addr, buffer, count) __outsl(PCI_IOBASE + (addr), buffer, count)
~~~~~~~~~~ ^
In file included from mm/page_reporting.c:4:
In file included from include/linux/page_reporting.h:6:
In file included from include/linux/scatterlist.h:9:
In file included from arch/riscv/include/asm/io.h:136:
include/asm-generic/io.h:1134:55: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
return (port > MMIO_UPPER_LIMIT) ? NULL : PCI_IOBASE + port;
~~~~~~~~~~ ^
>> mm/page_reporting.c:360:5: error: member reference base type 'unsigned int' is not a structure or union
lockdep_is_held(&page_reporting_order))) {
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/linux/lockdep.h:286:52: note: expanded from macro 'lockdep_is_held'
#define lockdep_is_held(lock) lock_is_held(&(lock)->dep_map)
^ ~~~~~~~
include/linux/rcupdate.h:664:54: note: expanded from macro 'rcu_dereference_protected'
__rcu_dereference_protected((p), __UNIQUE_ID(rcu), (c), __rcu)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~
include/linux/rcupdate.h:460:21: note: expanded from macro '__rcu_dereference_protected'
RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_protected() usage"); \
~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/linux/rcupdate.h:409:52: note: expanded from macro 'RCU_LOCKDEP_WARN'
#define RCU_LOCKDEP_WARN(c, s) do { } while (0 && (c))
^
13 warnings and 1 error generated.

vim +360 mm/page_reporting.c

351
352 int page_reporting_register(struct page_reporting_dev_info *prdev)
353 {
354 int err = 0;
355
356 mutex_lock(&page_reporting_mutex);
357
358 /* nothing to do if already in use */
359 if (rcu_dereference_protected(pr_dev_info,
> 360 lockdep_is_held(&page_reporting_order))) {
361 err = -EBUSY;
362 goto err_out;
363 }
364
365 /*
366 * If the page_reporting_order value is not set, we check if
367 * an order is provided from the driver that is performing the
368 * registration. If that is not provided either, we default to
369 * pageblock_order.
370 */
371
372 if (page_reporting_order == -1) {
373 if (prdev->order > 0 && prdev->order <= MAX_ORDER)
374 page_reporting_order = prdev->order;
375 else
376 page_reporting_order = pageblock_order;
377 }
378
379 /* initialize state and work structures */
380 atomic_set(&prdev->state, PAGE_REPORTING_IDLE);
381 INIT_DELAYED_WORK(&prdev->work, &page_reporting_process);
382
383 /* Begin initial flush of zones */
384 __page_reporting_request(prdev);
385
386 /* Assign device to allow notifications */
387 rcu_assign_pointer(pr_dev_info, prdev);
388
389 /* enable page reporting notification */
390 if (!static_key_enabled(&page_reporting_enabled)) {
391 static_branch_enable(&page_reporting_enabled);
392 pr_info("Free page reporting enabled\n");
393 }
394 err_out:
395 mutex_unlock(&page_reporting_mutex);
396
397 return err;
398 }
399 EXPORT_SYMBOL_GPL(page_reporting_register);
400

--
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https://01.org/lkp

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