diff mbox series

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

Message ID 20221228012902.1510901-1-sj@kernel.org
State New
Headers
Series [v2] mm/page_reporting: replace rcu_access_pointer() with rcu_dereference_protected() |

Commit Message

SeongJae Park Dec. 28, 2022, 1:29 a.m. UTC 
  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 <sj@kernel.org>
---
Changes from v1
(https://lore.kernel.org/linux-mm/20221227192158.2553-1-sj@kernel.org/)
- Explicitly set the protection condition (Matthew Wilcox)

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

Comments

Miaohe Lin Dec. 28, 2022, 1:45 a.m. UTC | #1 
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 <sj@kernel.org>
> ---
> Changes from v1
> (https://lore.kernel.org/linux-mm/20221227192158.2553-1-sj@kernel.org/)
> - 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
SeongJae Park Dec. 28, 2022, 2:09 a.m. UTC | #2 
On Wed, 28 Dec 2022 09:45:00 +0800 Miaohe Lin <linmiaohe@huawei.com> 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 <sj@kernel.org>
> > ---
> > Changes from v1
> > (https://lore.kernel.org/linux-mm/20221227192158.2553-1-sj@kernel.org/)
> > - 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
kernel test robot Dec. 28, 2022, 4:55 a.m. UTC | #3 
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: x86_64-randconfig-a002-20221226
compiler: clang version 14.0.6 (https://github.com/llvm/llvm-project f28c006a5895fc0e329fe15fead81e37457cb1d1)
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
        # 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=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=x86_64 olddefconfig
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash

If you fix the issue, kindly add following tag where applicable
| Reported-by: kernel test robot <lkp@intel.com>

All errors (new ones prefixed by >>):

>> mm/page_reporting.c:360:5: error: member reference base type 'typeof (page_reporting_order)' (aka '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:381:8: note: expanded from macro 'RCU_LOCKDEP_WARN'
                   if ((c) && debug_lockdep_rcu_enabled() && !__warned) {  \
                        ^
   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
kernel test robot Dec. 28, 2022, 6:47 a.m. UTC | #4 
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 <lkp@intel.com>

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
diff mbox series

Patch

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();