[PATCH] rwsem: fix missed wakeup due to reordering of load

[PATCH] rwsem: fix missed wakeup due to reordering of load

[Prateek Sood]

If a spinner is present, there is a chance that the load of
rwsem_has_spinner() in rwsem_wake() can be reordered with
respect to decrement of rwsem count in __up_write() leading
to wakeup being missed.

 spinning writer                  up_write caller
 ---------------                  -----------------------
 [S] osq_unlock()                 [L] osq
  spin_lock(wait_lock)
  sem->count=0xFFFFFFFF00000001
            +0xFFFFFFFF00000000
  count=sem->count
  MB
                                   sem->count=0xFFFFFFFE00000001
                                             -0xFFFFFFFF00000001
                                   spin_trylock(wait_lock)
                                   return
 rwsem_try_write_lock(count)
 spin_unlock(wait_lock)
 schedule()

Reordering of atomic_long_sub_return_release() in __up_write()
and rwsem_has_spinner() in rwsem_wake() can cause missing of
wakeup in up_write() context. In spinning writer, sem->count
and local variable count is 0XFFFFFFFE00000001. It would result
in rwsem_try_write_lock() failing to acquire rwsem and spinning
writer going to sleep in rwsem_down_write_failed().

The smp_rmb() will make sure that the spinner state is
consulted after sem->count is updated in up_write context.

Signed-off-by: Prateek Sood <prsood@codeaurora.org>
---
 kernel/locking/rwsem-xadd.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 02f6606..1fefe6d 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -613,6 +613,33 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
 	DEFINE_WAKE_Q(wake_q);
 
 	/*
+	* __rwsem_down_write_failed_common(sem)
+	*   rwsem_optimistic_spin(sem)
+	*     osq_unlock(sem->osq)
+	*   ...
+	*   atomic_long_add_return(&sem->count)
+	*
+	*      - VS -
+	*
+	*              __up_write()
+	*                if (atomic_long_sub_return_release(&sem->count) < 0)
+	*                  rwsem_wake(sem)
+	*                    osq_is_locked(&sem->osq)
+	*
+	* And __up_write() must observe !osq_is_locked() when it observes the
+	* atomic_long_add_return() in order to not miss a wakeup.
+	*
+	* This boils down to:
+	*
+	* [S.rel] X = 1                [RmW] r0 = (Y += 0)
+	*         MB                         RMB
+	* [RmW]   Y += 1               [L]   r1 = X
+	*
+	* exists (r0=1 /\ r1=0)
+	*/
+	smp_rmb();
+
+	/*
 	 * If a spinner is present, it is not necessary to do the wakeup.
 	 * Try to do wakeup only if the trylock succeeds to minimize
 	 * spinlock contention which may introduce too much delay in the
-- 
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc., 
is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.

Re: [PATCH] rwsem: fix missed wakeup due to reordering of load

[Andrea Parri]

On Thu, Sep 07, 2017 at 08:00:58PM +0530, Prateek Sood wrote:
> If a spinner is present, there is a chance that the load of
> rwsem_has_spinner() in rwsem_wake() can be reordered with
> respect to decrement of rwsem count in __up_write() leading
> to wakeup being missed.
> 
>  spinning writer                  up_write caller
>  ---------------                  -----------------------
>  [S] osq_unlock()                 [L] osq
>   spin_lock(wait_lock)
>   sem->count=0xFFFFFFFF00000001
>             +0xFFFFFFFF00000000
>   count=sem->count
>   MB
>                                    sem->count=0xFFFFFFFE00000001
>                                              -0xFFFFFFFF00000001
>                                    spin_trylock(wait_lock)
>                                    return
>  rwsem_try_write_lock(count)
>  spin_unlock(wait_lock)
>  schedule()
> 
> Reordering of atomic_long_sub_return_release() in __up_write()
> and rwsem_has_spinner() in rwsem_wake() can cause missing of
> wakeup in up_write() context. In spinning writer, sem->count
> and local variable count is 0XFFFFFFFE00000001. It would result
> in rwsem_try_write_lock() failing to acquire rwsem and spinning
> writer going to sleep in rwsem_down_write_failed().
> 
> The smp_rmb() will make sure that the spinner state is
> consulted after sem->count is updated in up_write context.
> 
> Signed-off-by: Prateek Sood <prsood@codeaurora.org>

Reviewed-by: Andrea Parri <parri.andrea@gmail.com>

I understand that the merge window and LPC made this stalls for
a while... what am I missing? are there other changes that need
to be considered for this patch?

  Andrea


> ---
>  kernel/locking/rwsem-xadd.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index 02f6606..1fefe6d 100644
> --- a/kernel/locking/rwsem-xadd.c
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -613,6 +613,33 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
>  	DEFINE_WAKE_Q(wake_q);
>  
>  	/*
> +	* __rwsem_down_write_failed_common(sem)
> +	*   rwsem_optimistic_spin(sem)
> +	*     osq_unlock(sem->osq)
> +	*   ...
> +	*   atomic_long_add_return(&sem->count)
> +	*
> +	*      - VS -
> +	*
> +	*              __up_write()
> +	*                if (atomic_long_sub_return_release(&sem->count) < 0)
> +	*                  rwsem_wake(sem)
> +	*                    osq_is_locked(&sem->osq)
> +	*
> +	* And __up_write() must observe !osq_is_locked() when it observes the
> +	* atomic_long_add_return() in order to not miss a wakeup.
> +	*
> +	* This boils down to:
> +	*
> +	* [S.rel] X = 1                [RmW] r0 = (Y += 0)
> +	*         MB                         RMB
> +	* [RmW]   Y += 1               [L]   r1 = X
> +	*
> +	* exists (r0=1 /\ r1=0)
> +	*/
> +	smp_rmb();
> +
> +	/*
>  	 * If a spinner is present, it is not necessary to do the wakeup.
>  	 * Try to do wakeup only if the trylock succeeds to minimize
>  	 * spinlock contention which may introduce too much delay in the
> -- 
> Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc., 
> is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
> 

Re: [PATCH] rwsem: fix missed wakeup due to reordering of load

[Davidlohr Bueso]

On Thu, 07 Sep 2017, Prateek Sood wrote:
> 	/*
>+	* __rwsem_down_write_failed_common(sem)
>+	*   rwsem_optimistic_spin(sem)
>+	*     osq_unlock(sem->osq)
>+	*   ...
>+	*   atomic_long_add_return(&sem->count)
>+	*
>+	*      - VS -
>+	*
>+	*              __up_write()
>+	*                if (atomic_long_sub_return_release(&sem->count) < 0)
>+	*                  rwsem_wake(sem)
>+	*                    osq_is_locked(&sem->osq)
>+	*
>+	* And __up_write() must observe !osq_is_locked() when it observes the
>+	* atomic_long_add_return() in order to not miss a wakeup.
>+	*
>+	* This boils down to:
>+	*
>+	* [S.rel] X = 1                [RmW] r0 = (Y += 0)
>+	*         MB                         RMB
>+	* [RmW]   Y += 1               [L]   r1 = X
>+	*
>+	* exists (r0=1 /\ r1=0)
>+	*/
>+	smp_rmb();

Instead, how about just removing the release from atomic_long_sub_return_release()
such that the osq load is not hoisted over the atomic compound (along with Peter's
comment):

diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
index 6c6a2141f271..487ce31078ff 100644
--- a/include/asm-generic/rwsem.h
+++ b/include/asm-generic/rwsem.h
@@ -101,7 +101,7 @@ static inline void __up_read(struct rw_semaphore *sem)
  */
 static inline void __up_write(struct rw_semaphore *sem)
 {
-	if (unlikely(atomic_long_sub_return_release(RWSEM_ACTIVE_WRITE_BIAS,
+	if (unlikely(atomic_long_sub_return(RWSEM_ACTIVE_WRITE_BIAS,
 						    &sem->count) < 0))
 		rwsem_wake(sem);
 }

Re: [PATCH] rwsem: fix missed wakeup due to reordering of load

[Andrea Parri]

On Wed, Sep 20, 2017 at 07:52:54AM -0700, Davidlohr Bueso wrote:
> On Thu, 07 Sep 2017, Prateek Sood wrote:
> >	/*
> >+	* __rwsem_down_write_failed_common(sem)
> >+	*   rwsem_optimistic_spin(sem)
> >+	*     osq_unlock(sem->osq)
> >+	*   ...
> >+	*   atomic_long_add_return(&sem->count)
> >+	*
> >+	*      - VS -
> >+	*
> >+	*              __up_write()
> >+	*                if (atomic_long_sub_return_release(&sem->count) < 0)
> >+	*                  rwsem_wake(sem)
> >+	*                    osq_is_locked(&sem->osq)
> >+	*
> >+	* And __up_write() must observe !osq_is_locked() when it observes the
> >+	* atomic_long_add_return() in order to not miss a wakeup.
> >+	*
> >+	* This boils down to:
> >+	*
> >+	* [S.rel] X = 1                [RmW] r0 = (Y += 0)
> >+	*         MB                         RMB
> >+	* [RmW]   Y += 1               [L]   r1 = X
> >+	*
> >+	* exists (r0=1 /\ r1=0)
> >+	*/
> >+	smp_rmb();
> 
> Instead, how about just removing the release from atomic_long_sub_return_release()
> such that the osq load is not hoisted over the atomic compound (along with Peter's
> comment):

This solution will actually enforce a stronger (full) ordering w.r.t. the
solution described by Prateek and Peter. Also, it will "trade" two lwsync
for two sync (powerpc), one dmb.ld for one dmb (arm64).

What are the reasons you would prefer this?

  Andrea


> 
> diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
> index 6c6a2141f271..487ce31078ff 100644
> --- a/include/asm-generic/rwsem.h
> +++ b/include/asm-generic/rwsem.h
> @@ -101,7 +101,7 @@ static inline void __up_read(struct rw_semaphore *sem)
>  */
> static inline void __up_write(struct rw_semaphore *sem)
> {
> -	if (unlikely(atomic_long_sub_return_release(RWSEM_ACTIVE_WRITE_BIAS,
> +	if (unlikely(atomic_long_sub_return(RWSEM_ACTIVE_WRITE_BIAS,
> 						    &sem->count) < 0))
> 		rwsem_wake(sem);
> }

Re: [PATCH] rwsem: fix missed wakeup due to reordering of load

[Prateek Sood]

On 09/07/2017 08:00 PM, Prateek Sood wrote:
> If a spinner is present, there is a chance that the load of
> rwsem_has_spinner() in rwsem_wake() can be reordered with
> respect to decrement of rwsem count in __up_write() leading
> to wakeup being missed.
> 
>  spinning writer                  up_write caller
>  ---------------                  -----------------------
>  [S] osq_unlock()                 [L] osq
>   spin_lock(wait_lock)
>   sem->count=0xFFFFFFFF00000001
>             +0xFFFFFFFF00000000
>   count=sem->count
>   MB
>                                    sem->count=0xFFFFFFFE00000001
>                                              -0xFFFFFFFF00000001
>                                    spin_trylock(wait_lock)
>                                    return
>  rwsem_try_write_lock(count)
>  spin_unlock(wait_lock)
>  schedule()
> 
> Reordering of atomic_long_sub_return_release() in __up_write()
> and rwsem_has_spinner() in rwsem_wake() can cause missing of
> wakeup in up_write() context. In spinning writer, sem->count
> and local variable count is 0XFFFFFFFE00000001. It would result
> in rwsem_try_write_lock() failing to acquire rwsem and spinning
> writer going to sleep in rwsem_down_write_failed().
> 
> The smp_rmb() will make sure that the spinner state is
> consulted after sem->count is updated in up_write context.
> 
> Signed-off-by: Prateek Sood <prsood@codeaurora.org>
> ---
>  kernel/locking/rwsem-xadd.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index 02f6606..1fefe6d 100644
> --- a/kernel/locking/rwsem-xadd.c
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -613,6 +613,33 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
>  	DEFINE_WAKE_Q(wake_q);
>  
>  	/*
> +	* __rwsem_down_write_failed_common(sem)
> +	*   rwsem_optimistic_spin(sem)
> +	*     osq_unlock(sem->osq)
> +	*   ...
> +	*   atomic_long_add_return(&sem->count)
> +	*
> +	*      - VS -
> +	*
> +	*              __up_write()
> +	*                if (atomic_long_sub_return_release(&sem->count) < 0)
> +	*                  rwsem_wake(sem)
> +	*                    osq_is_locked(&sem->osq)
> +	*
> +	* And __up_write() must observe !osq_is_locked() when it observes the
> +	* atomic_long_add_return() in order to not miss a wakeup.
> +	*
> +	* This boils down to:
> +	*
> +	* [S.rel] X = 1                [RmW] r0 = (Y += 0)
> +	*         MB                         RMB
> +	* [RmW]   Y += 1               [L]   r1 = X
> +	*
> +	* exists (r0=1 /\ r1=0)
> +	*/
> +	smp_rmb();
> +
> +	/*
>  	 * If a spinner is present, it is not necessary to do the wakeup.
>  	 * Try to do wakeup only if the trylock succeeds to minimize
>  	 * spinlock contention which may introduce too much delay in the
> 

Hi Folks,

Do you have any more suggestion/feedback on this patch.


Regards
Prateek

-- 
Qualcomm India Private Limited, on behalf of Qualcomm Innovation
Center, Inc., is a member of Code Aurora Forum, a Linux Foundation
Collaborative Project

Re: [PATCH] rwsem: fix missed wakeup due to reordering of load

[Davidlohr Bueso]

On Wed, 20 Sep 2017, Andrea Parri wrote:

>> Instead, how about just removing the release from atomic_long_sub_return_release()
>> such that the osq load is not hoisted over the atomic compound (along with Peter's
>> comment):
>
>This solution will actually enforce a stronger (full) ordering w.r.t. the
>solution described by Prateek and Peter. Also, it will "trade" two lwsync
>for two sync (powerpc), one dmb.ld for one dmb (arm64).
>
>What are the reasons you would prefer this?

It was mainly to maintain consistency about dealing with sem->count, but sure
I won't argue with the above.

[tip:locking/urgent] locking/rwsem-xadd: Fix missed wakeup due to reordering of load

[tip-bot for Prateek Sood]

Commit-ID:  9c29c31830a4eca724e137a9339137204bbb31be
Gitweb:     https://git.kernel.org/tip/9c29c31830a4eca724e137a9339137204bbb31be
Author:     Prateek Sood <prsood@codeaurora.org>
AuthorDate: Thu, 7 Sep 2017 20:00:58 +0530
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 29 Sep 2017 10:10:20 +0200

locking/rwsem-xadd: Fix missed wakeup due to reordering of load

If a spinner is present, there is a chance that the load of
rwsem_has_spinner() in rwsem_wake() can be reordered with
respect to decrement of rwsem count in __up_write() leading
to wakeup being missed:

 spinning writer                  up_write caller
 ---------------                  -----------------------
 [S] osq_unlock()                 [L] osq
  spin_lock(wait_lock)
  sem->count=0xFFFFFFFF00000001
            +0xFFFFFFFF00000000
  count=sem->count
  MB
                                   sem->count=0xFFFFFFFE00000001
                                             -0xFFFFFFFF00000001
                                   spin_trylock(wait_lock)
                                   return
 rwsem_try_write_lock(count)
 spin_unlock(wait_lock)
 schedule()

Reordering of atomic_long_sub_return_release() in __up_write()
and rwsem_has_spinner() in rwsem_wake() can cause missing of
wakeup in up_write() context. In spinning writer, sem->count
and local variable count is 0XFFFFFFFE00000001. It would result
in rwsem_try_write_lock() failing to acquire rwsem and spinning
writer going to sleep in rwsem_down_write_failed().

The smp_rmb() will make sure that the spinner state is
consulted after sem->count is updated in up_write context.

Signed-off-by: Prateek Sood <prsood@codeaurora.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dave@stgolabs.net
Cc: longman@redhat.com
Cc: parri.andrea@gmail.com
Cc: sramana@codeaurora.org
Link: http://lkml.kernel.org/r/1504794658-15397-1-git-send-email-prsood@codeaurora.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/locking/rwsem-xadd.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 02f6606..1fefe6d 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -613,6 +613,33 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
 	DEFINE_WAKE_Q(wake_q);
 
 	/*
+	* __rwsem_down_write_failed_common(sem)
+	*   rwsem_optimistic_spin(sem)
+	*     osq_unlock(sem->osq)
+	*   ...
+	*   atomic_long_add_return(&sem->count)
+	*
+	*      - VS -
+	*
+	*              __up_write()
+	*                if (atomic_long_sub_return_release(&sem->count) < 0)
+	*                  rwsem_wake(sem)
+	*                    osq_is_locked(&sem->osq)
+	*
+	* And __up_write() must observe !osq_is_locked() when it observes the
+	* atomic_long_add_return() in order to not miss a wakeup.
+	*
+	* This boils down to:
+	*
+	* [S.rel] X = 1                [RmW] r0 = (Y += 0)
+	*         MB                         RMB
+	* [RmW]   Y += 1               [L]   r1 = X
+	*
+	* exists (r0=1 /\ r1=0)
+	*/
+	smp_rmb();
+
+	/*
 	 * If a spinner is present, it is not necessary to do the wakeup.
 	 * Try to do wakeup only if the trylock succeeds to minimize
 	 * spinlock contention which may introduce too much delay in the