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

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  [PATCH] rwsem: fix missed wakeup due to reordering of load Prateek Sood <prsood@codeaurora.org> - 2017-08-23 13:30 +0200
    Re: [PATCH] rwsem: fix missed wakeup due to reordering of load Peter Zijlstra <peterz@infradead.org> - 2017-08-24 13:30 +0200
      Re: [PATCH] rwsem: fix missed wakeup due to reordering of load Peter Zijlstra <peterz@infradead.org> - 2017-08-24 14:40 +0200
        Re: [PATCH] rwsem: fix missed wakeup due to reordering of load Peter Zijlstra <peterz@infradead.org> - 2017-08-24 15:00 +0200

#1718261 — [PATCH] rwsem: 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
                                   RMB
                                   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 | 45 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 34e727f..5c687f6 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -586,6 +586,51 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
 	DEFINE_WAKE_Q(wake_q);
 
 	/*
+	 * 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
+	 *                                   RMB
+	 *                                   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 RMB in below example is to make sure that the spinner state is
+	 * consulted after sem->count is updated in up_write context.
+	 * This would guarantee trylock on rwsem is successful
+	 * in rwsem_down_write_failed().
+	 * spinning writer                  up_write caller
+	 * ---------------                  -----------------------
+	 * [S] osq_unlock()                 atomic_update(sem->count)
+	 *                                  RMB
+	 * atomic_update(sem->count)        [L] osq
+	 * MB
+	 * rwsem_try_write_lock(count)
+	 */
+	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.

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#1719168

WTH did you not Cc the people that commented on your patch last time?

On Wed, Aug 23, 2017 at 04:58:55PM +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
>                                    RMB

This doesn't make sense, it appears to order a STORE against something
else.

>                                    spin_trylock(wait_lock)
>                                    return
>  rwsem_try_write_lock(count)
>  spin_unlock(wait_lock)
>  schedule()

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#1719197

On Thu, Aug 24, 2017 at 01:29:27PM +0200, Peter Zijlstra wrote:
> 
> WTH did you not Cc the people that commented on your patch last time?
> 
> On Wed, Aug 23, 2017 at 04:58:55PM +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
> >                                    RMB
> 
> This doesn't make sense, it appears to order a STORE against something
> else.
> 
> >                                    spin_trylock(wait_lock)
> >                                    return
> >  rwsem_try_write_lock(count)
> >  spin_unlock(wait_lock)
> >  schedule()

Is this what you wanted to write?

---
 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 02f660666ab8..813b5d3654ce 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

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#1719212

On Thu, Aug 24, 2017 at 02:33:04PM +0200, Peter Zijlstra wrote:
> On Thu, Aug 24, 2017 at 01:29:27PM +0200, Peter Zijlstra wrote:
> > 
> > WTH did you not Cc the people that commented on your patch last time?
> > 
> > On Wed, Aug 23, 2017 at 04:58:55PM +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
> > >                                    RMB
> > 
> > This doesn't make sense, it appears to order a STORE against something
> > else.
> > 
> > >                                    spin_trylock(wait_lock)
> > >                                    return
> > >  rwsem_try_write_lock(count)
> > >  spin_unlock(wait_lock)
> > >  schedule()
> 
> Is this what you wanted to write?

And ideally there should be a comment near the atomic_long_add_return()
in __rwsem_down_write_failed_common() to indicate we rely on the implied
smp_mb() before it -- just in case someone goes and makes it
atomic_long_add_return_relaxed().

And I suppose someone should look at the waiting branch of that thing
too.. because I'm not sure what happens if waiting is true but count
isn't big enough.

I bloody hate the rwsem code, that BIAS stuff forever confuses me. I
have a start at rewriting the thing to put the owner in the lock word
just like we now do for mutex, but never seem to get around to finishing
it.

> ---
>  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 02f660666ab8..813b5d3654ce 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

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