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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> - 2017-07-26 22:20 +0200
    Re: [PATCH] rwsem: fix missed wakeup due to reordering of load Waiman Long <longman@redhat.com> - 2017-07-27 17:50 +0200
      Re: [PATCH] rwsem: fix missed wakeup due to reordering of load Peter Zijlstra <peterz@infradead.org> - 2017-07-27 19:00 +0200

#1697544 — [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
                                   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 | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 34e727f..21c111a 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -585,6 +585,40 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
 	unsigned long flags;
 	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
+         *                                   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() here is to make sure that the spinner state is
+         * consulted after sem->count is updated in up_write context.
+         */
+        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
-- 
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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#1698115

On 07/26/2017 04:17 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>

Did you actually observe that the reordering happens?

I am not sure if some architectures can actually speculatively execute
instruction ahead of a branch and then ahead into a function call. I
know it can happen if the function call is inlined, but rwsem_wake()
will not be inlined into __up_read() or __up_write().

Even if that is the case, I am not sure if smp_rmb() alone is enough to
guarantee the ordering as I think it will depend on how the
atomic_long_sub_return_release() is implmented.

Cheers,
Longman

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#1698195

On Thu, Jul 27, 2017 at 11:48:53AM -0400, Waiman Long wrote:
> atomic_long_sub_return_release() is implmented.

I've not had time to really thing about the problem at hand, but this I
can answer:

TSO (x86, s390, sparc): fully serialized
PPC: lwsync; ll/sc	(RCpc)
ARM64: ll/sc-release	(RCsc)
other: smp_mb(); atomic_sub_return_relaxed();

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