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From: Waiman Long <longman@redhat.com>
To: Prateek Sood <prsood@codeaurora.org>,
	peterz@infradead.org, mingo@redhat.com
Cc: sramana@codeaurora.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] rwsem: fix missed wakeup due to reordering of load
Date: Thu, 27 Jul 2017 11:48:53 -0400
Message-ID: <680a3f82-b36b-e09f-b1db-fa23d9f21fc2@redhat.com> (raw)
In-Reply-To: <1501100272-16338-1-git-send-email-prsood@codeaurora.org>

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

  reply index

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-07-26 20:17 Prateek Sood
2017-07-27 15:48 ` Waiman Long [this message]
2017-07-27 16:59   ` Peter Zijlstra
2017-08-10  8:32   ` Andrea Parri
2017-08-10 10:41     ` Peter Zijlstra
2017-08-10 10:44 ` Peter Zijlstra
2017-08-23 11:28 Prateek Sood
2017-08-24 11:29 ` Peter Zijlstra
2017-08-24 12:33   ` Peter Zijlstra
2017-08-24 12:52     ` Peter Zijlstra
2017-09-07 14:08       ` Prateek Sood
2017-09-07 14:30 Prateek Sood
2017-09-19 14:05 ` Andrea Parri
2017-09-20 14:52 ` Davidlohr Bueso
2017-09-20 21:17   ` Andrea Parri
2017-09-27 21:20     ` Davidlohr Bueso
2017-09-26 18:37 ` Prateek Sood

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