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

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