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SubjectRe: [PATCH] rwsem: fix missed wakeup due to reordering of load
On Thu, Jul 27, 2017 at 01:47:52AM +0530, Prateek Sood wrote:
> 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.

I feel that comment can use help.. for example the RMB you add below is
not present at all.

> + */
> + 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

Your patch is whitespace damaged, all the indentation on the + lines is
with spaces. Please resend with \t.

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