Re: [PATCH] locking/rwsem: Fix lock optimistic spinning when owner is not running

[Sasha Levin <sasha.levin@oracle.com>]

On 03/07/2015 02:45 AM, Jason Low wrote:
> Fixes tip commit b3fd4f03ca0b (locking/rwsem: Avoid deceiving lock spinners).
> 
> Ming reported soft lockups occurring when running xfstest due to
> commit b3fd4f03ca0b.
> 
> When doing optimistic spinning in rwsem, threads should stop spinning when
> the lock owner is not running. While a thread is spinning on owner, if
> the owner reschedules, owner->on_cpu returns false and we stop spinning.
> 
> However, commit b3fd4f03ca0b essentially caused the check to get ignored
> because when we break out of the spin loop due to !on_cpu, we continue
> spinning if sem->owner != NULL.
> 
> This patch fixes this by making sure we stop spinning if the owner is not
> running. Furthermore, just like with mutexes, refactor the code such that
> we don't have separate checks for owner_running(). This makes it more
> straightforward in terms of why we exit the spin on owner loop and we
> would also avoid needing to "guess" why we broke out of the loop to make
> this more readable.

That seems to solve the hangs I'm seeing as well.


Thanks,
Sasha