On Wed, Oct 19, 2022 at 08:06:26AM -0700, Hugh Dickins wrote:
> On Wed, 19 Oct 2022, Greg Kroah-Hartman wrote:
>
> > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> >
> > [ Upstream commit 040b83fcecfb86f3225d3a5de7fd9b3fbccf83b4 ]
> >
> > There are two problems can lead to lost wakeup:
> >
> > 1) invalid wakeup on the wrong waitqueue:
> >
> > For example, 2 * wake_batch tags are put, while only wake_batch threads
> > are woken:
> >
> > __sbq_wake_up
> > atomic_cmpxchg -> reset wait_cnt
> > __sbq_wake_up -> decrease wait_cnt
> > ...
> > __sbq_wake_up -> wait_cnt is decreased to 0 again
> > atomic_cmpxchg
> > sbq_index_atomic_inc -> increase wake_index
> > wake_up_nr -> wake up and waitqueue might be empty
> > sbq_index_atomic_inc -> increase again, one waitqueue is skipped
> > wake_up_nr -> invalid wake up because old wakequeue might be empty
> >
> > To fix the problem, increasing 'wake_index' before resetting 'wait_cnt'.
> >
> > 2) 'wait_cnt' can be decreased while waitqueue is empty
> >
> > As pointed out by Jan Kara, following race is possible:
> >
> > CPU1 CPU2
> > __sbq_wake_up __sbq_wake_up
> > sbq_wake_ptr() sbq_wake_ptr() -> the same
> > wait_cnt = atomic_dec_return()
> > /* decreased to 0 */
> > sbq_index_atomic_inc()
> > /* move to next waitqueue */
> > atomic_set()
> > /* reset wait_cnt */
> > wake_up_nr()
> > /* wake up on the old waitqueue */
> > wait_cnt = atomic_dec_return()
> > /*
> > * decrease wait_cnt in the old
> > * waitqueue, while it can be
> > * empty.
> > */
> >
> > Fix the problem by waking up before updating 'wake_index' and
> > 'wait_cnt'.
> >
> > With this patch, noted that 'wait_cnt' is still decreased in the old
> > empty waitqueue, however, the wakeup is redirected to a active waitqueue,
> > and the extra decrement on the old empty waitqueue is not handled.
> >
> > Fixes: 88459642cba4 ("blk-mq: abstract tag allocation out into sbitmap library")
> > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> > Reviewed-by: Jan Kara <jack@xxxxxxx>
> > Link: https://lore.kernel.org/r/20220803121504.212071-1-yukuai1@xxxxxxxxxxxxxxx
> > Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
> > Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
>
> I have no authority on linux-block, but I'll say NAK to this one
> (and 517/862), and let Jens and Jan overrule me if they disagree.
>
> This was the first of several 6.1-rc1 commits which had given me lost
> wakeups never suffered before; was not tagged Cc stable; and (unless I've
> missed it on lore) never had AUTOSEL posted to linux-block or linux-kernel.
Ok, thanks for the review. I'll drop both of the sbitmap.c changes and
if people report issues and want them back, I'll be glad to revisit them
then.
greg k-h
