Messages in this thread | | | Date | Tue, 16 Nov 2021 10:14:20 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v5] locking/rwsem: Make handoff bit handling more consistent |
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On Tue, Nov 16, 2021 at 10:52:42AM +0800, Aiqun(Maria) Yu wrote: > On 11/16/2021 9:29 AM, Waiman Long wrote: > > There are some inconsistency in the way that the handoff bit is being > > handled in readers and writers. > > > > Firstly, when a queue head writer set the handoff bit, it will clear it > > when the writer is being killed or interrupted on its way out without > > acquiring the lock. That is not the case for a queue head reader. The > > handoff bit will simply be inherited by the next waiter. > > > > Secondly, in the out_nolock path of rwsem_down_read_slowpath(), both > > the waiter and handoff bits are cleared if the wait queue becomes empty. > > For rwsem_down_write_slowpath(), however, the handoff bit is not checked > > and cleared if the wait queue is empty. This can potentially make the > > handoff bit set with empty wait queue. > > > > To make the handoff bit handling more consistent and robust, extract > > out handoff bit clearing code into the new rwsem_del_waiter() helper > > function. The common function will only use atomic_long_andnot() to > > clear bits when the wait queue is empty to avoid possible race condition. > we do have race condition needed to be fixed with this change.
Indeed, let me edit the changelog to reflect that. Also, I think, it needs a Reported-by:.
> > If the first waiter with handoff bit set is killed or interrupted to > > exit the slowpath without acquiring the lock, the next waiter will > > inherit the handoff bit. > > > > While at it, simplify the trylock for loop in rwsem_down_write_slowpath() > > to make it easier to read. > > > > Fixes: 4f23dbc1e657 ("locking/rwsem: Implement lock handoff to prevent lock starvation") > > Suggested-by: Peter Zijlstra <peterz@infradead.org> > > Signed-off-by: Waiman Long <longman@redhat.com>
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