Re: [PATCH] locking/mutex: fix the MUTEX_FLAG_HANDOFF bit is cleared unexpected

[Xu, Yanfei]

On 6/30/21 2:20 PM, Xu, Yanfei wrote:
> > Oh, you are right. The current code doesn't actually prevent lock
> > stealer from actually stealing the lock in the special case that the
> > lock is in the unlock state when the HANDOFF bit is set. In this case,
>
> How about setting the HANDOFF bit before the top-waiter first give up
> cpu and fall asleep. Then It must can get the lock after being woken up,
> and there is no chance happen stealing lock.  And I sent a v2 with this.

Please ignore this, It is incorrect. Lock stealing is a performance
optimization.

Yanfei

> > it is free for all and whoever gets the lock will also clear the the
> > HANDOFF bit. The comment in __mutex_trylock_or_owner() about "We set the
> > HANDOFF bit" isn't quite right.