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Subject: Re: [PATCH] locking/mutex: fix the MUTEX_FLAG_HANDOFF bit is cleared
 unexpected
From:   "Xu, Yanfei" <yanfei.xu@windriver.com>
To:     Waiman Long <llong@redhat.com>, peterz@infradead.org,
        mingo@redhat.com, boqun.feng@gmail.com
Cc:     linux-kernel@vger.kernel.org
References: <20210628155155.11623-1-yanfei.xu@windriver.com>
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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.