linux-kernel - Re: [PATCH] locking/mutex: fix the MUTEX_FLAG

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Date:   Wed, 30 Jun 2021 18:06:05 +0800
From:   "Xu, Yanfei" <yanfei.xu@...driver.com>
To:     Waiman Long <llong@...hat.com>, peterz@...radead.org,
        mingo@...hat.com, boqun.feng@...il.com
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] locking/mutex: fix the MUTEX_FLAG_HANDOFF bit is cleared
 unexpected



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.