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Date: Sat, 16 Jul 2016 13:03:22 -0700
From: Davidlohr Bueso <dave@stgolabs.net>
To: Wanpeng Li <kernellwp@gmail.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Wanpeng Li <wanpeng.li@hotmail.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>, Waiman Long <Waiman.Long@hpe.com>
Subject: Re: [PATCH v4] locking/pvqspinlock: Fix double hash race
Message-ID: <20160716200321.GA3241@linux-80c1.suse>
References: <1468633869-4066-1-git-send-email-wanpeng.li@hotmail.com>
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On Sat, 16 Jul 2016, Wanpeng Li wrote:

>From: Wanpeng Li <wanpeng.li@hotmail.com>
>
>When the lock holder vCPU is racing with the queue head vCPU:
>
>lock holder vCPU             queue head vCPU
>=====================        ==================
>
>node->locked = 1;
><preemption>                 READ_ONCE(node->locked)
>   ...                       pv_wait_head_or_lock():
>                               SPIN_THRESHOLD loop;
>                               pv_hash();
>                               lock->locked = _Q_SLOW_VAL;
>                               node->state  = vcpu_hashed;
>pv_kick_node():
>  cmpxchg(node->state,
>     vcpu_halted, vcpu_hashed);
>  lock->locked = _Q_SLOW_VAL;
>  pv_hash();
>
>With preemption at the right moment, it is possible that both the
>lock holder and queue head vCPUs can be racing to set node->state
>which can result in hash entry race. Making sure the state is never
>set to vcpu_halted will prevent this racing from happening.
>
>This patch fix it by setting vcpu_hashed after we did all hash thing.
>
>Reviewed-by: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>
>Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
>Cc: Ingo Molnar <mingo@kernel.org>
>Cc: Waiman Long <Waiman.Long@hpe.com>
>Cc: Davidlohr Bueso <dave@stgolabs.net>
>Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>

Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>