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SubjectRe: [PATCH v4] locking/pvqspinlock: Fix double hash race
Ping Ingo, Peterz. :)
2016-07-17 4:03 GMT+08:00 Davidlohr Bueso <dave@stgolabs.net>:
> 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>

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