linux-kernel - Re: [PATCH RESEND v4] locking/pvqspinlock: Fix double hash race

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Date:	Wed, 3 Aug 2016 16:10:26 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Waiman Long <waiman.long@....com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>, Wanpeng Li <wanpeng.li@...mail.com>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH RESEND v4] locking/pvqspinlock: Fix double hash race

2016-08-03 11:28 GMT+08:00 Waiman Long <waiman.long@....com>:
> On 07/27/2016 07:30 AM, Wanpeng Li wrote:
>>
>> From: Wanpeng Li<wanpeng.li@...mail.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: Davidlohr Bueso<dave@...olabs.net>
>> Reviewed-by: Pan Xinhui<xinhui.pan@...ux.vnet.ibm.com>
>> Cc: Peter Zijlstra (Intel)<peterz@...radead.org>
>> Cc: Ingo Molnar<mingo@...nel.org>
>> Cc: Waiman Long<Waiman.Long@....com>
>> Cc: Davidlohr Bueso<dave@...olabs.net>
>> Signed-off-by: Wanpeng Li<wanpeng.li@...mail.com>
>> ---
>> v3 ->  v4:
>>   * update patch subject
>>   * add code comments
>> v2 ->  v3:
>>   * fix typo in patch description
>> v1 ->  v2:
>>   * adjust patch description
>>
>>   kernel/locking/qspinlock_paravirt.h | 23 ++++++++++++++++++++++-
>>   1 file changed, 22 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/locking/qspinlock_paravirt.h
>> b/kernel/locking/qspinlock_paravirt.h
>> index 21ede57..ca96db4 100644
>> --- a/kernel/locking/qspinlock_paravirt.h
>> +++ b/kernel/locking/qspinlock_paravirt.h
>> @@ -450,7 +450,28 @@ pv_wait_head_or_lock(struct qspinlock *lock, struct
>> mcs_spinlock *node)
>>                                 goto gotlock;
>>                         }
>>                 }
>> -               WRITE_ONCE(pn->state, vcpu_halted);
>> +               /*
>> +                * 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.
>> +                * Making sure the state is never set to vcpu_halted will
>> prevent this
>> +                * racing from happening.
>> +                */
>> +               WRITE_ONCE(pn->state, vcpu_hashed);
>>                 qstat_inc(qstat_pv_wait_head, true);
>>                 qstat_inc(qstat_pv_wait_again, waitcnt);
>>                 pv_wait(&l->locked, _Q_SLOW_VAL);
>
>
> Acked-by: Waiman Long <Waiman.Long@....com>

Thanks. :)

Regards,
Wanpeng Li