diff mbox

[RESEND,v4] locking/pvqspinlock: Fix double hash race

Message ID 1469619037-13826-1-git-send-email-wanpeng.li@hotmail.com (mailing list archive)
State New, archived
Headers show

Commit Message

Wanpeng Li July 27, 2016, 11:30 a.m. UTC 
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

Comments

Waiman Long Aug. 3, 2016, 3:28 a.m. UTC | #1 
On 07/27/2016 07:30 AM, 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: Davidlohr Bueso<dave@stgolabs.net>
> 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>
> ---
> 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@hpe.com>

Cheers,
Longman
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Wanpeng Li Aug. 3, 2016, 8:10 a.m. UTC | #2 
2016-08-03 11:28 GMT+08:00 Waiman Long <waiman.long@hpe.com>:
> On 07/27/2016 07:30 AM, 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: Davidlohr Bueso<dave@stgolabs.net>
>> 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>
>> ---
>> 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@hpe.com>

Thanks. :)

Regards,
Wanpeng Li
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diff mbox

Patch

=====================        ==================

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@stgolabs.net>
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>
---
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);