[PATCH v4] locking/pvqspinlock: Fix double hash race

[PATCH v4] locking/pvqspinlock: Fix double hash race

[Wanpeng Li]

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>
---
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);
-- 
2.1.0

Re: [PATCH v4] locking/pvqspinlock: Fix double hash race

[Davidlohr Bueso]

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>

Re: [PATCH v4] locking/pvqspinlock: Fix double hash race

[Wanpeng Li]

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>

Re: [PATCH v4] locking/pvqspinlock: Fix double hash race

[Wanpeng Li]

Sorry for the quick ping, but could we catch the merge window? Ingo. :)
2016-07-16 9:51 GMT+08:00 Wanpeng Li <kernellwp@gmail.com>:
> 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>
> ---
> 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);
> --
> 2.1.0
>



-- 
Regards,
Wanpeng Li