linux-kernel - Re: [PATCH v3] locking/pvqspinlock: restore/set vcpu_hashed state after failing adaptive locking spinning

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Date:	Thu, 14 Jul 2016 10:52:08 -0400
From:	Waiman Long <waiman.long@....com>
To:	Wanpeng Li <kernellwp@...il.com>
CC:	<linux-kernel@...r.kernel.org>,
	Wanpeng Li <wanpeng.li@...mail.com>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v3] locking/pvqspinlock: restore/set vcpu_hashed state
 after failing adaptive locking spinning

On 07/14/2016 07:39 AM, Wanpeng Li wrote:
> From: Wanpeng Li<wanpeng.li@...mail.com>
>
> When the lock holder vCPU is racing with the queue head:
>
>     CPU 0 (lock holder)    CPU 1 (queue head)
>     ===================    =================
>     spin_lock();           spin_lock();
>      pv_kick_node():        pv_wait_head_or_lock():
>                              if (!lp) {
>                               lp = pv_hash(lock, pn);
>                               xchg(&l->locked, _Q_SLOW_VAL);
>                              }
>                              WRITE_ONCE(pn->state, vcpu_halted);
>       cmpxchg(&pn->state,
>        vcpu_halted, vcpu_hashed);
>       WRITE_ONCE(l->locked, _Q_SLOW_VAL);
>       (void)pv_hash(lock, pn);
>
> In this case, lock holder inserts the pv_node of queue head into the
> hash table and set _Q_SLOW_VAL which can result in hash entry leak.
> This patch avoids it by restoring/setting vcpu_hashed state after
> failing adaptive locking spinning.
>
> 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>
> ---
> v2 ->  v3:
>   * fix typo in patch description
> v1 ->  v2:
>   * adjust patch description
>
>   kernel/locking/qspinlock_paravirt.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
> index 21ede57..ac7d20b 100644
> --- a/kernel/locking/qspinlock_paravirt.h
> +++ b/kernel/locking/qspinlock_paravirt.h
> @@ -450,7 +450,7 @@ pv_wait_head_or_lock(struct qspinlock *lock, struct mcs_spinlock *node)
>   				goto gotlock;
>   			}
>   		}
> -		WRITE_ONCE(pn->state, vcpu_halted);
> +		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);

As pv_kick_node() is called immediately after designating the next node 
as the queue head, the chance of this racing is possible, but is not 
likely unless the lock holder vCPU gets preempted for a long time at 
that right moment. This change does not do any harm though, so I am OK 
with that. However, I do want you to add a comment about the possible 
race in the code as it isn't that obvious or likely.

Cheers,
Longman