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

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Date:	Tue, 19 Jul 2016 14:53:18 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>
Cc:	Wanpeng Li <wanpeng.li@...mail.com>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Waiman Long <Waiman.Long@....com>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v4] locking/pvqspinlock: Fix double hash race

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



-- 
Regards,
Wanpeng Li