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SubjectRe: [PATCH] locking/pvqspinlock: restore/set vcpu_hashed state after failing adaptive locking spinning


On 2016年07月14日 15:53, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@hotmail.com>
>
> When the lock holder vCPU is racing with the queue head:
>
> CPU 0 (lock holder) CPU1 (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 unnecessary. This patch avoids it by
> restoring/setting vcpu_halted state after failing adaptive locking
> spinning.
>

good catching. this is hash entry leak.

After apply Waiman's pathset "locking/pvqspinlock: Fix missed PV wakeup & support PPC", this is not the big problem.
But we can still save another cmpxchg.

So
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>
> ---
> 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);
>

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