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SubjectRe: [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@hotmail.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@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>
> ---
> 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

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