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SubjectRe: [PATCH RESEND v4] locking/pvqspinlock: Fix double hash race
2016-08-03 11:28 GMT+08:00 Waiman Long <waiman.long@hpe.com>:
> On 07/27/2016 07:30 AM, 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: Davidlohr Bueso<dave@stgolabs.net>
>> 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);
>
>
> Acked-by: Waiman Long <Waiman.Long@hpe.com>

Thanks. :)

Regards,
Wanpeng Li

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