[PATCH v2] locking/pvqspinlock: restore/set vcpu_hashed state after failing adaptive locking spinning

[PATCH v2] locking/pvqspinlock: restore/set vcpu_hashed state after failing adaptive locking spinning

[Wanpeng Li]

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.

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>
---
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);
-- 
1.9.1

Re: [PATCH v2] locking/pvqspinlock: restore/set vcpu_hashed state after failing adaptive locking spinning

[Peter Zijlstra]

On Thu, Jul 14, 2016 at 04:15:56PM +0800, Wanpeng Li wrote:
> 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.
> 

> -		WRITE_ONCE(pn->state, vcpu_halted);
> +		WRITE_ONCE(pn->state, vcpu_hashed);
                                           ^^^^^^

The Changelog meant so say vcpu_hashed, surely?

Re: [PATCH v2] locking/pvqspinlock: restore/set vcpu_hashed state after failing adaptive locking spinning

[Wanpeng Li]

2016-07-14 19:26 GMT+08:00 Peter Zijlstra <peterz@infradead.org>:
> On Thu, Jul 14, 2016 at 04:15:56PM +0800, Wanpeng Li wrote:
>> 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.
>>
>
>> -             WRITE_ONCE(pn->state, vcpu_halted);
>> +             WRITE_ONCE(pn->state, vcpu_hashed);
>                                            ^^^^^^
>
> The Changelog meant so say vcpu_hashed, surely?

Oh, there is a typo in changelog, I will fix it.

Regards,
Wanpeng Li

Re: [PATCH v2] locking/pvqspinlock: restore/set vcpu_hashed state after failing adaptive locking spinning

[Waiman Long]

On 07/14/2016 07:26 AM, Peter Zijlstra wrote:
> On Thu, Jul 14, 2016 at 04:15:56PM +0800, Wanpeng Li wrote:
>> 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.
>>
>> -		WRITE_ONCE(pn->state, vcpu_halted);
>> +		WRITE_ONCE(pn->state, vcpu_hashed);
>                                             ^^^^^^
>
> The Changelog meant so say vcpu_hashed, surely?

Do you have chance to review the "Fix missed PV wakeup & support PPC" 
patch (https://lkml.org/lkml/2016/5/31/677) that I had sent out a while 
ago? That patch addressed a similar pv_hash leakage and collision problem.

Cheers,
Longman

[tip:locking/core] locking/pvqspinlock: Fix double hash race

[tip-bot for Wanpeng Li]

Commit-ID:  229ce631574761870a2ac938845fadbd07f35caa
Gitweb:     http://git.kernel.org/tip/229ce631574761870a2ac938845fadbd07f35caa
Author:     Wanpeng Li <wanpeng.li@hotmail.com>
AuthorDate: Thu, 14 Jul 2016 16:15:56 +0800
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 10 Aug 2016 14:13:28 +0200

locking/pvqspinlock: Fix double hash race

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_hashed state after failing adaptive locking
spinning.

Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman Long <Waiman.Long@hpe.com>
Link: http://lkml.kernel.org/r/1468484156-4521-1-git-send-email-wanpeng.li@hotmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 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 37649e6..8a99abf 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);