linux-kernel - Re: [PATCH 2/2] locking/qspinlock: Limit # of spins in _Q_PENDING

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Date:   Wed, 11 Apr 2018 14:06:12 -0400
From:   Waiman Long <longman@...hat.com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Will Deacon <will.deacon@....com>,
        boqun.feng@...il.com,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 2/2] locking/qspinlock: Limit # of spins in _Q_PENDING_VAL
 wait loop

On 04/11/2018 11:22 AM, Catalin Marinas wrote:
> Hi Waiman,
>
> On Mon, Apr 09, 2018 at 02:08:52PM -0400, Waiman Long wrote:
>> @@ -311,13 +320,19 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>>  		return;
>>  
>>  	/*
>> -	 * wait for in-progress pending->locked hand-overs
>> +	 * wait for in-progress pending->locked hand-overs with a
>> +	 * limited number of spins.
>>  	 *
>>  	 * 0,1,0 -> 0,0,1
>>  	 */
>>  	if (val == _Q_PENDING_VAL) {
>> -		while ((val = atomic_read(&lock->val)) == _Q_PENDING_VAL)
>> +		int cnt = _Q_PENDING_LOOP;
>> +
>> +		while ((val = atomic_read(&lock->val)) == _Q_PENDING_VAL) {
>> +			if (!--cnt)
>> +				goto queue;
>>  			cpu_relax();
>> +		}
>>  	}
> In my model, the pathological case is not this loop but the following
> one (trylock || pending):
>
> P0:					P1:
> queued_spin_lock() fails		queued_spin_lock() succeeds
> queued_spin_lock_slowpath()
> 	val == _Q_LOCKED_VAL
> 	new = _Q_LOCKED_VAL |
> 		_Q_PENDING_VAL
> 					queued_spin_unlock()
> 						lock->val == 0
> 	cmpxchg(&lock->val, val, new)
> 		fails
> 	val = old (0)
> 	repeat for (;;) loop:
> 	new = _Q_LOCKED_VAL
> 					queued_spin_lock() succeeds
> 						lock->val == _Q_LOCKED_VAL
> 	cmpxchg(&lock->val, val, new)
> 		fails
> 	val = old (_Q_LOCKED_VAL)
> 	repeat for (;;) loop:
>
> 	... and we are back to the P0 state above when it first entered
> 	the loop, hence no progress. P1 never enters slowpath.

I don't see any problem in removing this second loop. Thanks for running
tool to check for problem.

-Longman