linux-kernel - Re: [PATCH v5 4/4] qrwlock: Use the mcs_spinlock helper functions for MCS queuing

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Date:	Fri, 08 Nov 2013 17:42:36 -0500
From:	Waiman Long <waiman.long@...com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	George Spelvin <linux@...izon.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>, "" <aswin@...com>,
	Scott J Norton <scott.norton@...com>
Subject: Re: [PATCH v5 4/4] qrwlock: Use the mcs_spinlock helper functions
 for MCS queuing

On 11/08/2013 04:21 PM, Paul E. McKenney wrote:
> On Mon, Nov 04, 2013 at 12:17:20PM -0500, Waiman Long wrote:
>> There is a pending patch in the rwsem patch series that adds a generic
>> MCS locking helper functions to do MCS-style locking. This patch
>> will enable the queue rwlock to use that generic MCS lock/unlock
>> primitives for internal queuing. This patch should only be merged
>> after the merging of that generic MCS locking patch.
>>
>> Signed-off-by: Waiman Long<Waiman.Long@...com>
> This one does might address at least some of the earlier memory-barrier
> issues, at least assuming that the MCS lock is properly memory-barriered.
>
> Then again, maybe not.  Please see below.
>
> 							Thanx, Paul
>
>>   	/*
>>   	 * At the head of the wait queue now, try to increment the reader
>> @@ -172,12 +103,36 @@ void queue_read_lock_slowpath(struct qrwlock *lock)
>>   		while (ACCESS_ONCE(lock->cnts.writer))
>>   			cpu_relax();
>>   	}
>> -	rspin_until_writer_unlock(lock, 1);
>> -	signal_next(lock,&node);
>> +	/*
>> +	 * Increment reader count&  wait until writer unlock
>> +	 */
>> +	cnts.rw = xadd(&lock->cnts.rw, QRW_READER_BIAS);
>> +	rspin_until_writer_unlock(lock, cnts);
>> +	mcs_spin_unlock(&lock->waitq,&node);
> But mcs_spin_unlock() is only required to do a RELEASE barrier, which
> could still allow critical-section leakage.

Yes, that is a problem. I will try to add an ACQUIRE barrier in reading 
the writer byte.

-Longman

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