linux-kernel - Re: [PATCH 5/8] locking, mutex: Cancelable MCS lock for adaptive spinning

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Mon, 10 Feb 2014 14:04:22 -0800
From:	Jason Low <jason.low2@...com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, Waiman Long <waiman.long@...com>,
	mingo@...nel.org, paulmck@...ux.vnet.ibm.com,
	torvalds@...ux-foundation.org, tglx@...utronix.de, riel@...hat.com,
	akpm@...ux-foundation.org, davidlohr@...com, hpa@...or.com,
	andi@...stfloor.org, aswin@...com, scott.norton@...com,
	chegu_vinod@...com
Subject: Re: [PATCH 5/8] locking, mutex: Cancelable MCS lock for adaptive
 spinning

On Mon, 2014-02-10 at 22:32 +0100, Peter Zijlstra wrote:
> On Mon, Feb 10, 2014 at 01:15:59PM -0800, Jason Low wrote:
> > On Mon, 2014-02-10 at 20:58 +0100, Peter Zijlstra wrote:
> > > +void osq_unlock(struct optimistic_spin_queue **lock)
> > > +{
> > > +	struct optimistic_spin_queue *node = this_cpu_ptr(&osq_node);
> > > +	struct optimistic_spin_queue *next;
> > > +
> > > +	/*
> > > +	 * Fast path for the uncontended case.
> > > +	 */
> > > +	if (likely(cmpxchg(lock, node, NULL) == node))
> > > +		return;
> > 
> > Can we can also add the following code here as I'm noticing next != NULL
> > is the much more likely scenario on my box:
> > 
> >         next = xchg(&node->next, NULL);
> >         if (next) {
> >                 ACCESS_ONCE(next->locked) = 1;
> >                 return;
> 
> Is adding that really much faster than the relatively straight path
> oqs_wait_next() would walk to bit the same exit?
> 
> The only reason I pulled out the above cmpxchg() is because its the
> uncontended fast path, which seems like a special enough case.

So it would avoid 2 extra checks (*lock == node) and (node->next) in the
oqs_wait_next() path, which aren't necessary when node->next != NULL.

And I think node->next != NULL can be considered a special enough case
after the cmpxchg() fails because in the contended case, we're expecting
the node->next to be pointing at something. The only times node->next is
NULL after cmpxchg() fails are during a very small race window with the
osq_lock(), and when the next node is unqueuing due to need_resched,
which is also a very small window.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/