linux-kernel - Re: [PATCH 4/5] futex: Avoid taking hb lock if nothing to wakeup

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]

Message-ID: <1385185326.29354.167.camel@dvhart-mobl4.amr.corp.intel.com>
Date:	Sat, 23 Nov 2013 05:42:07 +0000
From:	"Hart, Darren" <darren.hart@...el.com>
To:	Davidlohr Bueso <davidlohr@...com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"efault@....de" <efault@....de>, "jeffm@...e.com" <jeffm@...e.com>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"scott.norton@...com" <scott.norton@...com>,
	"tom.vaden@...com" <tom.vaden@...com>,
	"aswin@...com" <aswin@...com>,
	"Waiman.Long@...com" <Waiman.Long@...com>,
	"jason.low2@...com" <jason.low2@...com>
Subject: Re: [PATCH 4/5] futex: Avoid taking hb lock if nothing to wakeup

On Fri, 2013-11-22 at 21:40 -0800, Darren Hart wrote:
> On Fri, 2013-11-22 at 16:56 -0800, Davidlohr Bueso wrote:
> > In futex_wake() there is clearly no point in taking the hb->lock if
> > we know beforehand that there are no tasks to be woken. This comes
> > at the smaller cost of doing some atomic operations to keep track of
> > the list's size. Specifically, increment the counter when an element is
> > added to the list, and decrement when it is removed. Of course, if the
> > counter is 0, then there are no tasks blocked on a futex. Some special
> > considerations:
> > 
> > - increment the counter at queue_lock() as we always end up calling
> >   queue_me() which adds the element to the list. Upon any error,
> >   queue_unlock() is called for housekeeping, for which we decrement
> >   to mach the increment done in queue_lock().
> > 
> > - decrement the counter at __unqueue_me() to reflect when an element is
> >   removed from the queue for wakeup related purposes.
> 
> What is the problem you are trying to solve here?

Apologies, too quick on the trigger. I see plenty of detail in 0/5. Will
spend some time reviewing that.

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel