linux-kernel - Re: [PATCH][RFC]: mutex: adaptive spin

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:	Tue, 6 Jan 2009 17:54:09 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>,
	Chris Mason <chris.mason@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH][RFC]: mutex: adaptive spin


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Tue, 6 Jan 2009, Linus Torvalds wrote:
> > 
> > So it should be renamed. Something like "task_is_oncpu()" or whatever.
> 
> Another complaint, which is tangentially related in that it actually 
> concerns "current".
> 
> Right now, if some process deadlocks on a mutex, we get hung process, 
> but with a nice backtrace and hopefully other things (that don't need 
> that lock) still continue to work.
> 
> But if I read it correctly, the adaptive spin code will instead just 
> hang. Exactly because "task_is_current()" will also trigger for that 
> case, and now you get an infinite loop, with the process spinning until 
> it looses its own CPU, which obviously will never happen.
> 
> Yes, this is the behavior we get with spinlocks too, and yes, lock 
> debugging will talk about it, but it's a regression. We've historically 
> had a _lot_ more bad deadlocks on mutexes than we have had on spinlocks, 
> exactly because mutexes can be held over much more complex code. So 
> regressing on it and making it less debuggable is bad.
> 
> IOW, if we do this, then I think we need a
> 
> 	BUG_ON(task == owner);
> 
> in the waiting slow-path. I realize the test already exists for the 
> DEBUG case, but I think we just want it even for production kernels. 
> Especially since we'd only ever need it in the slow-path.

yeah, sounds good.

One thought:

BUG_ON()'s do_exit() shows a slightly misleading failure pattern to users: 
instead of a 'hanging' task, we'd get a misbehaving app due to one of its 
tasks exiting spuriously. It can even go completely unnoticed [users dont 
look at kernel logs normally] - while a hanging task generally does get 
noticed. (because there's no progress in processing)

So instead of the BUG_ON() we could emit a WARN_ONCE() perhaps, plus not 
do any spinning and just block - resulting in an uninterruptible task 
(that the user will probably notice) and a scary message in the syslog? 
[all in the slowpath]

So in this case WARN_ONCE() is both more passive (it does not run 
do_exit()), and shows the more intuitive failure pattern to users. No 
strong feelings though.

	Ingo
--
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/