linux-kernel - Re: BUG: spinlock trylock failure on UP, i.MX28 3.12.15-rt25

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 for Android: free password hash cracker in your pocket

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

Message-ID: <20140422134802.73fc1fa4@gandalf.local.home>
Date:	Tue, 22 Apr 2014 13:48:02 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	Stanislav Meduna <stano@...una.org>,
	"linux-rt-users@...r.kernel.org" <linux-rt-users@...r.kernel.org>,
	Linux ARM Kernel <linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: BUG: spinlock trylock failure on UP, i.MX28 3.12.15-rt25

On Tue, 22 Apr 2014 18:51:17 +0200
Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:

> On 04/22/2014 03:46 PM, Steven Rostedt wrote:
> > [ added Peter ]
> > 
> > On Tue, 22 Apr 2014 13:54:39 +0200
> > Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
> >  
> >> this is, erm, harmless. We grab the timer lock via trylock in hardirq
> >> context. If the lock is already taken then we fail to get it we go for
> >> plan B. According to lockdep a trylock should not fail on UP. This is
> >> true in general except for this timer case. I was thinking abour
> >> disabling this lockdep check…
> > 
> > trylock not failing on UP, can that be an issue? I mean, if a hardirq
> > does a trylock to see if it can grab a lock that is not protected by
> > disabling irqs, and will go to plan B if it fails, on UP, it will
> > always get it.  But the issue is still there. That would mean that a
> > hardirq could have preempted a critical section and doing a trylock
> > here would succeed when it really should have failed.
> 
> If you take a lock with irqs enabled and disabled then lockdep should
> complain about it.

There's nothing wrong with taking locks with irqs enabled and disabled.
It's only wrong if that lock (or a lock that is held when the lock is
taken) is also taking in interrupt *context*.

> 
> This is the ->wait_lock of the timer base lock. This (sleeping) lock is
> usually taken with interrupts enabled. Except here, in the timer
> callback, we check if the lock is available or not. And this lock may
> be a) taken (and the ->wait_lock unlocked) or b) in process to be taken
> but the caller only succeeded to acquire the ->wait_lock before the
> interrupt occurred. This is the case here and we can't acquire the
> ->wait_lock a second time the check if the lock is really taken. But
> since the wait_lock is occupied it is likely that the lock itself is
> occupied as well.

I need to take a deeper look into the actual code. But as trylocks on
UP are nops (always succeed), and if it expects to be able to do
something in a critical section that is protected by spinlocks (again
nops on UP), this would be broken for UP.

-- Steve

--
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/