linux-kernel - Re: [PATCH] kmemleak: Fix scheduling-while-atomic bug

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Message-Id: <1246441592.8492.38.camel@pc1117.cambridge.arm.com>
Date:	Wed, 01 Jul 2009 10:46:31 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	git-commits-head@...r.kernel.org
Subject: Re: [PATCH] kmemleak: Fix scheduling-while-atomic bug

On Wed, 2009-07-01 at 11:30 +0200, Ingo Molnar wrote:
> * Catalin Marinas <catalin.marinas@....com> wrote:
> 
> > > The minimal fix below removes scan_yield() and adds a 
> > > cond_resched() to the outmost (safe) place of the scanning 
> > > thread. This solves the regression.
> > 
> > With CONFIG_PREEMPT disabled it won't reschedule during the bss 
> > scanning but I don't see this as a real issue (task stacks 
> > scanning probably takes longer anyway).
> 
> Yeah. I suspect one more cond_resched() could be added - i just 
> didnt see an obvious place for it, given that scan_block() is being 
> called with asymetric held-locks contexts.

Yes, scan_block shouldn't call cond_resched(). The code is cleaner if
functions don't have too many side-effects. I can see about 1 sec of bss
scanning on an ARM board but with processor at < 500MHz and slow memory
system. On a standard x86 systems BSS scanning may not be noticeable
(and I think PREEMPT enabling is quite common these days).

Since we are at locking, I just noticed this on my x86 laptop when
running cat /sys/kernel/debug/kmemleak (I haven't got it on an ARM
board):

================================================
[ BUG: lock held when returning to user space! ]
------------------------------------------------
cat/3687 is leaving the kernel with locks still held!
1 lock held by cat/3687:
#0:  (scan_mutex){+.+.+.}, at: [<c01e0c5c>] kmemleak_open+0x3c/0x70

kmemleak_open() acquires scan_mutex and unconditionally releases it in
kmemleak_release(). The mutex seems to be released as a subsequent
acquiring works fine.

Is this caused just because cat may have exited without closing the file
descriptor (which should be done automatically anyway)?

Thanks.

-- 
Catalin

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