linux-kernel - Re: Exiting with locks still held (was Re: [PATCH] kmemleak: Fix scheduling-while-atomic bug)

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Date:	Fri, 3 Jul 2009 09:04:56 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Catalin Marinas <catalin.marinas@....com>
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: Exiting with locks still held (was Re: [PATCH] kmemleak: Fix
	scheduling-while-atomic bug)


* Catalin Marinas <catalin.marinas@....com> wrote:

> Hi Ingo,
> 
> On Wed, 2009-07-01 at 13:04 +0200, Ingo Molnar wrote:
> > * Catalin Marinas <catalin.marinas@....com> wrote:
> > > 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)?
> > 
> > This lockdep warning has a 0% false positives track record so 
> > far: all previous cases it triggered showed some real (and 
> > fatal) bug in the underlying code.
> 
> In this particular case, there is no fatal problem as the mutex is 
> released shortly after this message.

Maybe - but holding locks in user-space is almost always bad.

What happens if user-space opens a second file descriptor before 
closing the first one? We either lock up (which is bad and fatal) or 
you already have some _other_ exclusion mechanism that prevents this 
case, which calls into question the need to hold this particular 
mutex in user-space to begin with.

I've yet to see a valid 'need to hold this kernel lock in 
user-space' case, and this does not seem to be such a case either.

	Ingo
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