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

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Message-Id: <1246439937.8492.18.camel@pc1117.cambridge.arm.com>
Date:	Wed, 01 Jul 2009 10:18:57 +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 09:53 +0200, Ingo Molnar wrote:
> * Linux Kernel Mailing List <linux-kernel@...r.kernel.org> wrote:
> 
> > Gitweb:     http://git.kernel.org/linus/acf4968ec9dea49387ca8b3d36dfaa0850bdb2d5
> > Commit:     acf4968ec9dea49387ca8b3d36dfaa0850bdb2d5
> > Parent:     4698c1f2bbe44ce852ef1a6716973c1f5401a4c4
> > Author:     Catalin Marinas <catalin.marinas@....com>
> > AuthorDate: Fri Jun 26 17:38:29 2009 +0100
> > Committer:  Catalin Marinas <catalin.marinas@....com>
> > CommitDate: Fri Jun 26 17:38:29 2009 +0100
> > 
> >     kmemleak: Slightly change the policy on newly allocated objects
> 
> I think one of the kmemleak fixes that went upstream yesterday 
> caused the following scheduling-while-holding-the-tasklist-lock 
> regression/crash on x86:

Thanks for the patch. The bug was always there, only that the task stack
scanning is now enabled by default (and I probably have a small number
of tasks that the rescheduling didn't happen during stack scanning).

> 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).

> The background scanning thread could probably also be reniced
> to +10.

diff --git a/mm/kmemleak.c b/mm/kmemleak.c
index e766e1d..6006553 100644
--- a/mm/kmemleak.c
+++ b/mm/kmemleak.c
@@ -1039,6 +1039,7 @@ static int kmemleak_scan_thread(void *arg)
 	static int first_run = 1;
 
 	pr_info("Automatic memory scanning thread started\n");
+	set_user_nice(current, 10);
 
 	/*
 	 * Wait before the first scan to allow the system to fully initialize.

-- 
Catalin

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