linux-kernel - Re: [PATCH 1/2] workqueues: make get_online

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Message-Id: <20080702013147.126feb34.akpm@linux-foundation.org>
Date:	Wed, 2 Jul 2008 01:31:47 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Gautham R Shenoy <ego@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Max Krasnyansky <maxk@...lcomm.com>, Paul Jackson <pj@....com>,
	Paul Menage <menage@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Vegard Nossum <vegard.nossum@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] workqueues: make get_online_cpus() useable for
 work->func()

On Sun, 29 Jun 2008 20:51:31 +0400 Oleg Nesterov <oleg@...sign.ru> wrote:

> workqueue_cpu_callback(CPU_DEAD) flushes cwq->thread under
> cpu_maps_update_begin(). This means that the multithreaded workqueues can't
> use get_online_cpus() due to the possible deadlock, very bad and very old
> problem.
> 
> Introduce the new state, CPU_POST_DEAD, which is called after
> cpu_hotplug_done() but before cpu_maps_update_done().
> 
> Change workqueue_cpu_callback() to use CPU_POST_DEAD instead of CPU_DEAD.
> This means that create/destroy functions can't rely on get_online_cpus()
> any longer and should take cpu_add_remove_lock instead.

I know that Document/SubmitChecklist has a lot of stuff.  But a basic
allnoconfig only takes seconds and it's often the thing which breaks.

 include/linux/cpu.h |   15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff -puN include/linux/cpu.h~workqueues-make-get_online_cpus-useable-for-work-func-fix include/linux/cpu.h
--- a/include/linux/cpu.h~workqueues-make-get_online_cpus-useable-for-work-func-fix
+++ a/include/linux/cpu.h
@@ -69,10 +69,11 @@ static inline void unregister_cpu_notifi
 #endif
 
 int cpu_up(unsigned int cpu);
-
 extern void cpu_hotplug_init(void);
+extern void cpu_maps_update_begin(void);
+extern void cpu_maps_update_done(void);
 
-#else
+#else	/* CONFIG_SMP */
 
 static inline int register_cpu_notifier(struct notifier_block *nb)
 {
@@ -87,10 +88,16 @@ static inline void cpu_hotplug_init(void
 {
 }
 
+static inline void cpu_maps_update_begin(void)
+{
+}
+
+static inline void cpu_maps_update_done(void)
+{
+}
+
 #endif /* CONFIG_SMP */
 extern struct sysdev_class cpu_sysdev_class;
-extern void cpu_maps_update_begin(void);
-extern void cpu_maps_update_done(void);
 
 #ifdef CONFIG_HOTPLUG_CPU
 /* Stop CPUs going up and down. */
_

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