From: Anton Vorontsov <anton.vorontsov@linaro.org>
To: Andrew Morton <akpm@linux-foundation.org>,
Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Oleg Nesterov <oleg@redhat.com>,
Russell King <linux@arm.linux.org.uk>,
Mike Frysinger <vapier@gentoo.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Richard Weinberger <richard@nod.at>,
Paul Mundt <lethal@linux-sh.org>,
KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
John Stultz <john.stultz@linaro.org>,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org,
uclinux-dist-devel@blackfin.uclinux.org,
linuxppc-dev@lists.ozlabs.org, linux-sh@vger.kernel.org,
user-mode-linux-devel@lists.sourceforge.net,
linaro-kernel@lists.linaro.org, patches@linaro.org,
linux-mm@kvack.org
Subject: [PATCH] cpu: Document clear_tasks_mm_cpumask()
Date: Fri, 4 May 2012 18:47:28 -0700 [thread overview]
Message-ID: <20120505014728.GA9079@lizard> (raw)
In-Reply-To: <1335869133.13683.125.camel@twins>
This patch adds more comments on clear_tasks_mm_cpumask, plus adds
a runtime check: the function is only suitable for offlined CPUs,
and if called inappropriately, the kernel should scream aloud.
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Suggested-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---
On Tue, May 01, 2012 at 12:45:33PM +0200, Peter Zijlstra wrote:
> On Thu, 2012-04-26 at 16:59 -0700, Andrew Morton wrote:
> > > +void clear_tasks_mm_cpumask(int cpu)
> >
> > The operation of this function was presumably obvious to you at the
> > time you wrote it, but that isn't true of other people at later times.
> >
> > Please document it?
[...]
> > If someone tries to use this function for a different purpose, or
> > copies-and-modifies it for a different purpose, we just shot them in
> > the foot.
> >
> > They'd be pretty dumb to do that without reading the local comment,
> > but still...
>
> Methinks something simple like:
>
> WARN_ON(cpu_online(cpu));
>
> Ought to cure that worry, no? :-)
Yeah, this is all good ideas, thanks.
How about the following patch?
kernel/cpu.c | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)
diff --git a/kernel/cpu.c b/kernel/cpu.c
index ecdf499..1bfa26f 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -13,6 +13,7 @@
#include <linux/oom.h>
#include <linux/rcupdate.h>
#include <linux/export.h>
+#include <linux/bug.h>
#include <linux/kthread.h>
#include <linux/stop_machine.h>
#include <linux/mutex.h>
@@ -173,6 +174,18 @@ void __ref unregister_cpu_notifier(struct notifier_block *nb)
}
EXPORT_SYMBOL(unregister_cpu_notifier);
+/**
+ * clear_tasks_mm_cpumask - Safely clear tasks' mm_cpumask for a CPU
+ * @cpu: a CPU id
+ *
+ * This function walks up all processes, finds a valid mm struct for
+ * each one and then clears a corresponding bit in mm's cpumask. While
+ * this all sounds trivial, there are various non-obvious corner cases,
+ * which this function tries to solve in a safe manner.
+ *
+ * Also note that the function uses a somewhat relaxed locking scheme,
+ * so it may be called only for an already offlined CPU.
+ */
void clear_tasks_mm_cpumask(int cpu)
{
struct task_struct *p;
@@ -184,10 +197,15 @@ void clear_tasks_mm_cpumask(int cpu)
* Thus, we may use rcu_read_lock() here, instead of grabbing
* full-fledged tasklist_lock.
*/
+ WARN_ON(cpu_online(cpu));
rcu_read_lock();
for_each_process(p) {
struct task_struct *t;
+ /*
+ * Main thread might exit, but other threads may still have
+ * a valid mm. Find one.
+ */
t = find_lock_task_mm(p);
if (!t)
continue;
--
1.7.9.2
next prev parent reply other threads:[~2012-05-05 1:49 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-04-23 7:06 [PATCH v3 0/9] Fixes for common mistakes w/ for_each_process and task->mm Anton Vorontsov
2012-04-23 7:07 ` [PATCH 1/9] cpu: Introduce clear_tasks_mm_cpumask() helper Anton Vorontsov
2012-04-26 23:59 ` Andrew Morton
2012-05-01 10:45 ` Peter Zijlstra
2012-05-05 1:47 ` Anton Vorontsov [this message]
2012-05-05 1:47 ` Anton Vorontsov
2012-04-23 7:08 ` [PATCH 2/9] arm: Use clear_tasks_mm_cpumask() Anton Vorontsov
2012-04-23 7:08 ` [PATCH 3/9] powerpc: " Anton Vorontsov
2012-04-23 7:08 ` [PATCH 4/9] sh: " Anton Vorontsov
2012-04-23 7:09 ` [PATCH 5/9] blackfin: A couple of task->mm handling fixes Anton Vorontsov
2012-06-01 4:36 ` Mike Frysinger
2012-04-23 7:09 ` [PATCH 6/9] blackfin: Fix possible deadlock in decode_address() Anton Vorontsov
2012-04-23 7:09 ` [PATCH 7/9] um: Should hold tasklist_lock while traversing processes Anton Vorontsov
2012-04-23 14:57 ` Richard Weinberger
2012-04-23 15:40 ` Anton Vorontsov
2012-04-23 7:09 ` [PATCH 8/9] um: Fix possible race on task->mm Anton Vorontsov
2012-04-23 7:09 ` [PATCH 9/9] um: Properly check all process' threads for a live mm Anton Vorontsov
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