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um: fix possible race on task->mm

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Checking for task->mm is dangerous as ->mm might disappear (exit_mm()
assigns NULL under task_lock(), so tasklist lock is not enough).

We can't use get_task_mm()/mmput() pair as mmput() might sleep, so let's
take the task lock while we care about its mm.

Note that we should also use find_lock_task_mm() to check all process'
threads for a valid mm, but for uml we'll do it in a separate patch.

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
Cc: Richard Weinberger <richard@nod.at>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Anton Vorontsov 2012-05-31 16:26:25 -07:00 committed by Linus Torvalds
parent 9bd0a07712
commit 137d1a26c8
1 changed files with 5 additions and 2 deletions
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7
arch/um/kernel/reboot.c
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@ -25,10 +25,13 @@ static void kill_off_processes(void)
read_lock(&tasklist_lock);
for_each_process(p) {
if (p->mm == NULL)
task_lock(p);
if (!p->mm) {
task_unlock(p);
continue;
}
pid = p->mm->context.id.u.pid;
task_unlock(p);
os_kill_ptraced_process(pid, 1);
}
read_unlock(&tasklist_lock);