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[PATCH] run_posix_cpu_timers: remove a bogus BUG_ON()

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do_exit() clears ->it_##clock##_expires, but nothing prevents
another cpu to attach the timer to exiting process after that.
arm_timer() tries to protect against this race, but the check
is racy.

After exit_notify() does 'write_unlock_irq(&tasklist_lock)' and
before do_exit() calls 'schedule() local timer interrupt can find
tsk->exit_state != 0. If that state was EXIT_DEAD (or another cpu
does sys_wait4) interrupted task has ->signal == NULL.

At this moment exiting task has no pending cpu timers, they were
cleanuped in __exit_signal()->posix_cpu_timers_exit{,_group}(),
so we can just return from irq.

John Stultz recently confirmed this bug, see

	http://marc.theaimsgroup.com/?l=linux-kernel&m=115015841413687

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Oleg Nesterov 2006-06-15 20:11:43 +04:00 committed by Linus Torvalds
parent 8f17fc20bf
commit 30f1e3dd8c
2 changed files with 18 additions and 26 deletions
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8
kernel/exit.c
View File

@ -881,14 +881,6 @@ fastcall NORET_TYPE void do_exit(long code)
tsk->flags |= PF_EXITING;
/*
* Make sure we don't try to process any timer firings
* while we are already exiting.
*/
tsk->it_virt_expires = cputime_zero;
tsk->it_prof_expires = cputime_zero;
tsk->it_sched_expires = 0;
if (unlikely(in_atomic()))
printk(KERN_INFO "note: %s[%d] exited with preempt_count %d\n",
current->comm, current->pid,

36
kernel/posix-cpu-timers.c
View File

@ -1288,30 +1288,30 @@ void run_posix_cpu_timers(struct task_struct *tsk)
#undef UNEXPIRED
BUG_ON(tsk->exit_state);
/*
* Double-check with locks held.
*/
read_lock(&tasklist_lock);
spin_lock(&tsk->sighand->siglock);
if (likely(tsk->signal != NULL)) {
spin_lock(&tsk->sighand->siglock);
/*
* Here we take off tsk->cpu_timers[N] and tsk->signal->cpu_timers[N]
* all the timers that are firing, and put them on the firing list.
*/
check_thread_timers(tsk, &firing);
check_process_timers(tsk, &firing);
/*
* Here we take off tsk->cpu_timers[N] and tsk->signal->cpu_timers[N]
* all the timers that are firing, and put them on the firing list.
*/
check_thread_timers(tsk, &firing);
check_process_timers(tsk, &firing);
/*
* We must release these locks before taking any timer's lock.
* There is a potential race with timer deletion here, as the
* siglock now protects our private firing list. We have set
* the firing flag in each timer, so that a deletion attempt
* that gets the timer lock before we do will give it up and
* spin until we've taken care of that timer below.
*/
spin_unlock(&tsk->sighand->siglock);
/*
* We must release these locks before taking any timer's lock.
* There is a potential race with timer deletion here, as the
* siglock now protects our private firing list. We have set
* the firing flag in each timer, so that a deletion attempt
* that gets the timer lock before we do will give it up and
* spin until we've taken care of that timer below.
*/
spin_unlock(&tsk->sighand->siglock);
}
read_unlock(&tasklist_lock);
/*