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freezer: change ptrace_stop/do_signal_stop to use freezable_schedule()

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try_to_freeze_tasks() and cgroup_freezer rely on scheduler locks
to ensure that a task doing STOPPED/TRACED -> RUNNING transition
can't escape freezing. This mostly works, but ptrace_stop() does
not necessarily call schedule(), it can change task->state back to
RUNNING and check freezing() without any lock/barrier in between.

We could add the necessary barrier, but this patch changes
ptrace_stop() and do_signal_stop() to use freezable_schedule().
This fixes the race, freezer_count() and freezer_should_skip()
carefully avoid the race.

And this simplifies the code, try_to_freeze_tasks/update_if_frozen
no longer need to use task_is_stopped_or_traced() checks with the
non trivial assumptions. We can rely on the mechanism which was
specially designed to mark the sleeping task as "frozen enough".

v2: As Tejun pointed out, we can also change get_signal_to_deliver()
and move try_to_freeze() up before 'relock' label.

Signed-off-by: Oleg Nesterov <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>
This commit is contained in:
Oleg Nesterov 2012-10-26 19:46:06 +02:00 • committed by Tejun Heo
commit 5d8f72b55c
5 changed files with 13 additions and 41 deletions
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20
kernel/signal.c
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@ -1908,7 +1908,7 @@ static void ptrace_stop(int exit_code, int why, int clear_code, siginfo_t *info)
preempt_disable();
read_unlock(&tasklist_lock);
preempt_enable_no_resched();
schedule();
freezable_schedule();
} else {
/*
* By the time we got the lock, our tracer went away.
@ -1929,13 +1929,6 @@ static void ptrace_stop(int exit_code, int why, int clear_code, siginfo_t *info)
read_unlock(&tasklist_lock);
}
/*
* While in TASK_TRACED, we were considered "frozen enough".
* Now that we woke up, it's crucial if we're supposed to be
* frozen that we freeze now before running anything substantial.
*/
try_to_freeze();
/*
* We are back. Now reacquire the siglock before touching
* last_siginfo, so that we are sure to have synchronized with
@ -2092,7 +2085,7 @@ static bool do_signal_stop(int signr)
}
/* Now we don't run again until woken by SIGCONT or SIGKILL */
schedule();
freezable_schedule();
return true;
} else {
/*
@ -2200,15 +2193,14 @@ int get_signal_to_deliver(siginfo_t *info, struct k_sigaction *return_ka,
if (unlikely(uprobe_deny_signal()))
return 0;
relock:
/*
* We'll jump back here after any time we were stopped in TASK_STOPPED.
* While in TASK_STOPPED, we were considered "frozen enough".
* Now that we woke up, it's crucial if we're supposed to be
* frozen that we freeze now before running anything substantial.
* Do this once, we can't return to user-mode if freezing() == T.
* do_signal_stop() and ptrace_stop() do freezable_schedule() and
* thus do not need another check after return.
*/
try_to_freeze();
relock:
spin_lock_irq(&sighand->siglock);
/*
* Every stopped thread goes here after wakeup. Check to see if