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freezer: change ptrace_stop/do_signal_stop to use freezable_schedule() 

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 <oleg@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Oleg Nesterov 2012-10-26 19:46:06 +02:00 committed by Tejun Heo
parent ead5c47371
commit 5d8f72b55c
5 changed files with 13 additions and 41 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
include/linux/freezer.h
View File

@ -134,10 +134,9 @@ static inline bool freezer_should_skip(struct task_struct *p)
}
/*
* These macros are intended to be used whenever you want allow a task that's
* sleeping in TASK_UNINTERRUPTIBLE or TASK_KILLABLE state to be frozen. Note
* that neither return any clear indication of whether a freeze event happened
* while in this function.
* These macros are intended to be used whenever you want allow a sleeping
* task to be frozen. Note that neither return any clear indication of
* whether a freeze event happened while in this function.
*/
/* Like schedule(), but should not block the freezer. */

3
kernel/cgroup_freezer.c
View File

@ -198,8 +198,7 @@ static void update_if_frozen(struct cgroup *cgroup, struct freezer *freezer)
* completion. Consider it frozen in addition to
* the usual frozen condition.
*/
if (!frozen(task) && !task_is_stopped_or_traced(task) &&
!freezer_should_skip(task))
if (!frozen(task) && !freezer_should_skip(task))
goto notyet;
}
}

11
kernel/freezer.c
View File

@ -116,17 +116,10 @@ bool freeze_task(struct task_struct *p)
return false;
}
if (!(p->flags & PF_KTHREAD)) {
if (!(p->flags & PF_KTHREAD))
fake_signal_wake_up(p);
/*
* fake_signal_wake_up() goes through p's scheduler
* lock and guarantees that TASK_STOPPED/TRACED ->
* TASK_RUNNING transition can't race with task state
* testing in try_to_freeze_tasks().
*/
} else {
else
wake_up_state(p, TASK_INTERRUPTIBLE);
}
spin_unlock_irqrestore(&freezer_lock, flags);
return true;

13
kernel/power/process.c
View File

@ -48,18 +48,7 @@ static int try_to_freeze_tasks(bool user_only)
if (p == current || !freeze_task(p))
continue;
/*
* Now that we've done set_freeze_flag, don't
* perturb a task in TASK_STOPPED or TASK_TRACED.
* It is "frozen enough". If the task does wake
* up, it will immediately call try_to_freeze.
*
* Because freeze_task() goes through p's scheduler lock, it's
* guaranteed that TASK_STOPPED/TRACED -> TASK_RUNNING
* transition can't race with task state testing here.
*/
if (!task_is_stopped_or_traced(p) &&
!freezer_should_skip(p))
if (!freezer_should_skip(p))
todo++;
} while_each_thread(g, p);
read_unlock(&tasklist_lock);

20
kernel/signal.c
View File

@ -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