Messages in this thread | | | From | "Rafael J. Wysocki" <> | Subject | Re: [PATCH 02/14] freezer: fix a race during freezing of TASK_STOPPED tasks | Date | Fri, 26 Nov 2010 20:40:27 +0100 |
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On Friday, November 26, 2010, Tejun Heo wrote: > After calling freeze_task(), try_to_freeze_tasks() see whether the > task is stopped or traced and if so, considers it to be frozen; > however, nothing guarantees that either the task being frozen sees > TIF_FREEZE or the freezer sees TASK_STOPPED -> TASK_RUNNING > transition. The task being frozen may wake up and not see TIF_FREEZE > while the freezer fails to notice the transition and believes the task > is still stopped. > > This patch fixes the race by making freeze_task() always go through > fake_signal_wake_up() for applicable tasks. The function goes through > the target task's scheduler lock and thus guarantees that either the > target sees TIF_FREEZE or try_to_freeze_task() sees TASK_RUNNING.
Looks good. I'll take this one to my tree, if you don't mind.
Thanks, Rafael
> Signed-off-by: Tejun Heo <tj@kernel.org> > Cc: Oleg Nesterov <oleg@redhat.com> > Cc: Rafael J. Wysocki <rjw@sisk.pl> > --- > kernel/freezer.c | 9 +++++++-- > kernel/power/process.c | 6 ++++++ > 2 files changed, 13 insertions(+), 2 deletions(-) > > diff --git a/kernel/freezer.c b/kernel/freezer.c > index bd1d42b..66ecd2e 100644 > --- a/kernel/freezer.c > +++ b/kernel/freezer.c > @@ -104,8 +104,13 @@ bool freeze_task(struct task_struct *p, bool sig_only) > } > > if (should_send_signal(p)) { > - if (!signal_pending(p)) > - fake_signal_wake_up(p); > + 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 if (sig_only) { > return false; > } else { > diff --git a/kernel/power/process.c b/kernel/power/process.c > index e50b4c1..eb2c88a 100644 > --- a/kernel/power/process.c > +++ b/kernel/power/process.c > @@ -64,6 +64,12 @@ static int try_to_freeze_tasks(bool sig_only) > * 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 after setting TIF_FREEZE, it's > + * guaranteed that either we see TASK_RUNNING or > + * try_to_stop() after schedule() in ptrace/signal > + * stop sees TIF_FREEZE. > */ > if (!task_is_stopped_or_traced(p) && > !freezer_should_skip(p)) >
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