Messages in this thread | | | Date | Mon, 21 Oct 2019 14:51:02 +0200 | From | Christian Brauner <> | Subject | Re: KCSAN: data-race in exit_signals / prepare_signal |
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On Mon, Oct 21, 2019 at 02:00:30PM +0200, Oleg Nesterov wrote: > On 10/21, Christian Brauner wrote: > > > > This traces back to Oleg fixing a race between a group stop and a thread > > exiting before it notices that it has a pending signal or is in the middle of > > do_exit() already, causing group stop to get wacky. > > The original commit to fix this race is > > commit d12619b5ff56 ("fix group stop with exit race") which took sighand > > lock before setting PF_EXITING on the thread. > > Not really... sig_task_ignored() didn't check task->flags until the recent > 33da8e7c81 ("signal: Allow cifs and drbd to receive their terminating signals"). > But I think this doesn't matter, see below. > > > If the race really matters and given how tsk->flags is currently accessed > > everywhere the simple fix for now might be: > > > > diff --git a/kernel/signal.c b/kernel/signal.c > > index c4da1ef56fdf..cf61e044c4cc 100644 > > --- a/kernel/signal.c > > +++ b/kernel/signal.c > > @@ -2819,7 +2819,9 @@ void exit_signals(struct task_struct *tsk) > > cgroup_threadgroup_change_begin(tsk); > > > > if (thread_group_empty(tsk) || signal_group_exit(tsk->signal)) { > > + spin_lock_irq(&tsk->sighand->siglock); > > tsk->flags |= PF_EXITING; > > + spin_unlock_irq(&tsk->sighand->siglock); > > Well, exit_signals() tries to avoid ->siglock in this case.... > > But this doesn't matter. IIUC the problem is not that exit_signals() sets > PF_EXITING, the problem is that it writes to tsk->flags and kasan detects > the data race.
Right, that's the reason I said "If the race really matters". I thought that other writers/readers always take sighand lock. So the easy fix would have been to take sighand lock too. The alternative is that we need to fiddle with task_struct itself and replace flags with an atomic_t or sm which is more invasive and probably more controversial.
Christian
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