Messages in this thread | | | Date | Mon, 22 Sep 2014 17:24:05 -0400 | From | Steven Rostedt <> | Subject | Re: [PATCH 1/2] signal: simplify deadlock-avoidance in lock_task_sighand() |
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On Mon, 22 Sep 2014 21:11:30 +0200 Oleg Nesterov <oleg@redhat.com> wrote:
> On 09/22, Steven Rostedt wrote: > > > > On Mon, 22 Sep 2014 18:44:37 +0200 > > Oleg Nesterov <oleg@redhat.com> wrote: > > > > > __lock_task_sighand() does local_irq_save() to prevent the potential > > > deadlock, we can use preempt_disable() with the same effect. And in > > > this case we can do preempt_disable/enable + rcu_read_lock/unlock only > > > once outside of the main loop and simplify the code. This also shaves > > > 112 bytes from signal.o. > > > > > > With this patch the main loop runs with preemption disabled, but this > > > should be fine because restart is very unlikely: it can only happen if > > > we race with de_thread() and ->sighand is shared. And the latter is only > > > possible if CLONE_SIGHAND was used without CLONE_THREAD, most probably > > > nobody does this nowadays. > > > > > > Signed-off-by: Oleg Nesterov <oleg@redhat.com> > > > --- > > > kernel/signal.c | 31 +++++++++++++------------------ > > > 1 files changed, 13 insertions(+), 18 deletions(-) > > > > > > diff --git a/kernel/signal.c b/kernel/signal.c > > > index 8f0876f..61a1f55 100644 > > > --- a/kernel/signal.c > > > +++ b/kernel/signal.c > > > @@ -1261,30 +1261,25 @@ struct sighand_struct *__lock_task_sighand(struct task_struct *tsk, > > > unsigned long *flags) > > > { > > > struct sighand_struct *sighand; > > > - > > > + /* > > > + * We are going to do rcu_read_unlock() under spin_lock_irqsave(). > > > + * Make sure we can not be preempted after rcu_read_lock(), see > > > + * rcu_read_unlock() comment header for details. > > > + */ > > > + preempt_disable(); > > > > The sad part is, this is going to break -rt. > > Hmm, why??
Because in -rt, siglock is a mutex.
> > > That > > is, is -rt susceptible to this deadlock as well?
As siglock is a mutex, this shouldn't be a problem.
> > In fact this deadlock is not really possible in any case, scheduler locks > should be fine under ->siglock (for example, signal_wake_up() is called > under this lock). > > But, the comment above rcu_read_unlock() says: > > Given that the set of locks acquired by rt_mutex_unlock() might change > at any time, a somewhat more future-proofed approach is to make sure > that that preemption never happens ...
Hmm, I'm not sure we need to worry about this. As in -rt siglock is a mutex, which is rt_mutex() itself, I highly doubt we will have rt_mutex_unlock() grab siglock, otherwise that would cause havoc in -rt.
> > so this patch doesn't try to change the rules. > > But perhaps we can simply remove this preempt_disable/enable? > > Or. We can shift rcu_read_unlock() from lock_task_sighand() to > unlock_task_sighand(). This way we can avoid preempt_disable too, but > I'd prefer to not do this.
I really thing the preempt_disable/enable is not needed.
Paul, Thomas, care to comment?
-- Steve
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