On Mon, 22 Sep 2014 18:44:37 +0200 Oleg Nesterov <[email protected]> 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 <[email protected]> > --- > 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. Or is this something we can have preempt_disable_nort() with (for the -rt kernel that is). That is, is -rt susceptible to this deadlock as well? -- Steve > + rcu_read_lock(); > for (;;) { > - /* > - * Disable interrupts early to avoid deadlocks. > - * See rcu_read_unlock() comment header for details. > - */ > - local_irq_save(*flags); > - rcu_read_lock(); > sighand = rcu_dereference(tsk->sighand); > - if (unlikely(sighand == NULL)) { > - rcu_read_unlock(); > - local_irq_restore(*flags); > + if (unlikely(sighand == NULL)) > break; > - } > > - spin_lock(&sighand->siglock); > - if (likely(sighand == tsk->sighand)) { > - rcu_read_unlock(); > + spin_lock_irqsave(&sighand->siglock, *flags); > + if (likely(sighand == tsk->sighand)) > break; > - } > - spin_unlock(&sighand->siglock); > - rcu_read_unlock(); > - local_irq_restore(*flags); > + spin_unlock_irqrestore(&sighand->siglock, *flags); > } > + rcu_read_unlock(); > + preempt_enable(); > > return sighand; > } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
