Messages in this thread | | | Date | Sat, 16 Feb 2013 12:59:09 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 04/16] rcu: rcutiny: Prevent RCU stall |
| |
On Wed, Feb 13, 2013 at 05:11:59PM +0100, Sebastian Andrzej Siewior wrote: > From: Thomas Gleixner <tglx@linutronix.de> > > rcu_read_unlock_special() checks in_serving_softirq() and leaves early > when true. On RT this is obviously wrong as softirq processing context > can be preempted and therefor such a task can be on the gp_tasks > list. Leaving early here will leave the task on the list and therefor > block RCU processing forever. > > This cannot happen on mainline because softirq processing context > cannot be preempted and therefor this can never happen at all. > > In fact this check looks quite questionable in general. Neither irq > context nor softirq processing context in mainline can ever be > preempted in mainline so the special unlock case should not ever be > invoked in such context. Now the only explanation might be a > rcu_read_unlock() being interrupted and therefor leave the rcu nest > count at 0 before the special unlock bit has been cleared. That looks > fragile. At least it's missing a big fat comment. Paul ???? > > See mainline commits: ec433f0c5 and 8762705a for further enlightment. > > Reported-by: Kristian Lehmann <krleit00@hs-esslingen.de> > Signed-off-by: Thomas Gleixner <tglx@linutronix.de> > [bigeasy@linutronix: different in-irq check] > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> > --- > kernel/rcutiny_plugin.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/rcutiny_plugin.h b/kernel/rcutiny_plugin.h > index 2b0484a..bac1906 100644 > --- a/kernel/rcutiny_plugin.h > +++ b/kernel/rcutiny_plugin.h > @@ -552,7 +552,7 @@ static void rcu_read_unlock_special(struct task_struct *t) > rcu_preempt_cpu_qs(); > > /* Hardware IRQ handlers cannot block. */ > - if (in_irq()) { > + if (preempt_count() & (HARDIRQ_MASK | SOFTIRQ_OFFSET)) {
For whatever it is worth, in mainline this is:
if (in_irq() || in_serving_softirq()) {
The definition of in_serving_softirq() is a bit different:
#define in_serving_softirq() (softirq_count() & SOFTIRQ_OFFSET)
This might be due to differences between mainline and -rt, but thought it worth calling attention to.
Thanx, Paul
> local_irq_restore(flags); > return; > } > -- > 1.7.10.4 > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ >
| |