tasklet: Prevent tasklets from going into infinite spin in RT

tasklet: Prevent tasklets from going into infinite spin in RT

When CONFIG_PREEMPT_RT_FULL is enabled, tasklets run as threads,
and spinlocks turn are mutexes. But this can cause issues with
tasks disabling tasklets. A tasklet runs under ksoftirqd, and
if a tasklets are disabled with tasklet_disable(), the tasklet
count is increased. When a tasklet runs, it checks this counter
and if it is set, it adds itself back on the softirq queue and
returns.

The problem arises in RT because ksoftirq will see that a softirq
is ready to run (the tasklet softirq just re-armed itself), and will
not sleep, but instead run the softirqs again. The tasklet softirq
will still see that the count is non-zero and will not execute
the tasklet and requeue itself on the softirq again, which will
cause ksoftirqd to run it again and again and again.

It gets worse because ksoftirqd runs as a real-time thread.
If it preempted the task that disabled tasklets, and that task
has migration disabled, or can't run for other reasons, the tasklet
softirq will never run because the count will never be zero, and
ksoftirqd will go into an infinite loop. As an RT task, it this
becomes a big problem.

This is a hack solution to have tasklet_disable stop tasklets, and
when a tasklet runs, instead of requeueing the tasklet softirqd
it delays it. When tasklet_enable() is called, and tasklets are
waiting, then the tasklet_enable() will kick the tasklets to continue.
This prevents the lock up from ksoftirq going into an infinite loop.

[ rostedt@goodmis.org: ported to 3.0-rt ]

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>