net: Avoid livelock in net_tx_action() on RT

net: Avoid livelock in net_tx_action() on RT

qdisc_lock is taken w/o disabling interrupts or bottom halfs. So code
holding a qdisc_lock() can be interrupted and softirqs can run on the
return of interrupt in !RT.

The spin_trylock() in net_tx_action() makes sure, that the softirq
does not deadlock. When the lock can't be acquired q is requeued and
the NET_TX softirq is raised. That causes the softirq to run over and
over.

That works in mainline as do_softirq() has a retry loop limit and
leaves the softirq processing in the interrupt return path and
schedules ksoftirqd. The task which holds qdisc_lock cannot be
preempted, so the lock is released and either ksoftirqd or the next
softirq in the return from interrupt path can proceed. Though it's a
bit strange to actually run MAX_SOFTIRQ_RESTART (10) loops before it
decides to bail out even if it's clear in the first iteration :)

On RT all softirq processing is done in a FIFO thread and we don't
have a loop limit, so ksoftirqd preempts the lock holder forever and
unqueues and requeues until the reset button is hit.

Due to the forced threading of ksoftirqd on RT we actually cannot
deadlock on qdisc_lock because it's a "sleeping lock". So it's safe to
replace the spin_trylock() with a spin_lock(). When contended,
ksoftirqd is scheduled out and the lock holder can proceed.

[ tglx: Massaged changelog and code comments ]

Solved-by: Thomas Gleixner <tglx@linuxtronix.de>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Tested-by: Carsten Emde <cbe@osadl.org>
Cc: Clark Williams <williams@redhat.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Luis Claudio R. Goncalves <lclaudio@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>