[PATCH RT] seqlock/rt: Prevent livelocks with seqlocks in RT

[Steven Rostedt]

Thomas,

I was running my cpu hotplug stress test along with a kernel compile and
after about 40 minutes of running it locked up. It happened in the
read_seqcount_begin() that is called by d_lookup().

ksoftirqd was caught here:

static __always_inline unsigned read_seqbegin(const seqlock_t *sl)
{
	unsigned ret;

repeat:
	ret = ACCESS_ONCE(sl->sequence);
	if (unlikely(ret & 1)) {
		cpu_relax();
		goto repeat;
	}
	smp_rmb();

	return ret;
}

It preempted the holder of the seqlock that was held for write, and as
that holder had migrate disabled, it couldn't be scheduled. Then
ksoftirqd went into this infinite loop and the system locked up.

This patch fixes the issue by grabbing and releasing the write lock when
it detects contention. It only works with seqlocks and not seqcounts
that have their own locking. But we could add an api to include those
too if needed.

-- Steve