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

On Tue, 2012-03-06 at 09:18 +0100, Thomas Gleixner wrote:

Errm. rt15 has 

/*
 * Starvation safe read side for RT
 */
static inline unsigned read_seqbegin(seqlock_t *sl)
{
        unsigned ret;

repeat:
        ret = sl->seqcount.sequence;
        if (unlikely(ret & 1)) {
                /*
                 * Take the lock and let the writer proceed (i.e. evtl
                 * boost it), otherwise we could loop here forever.
                 */
                spin_lock(&sl->lock);
                spin_unlock(&sl->lock);
                goto repeat;
        }
        return ret;
}
#endif

You're right it does!

I didn't see this as the lock up showed it was locked up in
read_seqcount_begin(), it was late and I was tired.

Here's the real fix then:

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 723274d..29ffd4f 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h

@@ -177,7 +177,7 @@ typedef struct {
 /*
  * Read side functions for starting and finalizing a read side section.
  */
-#ifndef CONFIG_PREEMPT_RT
+#ifndef CONFIG_PREEMPT_RT_FULL
 static inline unsigned read_seqbegin(const seqlock_t *sl)
 {
 	return read_seqcount_begin(&sl->seqcount);