On 02/09/2013 05:14 AM, Paul E. McKenney wrote:

On Tue, Jan 22, 2013 at 01:04:11PM +0530, Srivatsa S. Bhat wrote:

quoted

If interrupt handlers can also be readers, then one of the ways to make
per-CPU rwlocks safe, is to disable interrupts at the reader side before
trying to acquire the per-CPU rwlock and keep it disabled throughout the
duration of the read-side critical section.

[...]

quoted

-void percpu_read_lock(struct percpu_rwlock *pcpu_rwlock)
+void percpu_read_lock_irqsafe(struct percpu_rwlock *pcpu_rwlock)
 {
 	preempt_disable();

 	/* First and foremost, let the writer know that a reader is active */
-	this_cpu_inc(*pcpu_rwlock->reader_refcnt);
+	this_cpu_add(*pcpu_rwlock->reader_refcnt, READER_PRESENT);

 	/*
 	 * If we are already using per-cpu refcounts, it is not safe to switch
 	 * the synchronization scheme. So continue using the refcounts.
 	 */
 	if (reader_nested_percpu(pcpu_rwlock)) {
-		goto out;
+		this_cpu_inc(*pcpu_rwlock->reader_refcnt);

Hmmm...  If the reader is nested, it -doesn't- need the memory barrier at
the end of this function.  If there is lots of nesting, it might be
worth getting rid of it.

Yes, good point! Will get rid of it.

Regards,
Srivatsa S. Bhat