On Mon, Jan 23, 2017 at 07:26:45PM -0800, Lance Roy wrote:
> > Yeah, we did have this same conversation awhile back, didn't we?
> >
> > Back then, did I think to ask if this could be minimized or even prevented
> > by adding memory barriers appropriately? ;-)
> >
> > Thanx, Paul
>
> Yes, it can be fixed by adding a memory barrier after incrementing ->completed
> inside srcu_flip(). The upper limit on NR_CPUS turns out to be more
> complicated
> than this, as it needs to deal with highly nested read side critical sections
> mixed with the critical section loops, but only the one memory barrier should
> be necessary.
Something like this, then?
Thanx, Paul
------------------------------------------------------------------------
commit 35be9e413dde662fc9661352e595105ac4b0b167
Author: Paul E. McKenney <paul...@linux.vnet.ibm.com>
Date: Tue Jan 24 08:51:34 2017 -0800
srcu: Reduce probability of SRCU ->unlock_count[] counter overflow
Because there are no memory barriers between the srcu_flip() ->completed
increment and the summation of the read-side ->unlock_count[] counters,
both the compiler and the CPU can reorder the summation with the
->completed increment. If the updater is preempted long enough during
this process, the read-side counters could overflow, resulting in a
too-short grace period.
This commit therefore adds a memory barrier just after the ->completed
increment, ensuring that if the summation misses an increment of
->unlock_count[] from __srcu_read_unlock(), the next __srcu_read_lock()
will see the new value of ->completed, thus bounding the number of
->unlock_count[] increments that can be missed to NR_CPUS. The actual
overflow computation is more complex due to the possibility of nesting
of __srcu_read_lock().
Reported-by: Lance Roy <ldr...@gmail.com>
Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
diff --git a/kernel/rcu/srcu.c b/kernel/rcu/srcu.c
index d3378ceb9762..aefe3ab20a6a 100644
--- a/kernel/rcu/srcu.c
+++ b/kernel/rcu/srcu.c
@@ -337,7 +337,16 @@ static bool try_check_zero(struct srcu_struct *sp, int
idx, int trycount)
*/
static void srcu_flip(struct srcu_struct *sp)
{
- sp->completed++;
+ WRITE_ONCE(sp->completed, sp->completed + 1);
+
+ /*
+ * Ensure that if the updater misses an __srcu_read_unlock()
+ * increment, that task's next __srcu_read_lock() will see the
+ * above counter update. Note that both this memory barrier
+ * and the one in srcu_readers_active_idx_check() provide the
+ * guarantee for __srcu_read_lock().
+ */
+ smp_mb(); /* D */ /* Pairs with C. */
}
/*
