blk-throttle: Correct the placement of smp_rmb()

o I was discussing what are the variable being updated without spin lock and
  why do we need barriers and Oleg pointed out that location of smp_rmb()
  should be between read of td->limits_changed and tg->limits_changed. This
  patch fixes it.

o Following is one possible sequence of events. Say cpu0 is executing
  throtl_update_blkio_group_read_bps() and cpu1 is executing
  throtl_process_limit_change().

 cpu0                                                cpu1

 tg->limits_changed = true;
 smp_mb__before_atomic_inc();
 atomic_inc(&td->limits_changed);

                                     if (!atomic_read(&td->limits_changed))
                                             return;

                                     if (tg->limits_changed)
                                             do_something;

 If cpu0 has updated tg->limits_changed and td->limits_changed, we want to
 make sure that if update to td->limits_changed is visible on cpu1, then
 update to tg->limits_changed should also be visible.

 Oleg pointed out to ensure that we need to insert an smp_rmb() between
 td->limits_changed read and tg->limits_changed read.

o I had erroneously put smp_rmb() before atomic_read(&td->limits_changed).
  This patch fixes it.

Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

1 files changed, 9 insertions, 14 deletions

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 2d134b7c40a3..381b09bb562b 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -725,26 +725,21 @@ static void throtl_process_limit_change(struct throtl_data *td)
         struct throtl_grp *tg;
         struct hlist_node *pos, *n;
 
-        /*
-         * Make sure atomic_inc() effects from
-         * throtl_update_blkio_group_read_bps(), group of functions are
-         * visible.
-         * Is this required or smp_mb__after_atomic_inc() was suffcient
-         * after the atomic_inc().
-         */
-        smp_rmb();
         if (!atomic_read(&td->limits_changed))
                 return;
 
         throtl_log(td, "limit changed =%d", atomic_read(&td->limits_changed));
 
-        hlist_for_each_entry_safe(tg, pos, n, &td->tg_list, tg_node) {
-                /*
-                 * Do I need an smp_rmb() here to make sure tg->limits_changed
-                 * update is visible. I am relying on smp_rmb() at the
-                 * beginning of function and not putting a new one here.
-                 */
+        /*
+         * Make sure updates from throtl_update_blkio_group_read_bps() group
+         * of functions to tg->limits_changed are visible. We do not
+         * want update td->limits_changed to be visible but update to
+         * tg->limits_changed not being visible yet on this cpu. Hence
+         * the read barrier.
+         */
+        smp_rmb();
 
+        hlist_for_each_entry_safe(tg, pos, n, &td->tg_list, tg_node) {
                 if (throtl_tg_on_rr(tg) && tg->limits_changed) {
                         throtl_log_tg(td, tg, "limit change rbps=%llu wbps=%llu"
                                 " riops=%u wiops=%u", tg->bps[READ],