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>
diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 2d134b7..381b09b 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -725,26 +725,21 @@
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],