net: sched: add barrier to ensure correct ordering for lockless qdisc

The spin_trylock() was assumed to contain the implicit
barrier needed to ensure the correct ordering between
STATE_MISSED setting/clearing and STATE_MISSED checking
in commit a90c57f2cedd ("net: sched: fix packet stuck
problem for lockless qdisc").

But it turns out that spin_trylock() only has load-acquire
semantic, for strongly-ordered system(like x86), the compiler
barrier implicitly contained in spin_trylock() seems enough
to ensure the correct ordering. But for weakly-orderly system
(like arm64), the store-release semantic is needed to ensure
the correct ordering as clear_bit() and test_bit() is store
operation, see queued_spin_lock().

So add the explicit barrier to ensure the correct ordering
for the above case.

Fixes: a90c57f2cedd ("net: sched: fix packet stuck problem for lockless qdisc")
Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index 1e625519ae9687608c087ceee6b30aac82513fca..57710303908c6a41194ac20514e66c9b1259420b 100644 (file)
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -163,6 +163,12 @@ static inline bool qdisc_run_begin(struct Qdisc *qdisc)
                if (spin_trylock(&qdisc->seqlock))
                        goto nolock_empty;
 
+               /* Paired with smp_mb__after_atomic() to make sure
+                * STATE_MISSED checking is synchronized with clearing
+                * in pfifo_fast_dequeue().
+                */
+               smp_mb__before_atomic();
+
                /* If the MISSED flag is set, it means other thread has
                 * set the MISSED flag before second spin_trylock(), so
                 * we can return false here to avoid multi cpus doing
@@ -180,6 +186,12 @@ static inline bool qdisc_run_begin(struct Qdisc *qdisc)
                 */
                set_bit(__QDISC_STATE_MISSED, &qdisc->state);
 
+               /* spin_trylock() only has load-acquire semantic, so use
+                * smp_mb__after_atomic() to ensure STATE_MISSED is set
+                * before doing the second spin_trylock().
+                */
+               smp_mb__after_atomic();
+
                /* Retry again in case other CPU may not see the new flag
                 * after it releases the lock at the end of qdisc_run_end().
                 */