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 --- V2: add the missing Fixes tag.
The above ordering issue can easily cause out of order packet problem when testing lockless qdisc bypass patchset [1] with two iperf threads and one netdev queue in arm64 system.
1. https://lkml.org/lkml/2021/6/2/1417 --- include/net/sch_generic.h | 12 ++++++++++++ 1 file changed, 12 insertions(+)
diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h index 1e62551..5771030 100644 --- 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(). */
On Thu, 17 Jun 2021 09:04:14 +0800 Yunsheng Lin wrote:
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
On Fri, 18 Jun 2021 17:30:47 -0700 Jakub Kicinski wrote:
On Thu, 17 Jun 2021 09:04:14 +0800 Yunsheng Lin wrote:
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
Actually.. do we really need the _before_atomic() barrier? I'd think we only need to make sure we re-check the lock after we set the bit, ordering of the first check doesn't matter.
On Fri, Jun 18, 2021 at 05:38:37PM -0700, Jakub Kicinski wrote:
On Fri, 18 Jun 2021 17:30:47 -0700 Jakub Kicinski wrote:
On Thu, 17 Jun 2021 09:04:14 +0800 Yunsheng Lin wrote:
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
Actually.. do we really need the _before_atomic() barrier? I'd think we only need to make sure we re-check the lock after we set the bit, ordering of the first check doesn't matter.
When debugging pointed to the misordering between STATE_MISSED setting/clearing and STATE_MISSED checking, only _after_atomic() was added first, and it did not fix the misordering problem, when both _before_atomic() and _after_atomic() were added, the misordering problem disappeared.
I suppose _before_atomic() matters because the STATE_MISSED setting and the lock rechecking is only done when first check of STATE_MISSED returns false. _before_atomic() is used to make sure the first check returns correct result, if it does not return the correct result, then we may have misordering problem too.
cpu0 cpu1 clear MISSED _after_atomic() dequeue enqueue first trylock() #false MISSED check #*true* ?
As above, even cpu1 has a _after_atomic() between clearing STATE_MISSED and dequeuing, we might stiil need a barrier to prevent cpu0 doing speculative MISSED checking before cpu1 clearing MISSED?
And the implicit load-acquire barrier contained in the first trylock() does not seems to prevent the above case too.
And there is no load-acquire barrier in pfifo_fast_dequeue() too, which possibly make the above case more likely to happen.
On Sat, 19 Jun 2021 10:30:09 +0000 Yunsheng Lin wrote:
When debugging pointed to the misordering between STATE_MISSED setting/clearing and STATE_MISSED checking, only _after_atomic() was added first, and it did not fix the misordering problem, when both _before_atomic() and _after_atomic() were added, the misordering problem disappeared.
I suppose _before_atomic() matters because the STATE_MISSED setting and the lock rechecking is only done when first check of STATE_MISSED returns false. _before_atomic() is used to make sure the first check returns correct result, if it does not return the correct result, then we may have misordering problem too.
cpu0 cpu1 clear MISSED _after_atomic() dequeue enqueuefirst trylock() #false MISSED check #*true* ?
As above, even cpu1 has a _after_atomic() between clearing STATE_MISSED and dequeuing, we might stiil need a barrier to prevent cpu0 doing speculative MISSED checking before cpu1 clearing MISSED?
And the implicit load-acquire barrier contained in the first trylock() does not seems to prevent the above case too.
And there is no load-acquire barrier in pfifo_fast_dequeue() too, which possibly make the above case more likely to happen.
Ah, you're right. The test_bit() was not in the patch context, I forgot it's there... Both barriers are indeed needed.
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Jakub Kicinski -
Yunsheng Lin -
Yunsheng Lin