On Fri, 18 Jun 2021 17:30:47 -0700 Jakub Kicinski wrote:

On Thu, 17 Jun 2021 09:04:14 +0800 Yunsheng Lin wrote:

quoted

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 <redacted>  

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.