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.