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@xxxxxxxxxx>
> >
> > Acked-by: Jakub Kicinski <kuba@xxxxxxxxxx>
>
> 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.
