linux-kernel - Re: [PATCH 3/3] fork, vhost: Use CLONE_THREAD to fix freezer/ps regression

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Date:   Wed, 31 May 2023 11:14:33 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Mike Christie <michael.christie@...cle.com>, linux@...mhuis.info,
        nicolas.dichtel@...nd.com, axboe@...nel.dk, ebiederm@...ssion.com,
        torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, mst@...hat.com,
        sgarzare@...hat.com, stefanha@...hat.com, brauner@...nel.org
Subject: Re: [PATCH 3/3] fork, vhost: Use CLONE_THREAD to fix freezer/ps
 regression

On 05/31, Jason Wang wrote:
>
> On Wed, May 31, 2023 at 3:25 PM Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > On 05/31, Jason Wang wrote:
> > >
> > > 在 2023/5/23 20:15, Oleg Nesterov 写道:
> > > >
> > > >             /* make sure flag is seen after deletion */
> > > >             smp_wmb();
> > > >             llist_for_each_entry_safe(work, work_next, node, node) {
> > > >                     clear_bit(VHOST_WORK_QUEUED, &work->flags);
> > > >
> > > >I am not sure about smp_wmb + clear_bit. Once we clear VHOST_WORK_QUEUED,
> > > >vhost_work_queue() can add this work again and change work->node->next.
> > > >
> > > >That is why we use _safe, but we need to ensure that llist_for_each_safe()
> > > >completes LOAD(work->node->next) before VHOST_WORK_QUEUED is cleared.
> > >
> > > This should be fine since store is not speculated, so work->node->next needs
> > > to be loaded before VHOST_WORK_QUEUED is cleared to meet the loop condition.
> >
> > I don't understand you. OK, to simplify, suppose we have 2 global vars
> >
> >         void *PTR = something_non_null;
> >         unsigned long FLAGS = -1ul;
> >
> > Now I think this code
> >
> >         CPU_0                           CPU_1
> >
> >         void *ptr = PTR;                if (!test_and_set_bit(0, FLAGS))
> >         clear_bit(0, FLAGS);                    PTR = NULL;
> >         BUG_ON(!ptr);
> >
> > is racy and can hit the BUG_ON(!ptr).
>
> This seems different to the above case?

not sure,

> And you can hit BUG_ON with
> the following execution sequence:
>
> [cpu 0] clear_bit(0, FLAGS);
> [cpu 1] if (!test_and_set_bit(0, FLAGS))
> [cpu 1] PTR = NULL;
> [cpu 0] BUG_ON(!ptr)

I don't understand this part... yes, we can hit this BUG_ON() without mb in
between, this is what I tried to say.

> In vhost code, there's a condition before the clear_bit() which sits
> inside llist_for_each_entry_safe():
>
> #define llist_for_each_entry_safe(pos, n, node, member)                        \
>         for (pos = llist_entry((node), typeof(*pos), member);                  \
>              member_address_is_nonnull(pos, member) &&                         \
>                 (n = llist_entry(pos->member.next, typeof(*n), member), true); \
>              pos = n)
>
> The clear_bit() is a store which is not speculated, so there's a
> control dependency, the store can't be executed until the condition
> expression is evaluated which requires pos->member.next
> (work->node.next) to be loaded.

But llist_for_each_entry_safe() doesn't check "n", I mean, it is not that we have
something like

	n = llist_entry(...);
	if (n)
		clear_bit(...);

so I do not see how can we rely on the load-store control dependency.

Oleg.