From: Oleg Nesterov <oleg@redhat.com>
To: Mike Christie <michael.christie@oracle.com>
Cc: linux@leemhuis.info, nicolas.dichtel@6wind.com, axboe@kernel.dk,
ebiederm@xmission.com, torvalds@linux-foundation.org,
linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org, mst@redhat.com,
sgarzare@redhat.com, jasowang@redhat.com, stefanha@redhat.com,
brauner@kernel.org
Subject: Re: [PATCH 3/3] fork, vhost: Use CLONE_THREAD to fix freezer/ps regression
Date: Tue, 23 May 2023 14:15:06 +0200 [thread overview]
Message-ID: <20230523121506.GA6562@redhat.com> (raw)
In-Reply-To: <20230522174757.GC22159@redhat.com>
On 05/22, Oleg Nesterov wrote:
>
> Right now I think that "int dead" should die,
No, probably we shouldn't call get_signal() if we have already dequeued SIGKILL.
> but let me think tomorrow.
May be something like this... I don't like it but I can't suggest anything better
right now.
bool killed = false;
for (;;) {
...
node = llist_del_all(&worker->work_list);
if (!node) {
schedule();
/*
* When we get a SIGKILL our release function will
* be called. That will stop new IOs from being queued
* and check for outstanding cmd responses. It will then
* call vhost_task_stop to tell us to return and exit.
*/
if (signal_pending(current)) {
struct ksignal ksig;
if (!killed)
killed = get_signal(&ksig);
clear_thread_flag(TIF_SIGPENDING);
}
continue;
}
-------------------------------------------------------------------------------
But let me ask a couple of questions. Let's forget this patch, let's look at the
current code:
node = llist_del_all(&worker->work_list);
if (!node)
schedule();
node = llist_reverse_order(node);
... process works ...
To me this looks a bit confusing. Shouldn't we do
if (!node) {
schedule();
continue;
}
just to make the code a bit more clear? If node == NULL then
llist_reverse_order() and llist_for_each_entry_safe() will do nothing.
But this is minor.
/* 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.
So it seems that smp_wmb() can't help and should be removed, instead we need
llist_for_each_entry_safe(...) {
smp_mb__before_atomic();
clear_bit(VHOST_WORK_QUEUED, &work->flags);
Also, if the work->fn pointer is not stable, we should read it before
smp_mb__before_atomic() as well.
No?
__set_current_state(TASK_RUNNING);
Why do we set TASK_RUNNING inside the loop? Does this mean that work->fn()
can return with current->state != RUNNING ?
work->fn(work);
Now the main question. Whatever we do, SIGKILL/SIGSTOP/etc can come right
before we call work->fn(). Is it "safe" to run this callback with
signal_pending() or fatal_signal_pending() ?
Finally. I never looked into drivers/vhost/ before so I don't understand
this code at all, but let me ask anyway... Can we change vhost_dev_flush()
to run the pending callbacks rather than wait for vhost_worker() ?
I guess we can't, ->mm won't be correct, but can you confirm?
Oleg.
next prev parent reply other threads:[~2023-05-23 12:16 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-22 2:51 [PATCH 0/3] vhost: Fix freezer/ps regressions Mike Christie
2023-05-22 2:51 ` [PATCH 1/3] signal: Don't always put SIGKILL in shared_pending Mike Christie
2023-05-23 15:30 ` Eric W. Biederman
2023-05-22 2:51 ` [PATCH 2/3] signal: Don't exit for PF_USER_WORKER tasks Mike Christie
2023-05-22 2:51 ` [PATCH 3/3] fork, vhost: Use CLONE_THREAD to fix freezer/ps regression Mike Christie
2023-05-22 12:30 ` Oleg Nesterov
2023-05-22 17:00 ` Mike Christie
2023-05-22 17:47 ` Oleg Nesterov
2023-05-23 12:15 ` Oleg Nesterov [this message]
2023-05-23 15:57 ` Eric W. Biederman
2023-05-24 14:10 ` Oleg Nesterov
2023-05-24 14:44 ` Eric W. Biederman
2023-05-25 11:55 ` Oleg Nesterov
2023-05-25 15:30 ` Eric W. Biederman
2023-05-25 16:20 ` Linus Torvalds
2023-05-27 9:49 ` Eric W. Biederman
2023-05-27 16:12 ` Linus Torvalds
2023-05-28 1:17 ` Eric W. Biederman
2023-05-28 1:21 ` Linus Torvalds
2023-05-29 11:19 ` Oleg Nesterov
2023-05-29 16:09 ` michael.christie
2023-05-29 17:46 ` Oleg Nesterov
2023-05-29 17:54 ` Oleg Nesterov
2023-05-29 19:03 ` Mike Christie
2023-05-29 19:35 ` Mike Christie
2023-05-29 19:46 ` michael.christie
2023-05-30 2:48 ` Eric W. Biederman
2023-05-30 2:38 ` Eric W. Biederman
2023-05-30 15:34 ` Mike Christie
2023-05-31 3:30 ` Mike Christie
2023-05-29 16:11 ` michael.christie
2023-05-30 14:15 ` Christian Brauner
2023-05-30 17:55 ` Oleg Nesterov
2023-05-30 15:01 ` Eric W. Biederman
2023-05-31 5:22 ` Jason Wang
2023-05-24 0:02 ` Mike Christie
2023-05-25 16:15 ` Mike Christie
2023-05-28 1:41 ` Eric W. Biederman
2023-05-28 19:29 ` Mike Christie
2023-05-31 5:22 ` Jason Wang
2023-05-31 7:25 ` Oleg Nesterov
2023-05-31 8:17 ` Jason Wang
2023-05-31 9:14 ` Oleg Nesterov
2023-06-01 2:44 ` Jason Wang
2023-06-01 7:43 ` Oleg Nesterov
2023-06-02 5:03 ` Jason Wang
2023-06-02 17:58 ` Oleg Nesterov
2023-06-02 20:07 ` Linus Torvalds
2023-06-05 14:20 ` Oleg Nesterov
2023-05-22 19:40 ` Michael S. Tsirkin
2023-05-23 15:39 ` Eric W. Biederman
2023-05-23 15:48 ` Mike Christie
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