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

From: Jason Wang
Date: Wed May 31 2023 - 01:23:23 EST

在 2023/5/23 20:15, Oleg Nesterov 写道:
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) {
* 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);



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)

node = llist_reverse_order(node);
... process works ...

To me this looks a bit confusing. Shouldn't we do

if (!node) {

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 */
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.

So it seems that smp_wmb() can't help and should be removed, instead we need

llist_for_each_entry_safe(...) {
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.

The fn won't be changed after it is initialized.



Why do we set TASK_RUNNING inside the loop? Does this mean that work->fn()
can return with current->state != RUNNING ?

It is because the state were set to TASK_INTERRUPTIBLE in the beginning of the loop otherwise it might be side effect while executing work->fn().


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() ?

It looks safe since:

1) vhost hold refcnt of the mm
2) release will sync with the worker

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?