On Tue, Aug 26, 2025 at 07:03:33AM -0700, Sean Christopherson wrote:
> On Tue, Aug 26, 2025, Michael S. Tsirkin wrote:
> > On Mon, Aug 25, 2025 at 05:40:09PM -0700, Sean Christopherson wrote:
> > > Provide an API in vhost task instead of forcing KVM to solve the problem,
> > > as KVM would literally just add an equivalent to VHOST_TASK_FLAGS_KILLED,
> > > along with a new lock to protect said flag. In general, forcing simple
> > > usage of vhost task to care about signals _and_ take non-trivial action to
> > > do the right thing isn't developer friendly, and is likely to lead to
> > > similar bugs in the future.
> > >
> > > Debugged-by: Sebastian Andrzej Siewior <[email protected]>
> > > Link: https://lore.kernel.org/all/[email protected]
> > > Link: https://lore.kernel.org/all/[email protected]
> > > Suggested-by: Sebastian Andrzej Siewior <[email protected]>
> > > Fixes: d96c77bd4eeb ("KVM: x86: switch hugepage recovery thread to
> > > vhost_task")
> > > Cc: [email protected]
> > > Signed-off-by: Sean Christopherson <[email protected]>
> >
> > OK but I dislike the API.
>
> FWIW, I don't love it either.
>
> > Default APIs should be safe. So vhost_task_wake_safe should be
> > vhost_task_wake
> >
> > This also reduces the changes to kvm.
> >
> >
> > It does not look like we need the "unsafe" variant, so pls drop it.
>
> vhost_vq_work_queue() calls
>
> vhost_worker_queue()
> |
> -> worker->ops->wakeup(worker)
> |
> -> vhost_task_wakeup()
> |
> -> vhost_task_wake()
>
> while holding RCU and so can't sleep.
>
> rcu_read_lock();
> worker = rcu_dereference(vq->worker);
> if (worker) {
> queued = true;
> vhost_worker_queue(worker, work);
> }
> rcu_read_unlock();
>
> And the call from __vhost_worker_flush() is done while holding a
> vhost_worker.mutex.
> That's probably ok? But there are many paths that lead to
> __vhost_worker_flush(),
> which makes it difficult to audit all flows. So even if there is an easy
> change
> for the RCU conflict, I wouldn't be comfortable adding a mutex_lock() to so
> many
> flows in a patch that needs to go to stable@.
>
> > If we do need it, it should be called __vhost_task_wake.
>
> I initially had that, but didn't like that vhost_task_wake() wouldn't call
> __vhost_task_wake(), i.e. wouldn't follow the semi-standard pattern of the
> no-underscores function being a wrapper for the double-underscores function.
Eh. that's not really a standard. the standard is that __ is an unsafe
variant.
> I'm definitely not opposed to that though (or any other naming options). Sans
> comments, this was my other idea for names:
>
>
> static void ____vhost_task_wake(struct vhost_task *vtsk)
That's way too many __. Just vhost_task_wake_up_process will do.
> {
> wake_up_process(vtsk->task);
> }
Pls add docs explaining the usage of __vhost_task_wake
and vhost_task_wake respectively.
> void __vhost_task_wake(struct vhost_task *vtsk)
> {
> WARN_ON_ONCE(!vtsk->handle_sigkill);
>
> if (WARN_ON_ONCE(test_bit(VHOST_TASK_FLAGS_KILLED, &vtsk->flags)))
> return;
Add comments here please explaining why we warn.
> ____vhost_task_wake(vtsk);
> }
> EXPORT_SYMBOL_GPL(__vhost_task_wake);
> void vhost_task_wake(struct vhost_task *vtsk)
> {
> guard(mutex)(&vtsk->exit_mutex);
>
> if (WARN_ON_ONCE(test_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags)))
Add comments here please explaining why we warn.
> return;
>
> if (test_bit(VHOST_TASK_FLAGS_KILLED, &vtsk->flags))
> return;
>
> ____vhost_task_wake(vtsk);
> }
> EXPORT_SYMBOL_GPL(vhost_task_wake);
