On 1/27/25 19:22, Keith Busch wrote:
It's not clear to me that calling vhost_task_wake() before vhost_task_start() is
allowed, which is why I deliberately waited until the task was started to make it
visible. Though FWIW, doing "vhost_task_wake(nx_thread)" before vhost_task_start()
doesn't explode.
Hm, it does look questionable to try to wake a process that hadn't been
started yet, but I think it may be okay: task state will be TASK_NEW
before vhost_task_start(), which looks like will cause wake_up_process()
to do nothing.
Yes, it's okay because both wake_up_new_task() and try_to_wake_up() take
p->pi_lock. try_to_wake_up() does not match either bit in TASK_NORMAL
(which is TASK_INTERRUPTIBLE | TASK_UNINTERRUPTIBLE) and does nothing.
I'm queuing the patch with the store before vhost_task_start, and
acquire/release instead of just READ_ONCE/WRITE_ONCE.
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 74c20dbb92da..6d5708146384 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -7127,7 +7127,8 @@ static void kvm_wake_nx_recovery_thread(struct kvm *kvm)
* may not be valid even though the VM is globally visible. Do nothing,
* as such a VM can't have any possible NX huge pages.
*/
- struct vhost_task *nx_thread = READ_ONCE(kvm->arch.nx_huge_page_recovery_thread);
+ struct vhost_task *nx_thread =
+ smp_load_acquire(&kvm->arch.nx_huge_page_recovery_thread);
if (nx_thread)
vhost_task_wake(nx_thread);
@@ -7474,10 +7475,10 @@ static void kvm_mmu_start_lpage_recovery(struct once *once)
if (!nx_thread)
return;
- vhost_task_start(nx_thread);
+ /* Make the task visible only once it is fully created. */
+ smp_store_release(&kvm->arch.nx_huge_page_recovery_thread, nx_thread);
- /* Make the task visible only once it is fully started. */
- WRITE_ONCE(kvm->arch.nx_huge_page_recovery_thread, nx_thread);
+ vhost_task_start(nx_thread);
}
int kvm_mmu_post_init_vm(struct kvm *kvm)
