Revert "KVM: async_pf: avoid recursive flushing of work items"

Now that KVM does NOT gift async #PF workers a "struct kvm" reference,
don't bother skipping "done" workers when flushing/canceling queued
workers, as the deadlock that was being fudged around can no longer occur.
When workers, i.e. async_pf_execute(), were gifted a referenced, it was
possible for a worker to put the last reference and trigger VM destruction,
i.e. trigger flushing of a workqueue from a worker in said workqueue.

Note, there is no actual lock, the deadlock was that a worker will be
stuck waiting for itself (the workqueue code simulates a lock/unlock via
lock_map_{acquire,release}()).

Skipping "done" workers isn't problematic per se, but using work->vcpu as
a "done" flag is confusing, e.g. it's not clear that async_pf.lock is
acquired to protect the work->vcpu, NOT the processing of async_pf.queue
(which is protected by vcpu->mutex).

This reverts commit 22583f0d.
Suggested-by: Xu Yilun <yilun.xu@linux.intel.com>
Link: https://lore.kernel.org/r/20240423191649.2885257-1-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

virt/kvm/async_pf.c

@@ -80,7 +80,6 @@ static void async_pf_execute(struct work_struct *work)
 	spin_lock(&vcpu->async_pf.lock);
 	first = list_empty(&vcpu->async_pf.done);
 	list_add_tail(&apf->link, &vcpu->async_pf.done);
-	apf->vcpu = NULL;
 	spin_unlock(&vcpu->async_pf.lock);
 
 	/*
@@ -120,8 +119,6 @@ static void kvm_flush_and_free_async_pf_work(struct kvm_async_pf *work)
 
 void kvm_clear_async_pf_completion_queue(struct kvm_vcpu *vcpu)
 {
-	spin_lock(&vcpu->async_pf.lock);
-
 	/* cancel outstanding work queue item */
 	while (!list_empty(&vcpu->async_pf.queue)) {
 		struct kvm_async_pf *work =
@@ -129,23 +126,15 @@ void kvm_clear_async_pf_completion_queue(struct kvm_vcpu *vcpu)
 					 typeof(*work), queue);
 		list_del(&work->queue);
 
-		/*
-		 * We know it's present in vcpu->async_pf.done, do
-		 * nothing here.
-		 */
-		if (!work->vcpu)
-			continue;
-
-		spin_unlock(&vcpu->async_pf.lock);
 #ifdef CONFIG_KVM_ASYNC_PF_SYNC
 		flush_work(&work->work);
 #else
 		if (cancel_work_sync(&work->work))
 			kmem_cache_free(async_pf_cache, work);
 #endif
-		spin_lock(&vcpu->async_pf.lock);
 	}
 
+	spin_lock(&vcpu->async_pf.lock);
 	while (!list_empty(&vcpu->async_pf.done)) {
 		struct kvm_async_pf *work =
 			list_first_entry(&vcpu->async_pf.done,