On 5/19/22 17:11, Sean Christopherson wrote:
AFAICT, kfree() is safe to call under a raw spinlock, so this? Compile tested
only...
Freeing outside the lock is not complicated enough to check if it is:
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 6aa1241a80b7..f849f7c9fbf2 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -229,12 +229,15 @@ void kvm_async_pf_task_wake(u32 token)
dummy->cpu = smp_processor_id();
init_swait_queue_head(&dummy->wq);
hlist_add_head(&dummy->link, &b->list);
+ dummy = NULL;
} else {
- kfree(dummy);
apf_task_wake_one(n);
}
raw_spin_unlock(&b->lock);
- return;
+
+ /* A dummy token might be allocated and ultimately not used. */
+ if (dummy)
+ kfree(dummy);
}
EXPORT_SYMBOL_GPL(kvm_async_pf_task_wake);
I queued your patch with the above fixup.
Paolo
--
From: Sean Christopherson <seanjc@xxxxxxxxxx>
Date: Thu, 19 May 2022 07:57:11 -0700
Subject: [PATCH] x86/kvm: Alloc dummy async #PF token outside of raw spinlock
Drop the raw spinlock in kvm_async_pf_task_wake() before allocating the
the dummy async #PF token, the allocator is preemptible on PREEMPT_RT
kernels and must not be called from truly atomic contexts.
Opportunistically document why it's ok to loop on allocation failure,
i.e. why the function won't get stuck in an infinite loop.
Reported-by: Yajun Deng <yajun.deng@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
arch/x86/kernel/kvm.c | 38 ++++++++++++++++++++++++--------------
1 file changed, 24 insertions(+), 14 deletions(-)
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index d0bb2b3fb305..5a4100896969 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -190,7 +190,7 @@ void kvm_async_pf_task_wake(u32 token)
{
u32 key = hash_32(token, KVM_TASK_SLEEP_HASHBITS);
struct kvm_task_sleep_head *b = &async_pf_sleepers[key];
- struct kvm_task_sleep_node *n;
+ struct kvm_task_sleep_node *n, *dummy = NULL;
if (token == ~0) {
apf_task_wake_all();
@@ -202,24 +202,34 @@ void kvm_async_pf_task_wake(u32 token)
n = _find_apf_task(b, token);
if (!n) {
/*
- * async PF was not yet handled.
- * Add dummy entry for the token.
+ * Async #PF not yet handled, add a dummy entry for the token.
+ * Allocating the token must be down outside of the raw lock
+ * as the allocator is preemptible on PREEMPT_RT kernels.
*/
- n = kzalloc(sizeof(*n), GFP_ATOMIC);
- if (!n) {
- /*
- * Allocation failed! Busy wait while other cpu
- * handles async PF.
- */
+ if (!dummy) {
raw_spin_unlock(&b->lock);
- cpu_relax();
+ dummy = kzalloc(sizeof(*dummy), GFP_KERNEL);
+
+ /*
+ * Continue looping on allocation failure, eventually
+ * the async #PF will be handled and allocating a new
+ * node will be unnecessary.
+ */
+ if (!dummy)
+ cpu_relax();
+
+ /*
+ * Recheck for async #PF completion before enqueueing
+ * the dummy token to avoid duplicate list entries.
+ */
goto again;
}
- n->token = token;
- n->cpu = smp_processor_id();
- init_swait_queue_head(&n->wq);
- hlist_add_head(&n->link, &b->list);
+ dummy->token = token;
+ dummy->cpu = smp_processor_id();
+ init_swait_queue_head(&dummy->wq);
+ hlist_add_head(&dummy->link, &b->list);
} else {
+ kfree(dummy);
apf_task_wake_one(n);
}
raw_spin_unlock(&b->lock);
base-commit: a3808d88461270c71d3fece5e51cc486ecdac7d0
--
