From e0d50520ece8f8a0c6acc9fbdda16e9094896a83 Mon Sep 17 00:00:00 2001 From: Sean Christopherson 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 Cc: stable@vger.kernel.org Signed-off-by: Sean Christopherson Signed-off-by: Paolo Bonzini --- arch/x86/kernel/kvm.c | 41 +++++++++++++++++++++++++++-------------- 1 file changed, 27 insertions(+), 14 deletions(-) diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c index b48ce07756ca8..35b3c5836703b 100644 --- a/arch/x86/kernel/kvm.c +++ b/arch/x86/kernel/kvm.c @@ -191,7 +191,7 @@ 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(); @@ -203,28 +203,41 @@ again: 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) { + if (!dummy) { + raw_spin_unlock(&b->lock); + dummy = kzalloc(sizeof(*dummy), GFP_KERNEL); + /* - * Allocation failed! Busy wait while other cpu - * handles async PF. + * 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. */ - raw_spin_unlock(&b->lock); - cpu_relax(); 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); + dummy = NULL; } else { 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); -- 2.39.5