linux-kernel - [PATCH] KVM: async_pf: Fix rcu_irq

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Message-Id: <1496837040-8200-1-git-send-email-wanpeng.li@hotmail.com>
Date:   Wed,  7 Jun 2017 05:04:00 -0700
From:   Wanpeng Li <kernellwp@...il.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: [PATCH] KVM: async_pf: Fix rcu_irq_enter/exit() usage

From: Wanpeng Li <wanpeng.li@...mail.com>

Commit 9b132fbe5419 (Add rcu user eqs exception hooks for async page fault) 
adds rcu_irq_enter/exit() to kvm_async_pf_task_wait() to exit cpu idle eqs 
when needed, to protect the code that needs use rcu. There is no need to call 
this pairs if async page fault is not triggered from idle task.

This patch fixes it by rcu irq exit if it is not triggered from idle task to 
avoid rcu hang after schedule() in the for loop.

Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Radim Krčmář <rkrcmar@...hat.com>
Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
---
 arch/x86/kernel/kvm.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 43e10d6..e70ed72 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -141,6 +141,8 @@ void kvm_async_pf_task_wait(u32 token)
 	n.token = token;
 	n.cpu = smp_processor_id();
 	n.halted = is_idle_task(current) || preempt_count() > 1;
+	if (!n.halted)
+		rcu_irq_exit();
 	init_swait_queue_head(&n.wq);
 	hlist_add_head(&n.link, &b->list);
 	raw_spin_unlock(&b->lock);
@@ -167,8 +169,9 @@ void kvm_async_pf_task_wait(u32 token)
 	}
 	if (!n.halted)
 		finish_swait(&n.wq, &wait);
+	else
+		rcu_irq_exit();
 
-	rcu_irq_exit();
 	return;
 }
 EXPORT_SYMBOL_GPL(kvm_async_pf_task_wait);
-- 
2.7.4