linux-kernel - Re: Regression: sched/preempt: Add static_key() to preempt

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Date:	Thu, 25 Jun 2015 14:09:49 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Pontus Fuchs <pontus.fuchs@...il.com>
Cc:	mingo@...hat.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	gleb@...nel.org
Subject: Re: Regression:  sched/preempt: Add static_key() to
 preempt_notifiers breaks my KVM

On Thu, Jun 25, 2015 at 02:00:02PM +0200, Pontus Fuchs wrote:
> Hi,
> 
> On 4.1+ kernels I can no longer start my KVM guest. Upon trying to start it
> I can see the following log message:
> 
> [   25.821060] BUG: sleeping function called from invalid context at
> kernel/locking/mutex.c:97
> [   25.821063] in_atomic(): 1, irqs_disabled(): 0, pid: 2113, name:
> qemu-system-x86
> [   25.821066] CPU: 0 PID: 2113 Comm: qemu-system-x86 Not tainted 4.1.0+ #88
> [   25.821067] Hardware name: Dell Inc.          Dell System XPS 15Z/00WW5M,
> BIOS A12 09/07/2012
> [   25.821068]  0000000000000061 ffff88021339bcd8 ffffffff816b8c81
> 0000000000000007
> [   25.821070]  ffff880231159d40 ffff88021339bcf8 ffffffff8107d163
> ffff88021339bd18
> [   25.821072]  ffffffff81a451bc ffff88021339bd28 ffffffff8107d1ed
> ffff8802133a0000
> [   25.821073] Call Trace:
> [   25.821078]  [<ffffffff816b8c81>] dump_stack+0x4c/0x65
> [   25.821081]  [<ffffffff8107d163>] ___might_sleep+0xd3/0x110
> [   25.821083]  [<ffffffff8107d1ed>] __might_sleep+0x4d/0x90
> [   25.821085]  [<ffffffff816bde74>] mutex_lock+0x24/0x50
> [   25.821087]  [<ffffffff81141ef7>] static_key_slow_inc+0x57/0xc0
> [   25.821089]  [<ffffffff8107cafd>] preempt_notifier_register+0x1d/0x60
> [   25.821099]  [<ffffffffa04f11fd>] vcpu_load+0x3d/0x70 [kvm]
> [   25.821108]  [<ffffffffa050699e>] kvm_arch_vcpu_setup+0x1e/0x50 [kvm]
> [   25.821115]  [<ffffffffa05066e1>] ? kvm_arch_vcpu_create+0x51/0x70 [kvm]
> [   25.821120]  [<ffffffffa04f29b2>] kvm_vm_ioctl+0x1d2/0x7a0 [kvm]
> [   25.821123]  [<ffffffff811b7881>] do_vfs_ioctl+0x301/0x550
> [   25.821124]  [<ffffffff811b7b49>] SyS_ioctl+0x79/0x90
> [   25.821127]  [<ffffffff816c0257>] entry_SYSCALL_64_fastpath+0x12/0x6a
> 

That seems pointless..

---
 virt/kvm/kvm_main.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 90977418aeb6..d7aafa0458a0 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -128,8 +128,9 @@ int vcpu_load(struct kvm_vcpu *vcpu)
 
 	if (mutex_lock_killable(&vcpu->mutex))
 		return -EINTR;
-	cpu = get_cpu();
 	preempt_notifier_register(&vcpu->preempt_notifier);
+
+	cpu = get_cpu();
 	kvm_arch_vcpu_load(vcpu, cpu);
 	put_cpu();
 	return 0;
@@ -139,8 +140,8 @@ void vcpu_put(struct kvm_vcpu *vcpu)
 {
 	preempt_disable();
 	kvm_arch_vcpu_put(vcpu);
-	preempt_notifier_unregister(&vcpu->preempt_notifier);
 	preempt_enable();
+	preempt_notifier_unregister(&vcpu->preempt_notifier);
 	mutex_unlock(&vcpu->mutex);
 }
 
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