[PATCH] virtio-scsi: Fix incorrect lock release order in virtscsi_kick_cmd

[PATCH] virtio-scsi: Fix incorrect lock release order in virtscsi_kick_cmd

[Nicholas A. Bellinger]

From: Nicholas Bellinger <nab@linux-iscsi.org>

This patch fixes a regression bug in virtscsi_kick_cmd() that relinquishes
the acquired spinlocks in the incorrect order using the wrong spin_unlock
macros, namely releasing vq->vq_lock before tgt->tgt_lock while invoking
the calls to virtio_ring.c:virtqueue_add_buf() and friends.

This bug was originally introduced in v3.5-rc7 code with:

commit 2bd37f0fde99cbf8b78fb55f1128e8c3a63cf1da
Author: Paolo Bonzini <pbonzini@redhat.com>
Date:   Wed Jun 13 16:56:34 2012 +0200

    [SCSI] virtio-scsi: split scatterlist per target

Go ahead and make sure that vq->vq_lock is relinquished w/ spin_unlock
first, then release tgt->tgt_lock w/ spin_unlock_irqrestore.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: James Bottomley <JBottomley@Parallels.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: stable@vger.kernel.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
---
 drivers/scsi/virtio_scsi.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
index 595af1a..b2abb8a 100644
--- a/drivers/scsi/virtio_scsi.c
+++ b/drivers/scsi/virtio_scsi.c
@@ -417,11 +417,11 @@ static int virtscsi_kick_cmd(struct virtio_scsi_target_state *tgt,
 
 	spin_lock(&vq->vq_lock);
 	ret = virtqueue_add_buf(vq->vq, tgt->sg, out_num, in_num, cmd, gfp);
-	spin_unlock(&tgt->tgt_lock);
+	spin_unlock(&vq->vq_lock);
 	if (ret >= 0)
 		ret = virtqueue_kick_prepare(vq->vq);
 
-	spin_unlock_irqrestore(&vq->vq_lock, flags);
+	spin_unlock_irqrestore(&tgt->tgt_lock, flags);
 
 	if (ret > 0)
 		virtqueue_notify(vq->vq);
-- 
1.7.2.5

Re: [PATCH] virtio-scsi: Fix incorrect lock release order in virtscsi_kick_cmd

[Wanlong Gao]

On 11/09/2012 02:29 PM, Nicholas A. Bellinger wrote:
> From: Nicholas Bellinger <nab@linux-iscsi.org>
> 
> This patch fixes a regression bug in virtscsi_kick_cmd() that relinquishes
> the acquired spinlocks in the incorrect order using the wrong spin_unlock
> macros, namely releasing vq->vq_lock before tgt->tgt_lock while invoking
> the calls to virtio_ring.c:virtqueue_add_buf() and friends.
> 
> This bug was originally introduced in v3.5-rc7 code with:
> 
> commit 2bd37f0fde99cbf8b78fb55f1128e8c3a63cf1da
> Author: Paolo Bonzini <pbonzini@redhat.com>
> Date:   Wed Jun 13 16:56:34 2012 +0200
> 
>     [SCSI] virtio-scsi: split scatterlist per target
> 
> Go ahead and make sure that vq->vq_lock is relinquished w/ spin_unlock
> first, then release tgt->tgt_lock w/ spin_unlock_irqrestore.

Did you hit any error? I don't think this order is wrong.

Thanks,
Wanlong Gao

> 
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: James Bottomley <JBottomley@Parallels.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: stable@vger.kernel.org
> Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
> ---
>  drivers/scsi/virtio_scsi.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
> index 595af1a..b2abb8a 100644
> --- a/drivers/scsi/virtio_scsi.c
> +++ b/drivers/scsi/virtio_scsi.c
> @@ -417,11 +417,11 @@ static int virtscsi_kick_cmd(struct virtio_scsi_target_state *tgt,
>  
>  	spin_lock(&vq->vq_lock);
>  	ret = virtqueue_add_buf(vq->vq, tgt->sg, out_num, in_num, cmd, gfp);
> -	spin_unlock(&tgt->tgt_lock);
> +	spin_unlock(&vq->vq_lock);
>  	if (ret >= 0)
>  		ret = virtqueue_kick_prepare(vq->vq);
>  
> -	spin_unlock_irqrestore(&vq->vq_lock, flags);
> +	spin_unlock_irqrestore(&tgt->tgt_lock, flags);
>  
>  	if (ret > 0)
>  		virtqueue_notify(vq->vq);
> 

Re: [PATCH] virtio-scsi: Fix incorrect lock release order in virtscsi_kick_cmd

[Paolo Bonzini]

Il 09/11/2012 07:29, Nicholas A. Bellinger ha scritto:
> From: Nicholas Bellinger <nab@linux-iscsi.org>
> 
> This patch fixes a regression bug in virtscsi_kick_cmd() that relinquishes
> the acquired spinlocks in the incorrect order using the wrong spin_unlock
> macros, namely releasing vq->vq_lock before tgt->tgt_lock while invoking
> the calls to virtio_ring.c:virtqueue_add_buf() and friends.
> 
> This bug was originally introduced in v3.5-rc7 code with:
> 
> commit 2bd37f0fde99cbf8b78fb55f1128e8c3a63cf1da
> Author: Paolo Bonzini <pbonzini@redhat.com>
> Date:   Wed Jun 13 16:56:34 2012 +0200
> 
>     [SCSI] virtio-scsi: split scatterlist per target
> 
> Go ahead and make sure that vq->vq_lock is relinquished w/ spin_unlock
> first, then release tgt->tgt_lock w/ spin_unlock_irqrestore.

That's done on purpose.  After you do virtqueue_add_buf, you don't need
the sg list anymore, nor the lock that protects it.  The cover letter is
at https://lkml.org/lkml/2012/6/13/295 and had this text:

  This series reorganizes the locking in virtio-scsi, introducing
  separate scatterlists for each target and "pipelining" the locks so
  that one command can be queued while the other is prepared.  This
  improves performance when there are multiple in-flight operations.

In fact, the patch _introduces_ wrong locking because
virtqueue_kick_prepare needs the vq_lock.

Perhaps what you want is separate local_irq_save/local_irq_restore?

Paolo

> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: James Bottomley <JBottomley@Parallels.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: stable@vger.kernel.org
> Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
> ---
>  drivers/scsi/virtio_scsi.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
> index 595af1a..b2abb8a 100644
> --- a/drivers/scsi/virtio_scsi.c
> +++ b/drivers/scsi/virtio_scsi.c
> @@ -417,11 +417,11 @@ static int virtscsi_kick_cmd(struct virtio_scsi_target_state *tgt,
>  
>  	spin_lock(&vq->vq_lock);
>  	ret = virtqueue_add_buf(vq->vq, tgt->sg, out_num, in_num, cmd, gfp);
> -	spin_unlock(&tgt->tgt_lock);
> +	spin_unlock(&vq->vq_lock);
>  	if (ret >= 0)
>  		ret = virtqueue_kick_prepare(vq->vq);
>  
> -	spin_unlock_irqrestore(&vq->vq_lock, flags);
> +	spin_unlock_irqrestore(&tgt->tgt_lock, flags);
>  
>  	if (ret > 0)
>  		virtqueue_notify(vq->vq);
> 

Re: [PATCH] virtio-scsi: Fix incorrect lock release order in virtscsi_kick_cmd

[Nicholas A. Bellinger]

Hi Paolo,

On Fri, 2012-11-09 at 09:42 +0100, Paolo Bonzini wrote:
> Il 09/11/2012 07:29, Nicholas A. Bellinger ha scritto:
> > From: Nicholas Bellinger <nab@linux-iscsi.org>
> > 
> > This patch fixes a regression bug in virtscsi_kick_cmd() that relinquishes
> > the acquired spinlocks in the incorrect order using the wrong spin_unlock
> > macros, namely releasing vq->vq_lock before tgt->tgt_lock while invoking
> > the calls to virtio_ring.c:virtqueue_add_buf() and friends.
> > 
> > This bug was originally introduced in v3.5-rc7 code with:
> > 
> > commit 2bd37f0fde99cbf8b78fb55f1128e8c3a63cf1da
> > Author: Paolo Bonzini <pbonzini@redhat.com>
> > Date:   Wed Jun 13 16:56:34 2012 +0200
> > 
> >     [SCSI] virtio-scsi: split scatterlist per target
> > 
> > Go ahead and make sure that vq->vq_lock is relinquished w/ spin_unlock
> > first, then release tgt->tgt_lock w/ spin_unlock_irqrestore.
> 
> That's done on purpose.  After you do virtqueue_add_buf, you don't need
> the sg list anymore, nor the lock that protects it.  The cover letter is
> at https://lkml.org/lkml/2012/6/13/295 and had this text:
> 
>   This series reorganizes the locking in virtio-scsi, introducing
>   separate scatterlists for each target and "pipelining" the locks so
>   that one command can be queued while the other is prepared.  This
>   improves performance when there are multiple in-flight operations.
> 
> In fact, the patch _introduces_ wrong locking because
> virtqueue_kick_prepare needs the vq_lock.
> 
> Perhaps what you want is separate local_irq_save/local_irq_restore?
> 

Ahh, that makes more sense now.

Just noticed this while reviewing code that using one spinlock flag's to
release the other looks suspicious, minus the ordering bit..

Using local_irq_* would probably be cleaner than swapping flags between
different locks, and a short comment here would be helpful to explain
the locking order context.

Anyways, no big deal.  Thanks for the explanation.

--nab

Re: [PATCH] virtio-scsi: Fix incorrect lock release order in virtscsi_kick_cmd

[Paolo Bonzini]

Il 09/11/2012 20:31, Nicholas A. Bellinger ha scritto:
>> That's done on purpose.  After you do virtqueue_add_buf, you don't need
>> the sg list anymore, nor the lock that protects it.  The cover letter is
>> at https://lkml.org/lkml/2012/6/13/295 and had this text:
>>
>>   This series reorganizes the locking in virtio-scsi, introducing
>>   separate scatterlists for each target and "pipelining" the locks so
>>   that one command can be queued while the other is prepared.  This
>>   improves performance when there are multiple in-flight operations.
>>
>> In fact, the patch _introduces_ wrong locking because
>> virtqueue_kick_prepare needs the vq_lock.
>>
>> Perhaps what you want is separate local_irq_save/local_irq_restore?
> 
> Ahh, that makes more sense now.
> 
> Just noticed this while reviewing code that using one spinlock flag's to
> release the other looks suspicious, minus the ordering bit..
> 
> Using local_irq_* would probably be cleaner than swapping flags between
> different locks, and a short comment here would be helpful to explain
> the locking order context.

Well, my plan is to improve the virtio API so I can reuse the higher
layer's scatterlist, and get rid of the lock (not just of the funny
order) altogether. :)  Queuing requests is really performance-sensitive,
and it can use any optimization.

But if I can't get to it quick, I'll queue a cleanup using local_irq_*.

Paolo