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SubjectRe: [PATCH] virtio-scsi: Fix incorrect lock release order in virtscsi_kick_cmd
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);
>



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