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@xxxxxxxxxxxxxxx> > > > > 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@xxxxxxxxxx> > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
