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address@hidden: [Qemu-devel] QEMU/KVM SCSI lock up]
From: |
Marcelo Tosatti |
Subject: |
address@hidden: [Qemu-devel] QEMU/KVM SCSI lock up] |
Date: |
Thu, 28 Aug 2008 10:59:45 -0300 |
User-agent: |
Mutt/1.5.18 (2008-05-17) |
Paul,
Please apply.
----- Forwarded message from Matteo Frigo <address@hidden> -----
From: Matteo Frigo <address@hidden>
Date: Wed, 02 Apr 2008 20:41:24 -0400
To: address@hidden
Subject: [Qemu-devel] QEMU/KVM SCSI lock up
X-detected-kernel: by monty-python.gnu.org: Linux 2.6, seldom 2.4 (older, 4)
X-Mailman-Approved-At: Thu, 03 Apr 2008 00:59:59 -0400
Reply-To: address@hidden
kvm-64 hangs under heavy disk I/O with scsi disks. To reproduce,
create a fresh qcow2 disk, boot linux, and execute
dd if=/dev/sdX of=/dev/null bs=1M
on the fresh disk. See also
https://sourceforge.net/tracker/?func=detail&atid=893831&aid=1895893&group_id=180599
I have attached a patch that appears to fix the problem. The bug
seems to be the following. scsi_read_data() does the following
bdrv_aio_read()
r->sector += n;
r->sector_count -= n;
For reasons that I do not fully understand, bdrv_aio_read() does
not return immediately, but instead it calls scsi_read_data()
recursively. Since ``r->sector += n;'' has not been executed
yet, the re-entrant call triggers a read of the same sector, which
breaks the producer-consumer lockstep. The fix is to swap the operations
as follows:
r->sector += n;
r->sector_count -= n;
bdrv_aio_read()
A similar fix applies to scsi_write_data().
Thanks for developing kvm, it is truly an amazing piece of software.
Regards,
Matteo Frigo
diff -aur kvm-64.old/qemu/hw/scsi-disk.c kvm-64.new/qemu/hw/scsi-disk.c
--- kvm-64.old/qemu/hw/scsi-disk.c 2008-03-26 08:49:35.000000000 -0400
+++ kvm-64.new/qemu/hw/scsi-disk.c 2008-03-30 08:37:25.000000000 -0400
@@ -196,12 +196,12 @@
n = SCSI_DMA_BUF_SIZE / 512;
r->buf_len = n * 512;
- r->aiocb = bdrv_aio_read(s->bdrv, r->sector, r->dma_buf, n,
+ r->sector += n;
+ r->sector_count -= n;
+ r->aiocb = bdrv_aio_read(s->bdrv, r->sector - n, r->dma_buf, n,
scsi_read_complete, r);
if (r->aiocb == NULL)
scsi_command_complete(r, SENSE_HARDWARE_ERROR);
- r->sector += n;
- r->sector_count -= n;
}
static void scsi_write_complete(void * opaque, int ret)
@@ -248,12 +248,12 @@
BADF("Data transfer already in progress\n");
n = r->buf_len / 512;
if (n) {
- r->aiocb = bdrv_aio_write(s->bdrv, r->sector, r->dma_buf, n,
+ r->sector += n;
+ r->sector_count -= n;
+ r->aiocb = bdrv_aio_write(s->bdrv, r->sector - n, r->dma_buf, n,
scsi_write_complete, r);
if (r->aiocb == NULL)
scsi_command_complete(r, SENSE_HARDWARE_ERROR);
- r->sector += n;
- r->sector_count -= n;
} else {
/* Invoke completion routine to fetch data from host. */
scsi_write_complete(r, 0);
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