Re: [Qemu-devel] [PATCH] block: Fix race in gluster_finish_aiocb

[Bharata B Rao <bharata@linux.vnet.ibm.com>]

On Thu, Aug 22, 2013 at 01:15:59PM +0200, Paolo Bonzini wrote:
> Il 22/08/2013 12:28, Bharata B Rao ha scritto:
> > On Thu, Aug 22, 2013 at 12:00:48PM +0200, Paolo Bonzini wrote:
> >> Il 22/08/2013 11:55, Bharata B Rao ha scritto:
> >>> This was the first apporach I had. I used to abort when writes to pipe
> >>> fail. But there were concerns raised about handling the failures gracefully
> >>> and hence we ended up doing all that error handling of completing the aio
> >>> with -EIO, closing the pipe and making the disk inaccessible.
> >>>
> >>>>> Under what circumstances could it happen?
> >>> Not very sure, I haven't seen that happening. I had to manually inject
> >>> faults to test this error path and verify the graceful recovery.
> >>
> >> Looking at write(2), it looks like it is impossible
> >>
> >>        EAGAIN or EWOULDBLOCK
> >>                can't happen, blocking file descriptor
> >>
> >>        EBADF, EPIPE
> >>                shouldn't happen since the device is drained before
> >>                calling qemu_gluster_close.
> >>
> >>        EDESTADDRREQ, EDQUOT, EFBIG, EIO, ENOSPC
> >>                cannot happen for pipes
> >>
> >>        EFAULT
> >>                abort would be fine
> > 
> > In the case where we have separate system and data disks and if error (EFAULT)
> > happens for the data disk, don't we want to keep the VM up by gracefully
> > disabling IO to the data disk ?
> 
> EFAULT means the buffer address is invalid, I/O error would be EIO, but...
> 
> > I remember this was one of the motivations to
> > handle this failure.
> 
> ... this write is on the pipe, not on a disk.

Right. Failure to complete the write on the pipe means that IO done to the
disk didn't complete and hence to the VM it is essentially a disk IO failure.
That's the reason we return -EIO and make the disk inaccessible when this
failure happens.

My question was if it is ok to abort the VM when IO to one of the disks fails ?

But, if you think it is not worth handling such errors then may be we can drop
this elaborate and race-prone error recovery and just abort.

Regards,
Bharata.