Re: [PATCH 01/10] vhost-user-blk: reconnect on any error during realize

[Roman Kagan]

On Fri, Nov 12, 2021 at 12:37:59PM +0100, Kevin Wolf wrote:
> Am 12.11.2021 um 08:39 hat Roman Kagan geschrieben:
> > On Thu, Nov 11, 2021 at 06:52:30PM +0100, Kevin Wolf wrote:
> > > Am 11.11.2021 um 16:33 hat Roman Kagan geschrieben:
> > > > vhost-user-blk realize only attempts to reconnect if the previous
> > > > connection attempt failed on "a problem with the connection and not an
> > > > error related to the content (which would fail again the same way in the
> > > > next attempt)".
> > > > 
> > > > However this distinction is very subtle, and may be inadvertently broken
> > > > if the code changes somewhere deep down the stack and a new error gets
> > > > propagated up to here.
> > > > 
> > > > OTOH now that the number of reconnection attempts is limited it seems
> > > > harmless to try reconnecting on any error.
> > > > 
> > > > So relax the condition of whether to retry connecting to check for any
> > > > error.
> > > > 
> > > > This patch amends a527e312b5 "vhost-user-blk: Implement reconnection
> > > > during realize".
> > > > 
> > > > Signed-off-by: Roman Kagan <rvkagan@yandex-team.ru>
> > > 
> > > It results in less than perfect error messages. With a modified export
> > > that just crashes qemu-storage-daemon during get_features, I get:
> > > 
> > > qemu-system-x86_64: -device vhost-user-blk-pci,chardev=c: Failed to read 
> > > msg header. Read 0 instead of 12. Original request 1.
> > > qemu-system-x86_64: -device vhost-user-blk-pci,chardev=c: Reconnecting 
> > > after error: vhost_backend_init failed: Protocol error
> > > qemu-system-x86_64: -device vhost-user-blk-pci,chardev=c: Reconnecting 
> > > after error: Failed to connect to '/tmp/vsock': Connection refused
> > > qemu-system-x86_64: -device vhost-user-blk-pci,chardev=c: Reconnecting 
> > > after error: Failed to connect to '/tmp/vsock': Connection refused
> > > qemu-system-x86_64: -device vhost-user-blk-pci,chardev=c: Failed to 
> > > connect to '/tmp/vsock': Connection refused
> > 
> > This patch doesn't change any error messages.  Which ones specifically
> > became less than perfect as a result of this patch?
> 
> But it adds error messages (for each retry), which are different from
> the first error message. As I said this is not the end of the world, but
> maybe a bit more confusing.

Ah, now I see what you mean: it adds reconnection attempts where there
used to be immediate failure return, so now every failed attempt logs
its own message.

> > > I guess this might be tolerable. On the other hand, the patch doesn't
> > > really fix anything either, but just gets rid of possible subtleties.
> > 
> > The remaining patches in the series make other errors beside -EPROTO
> > propagate up to this point, and some (most) of them are retryable.  This
> > was the reason to include this patch at the beginning of the series (I
> > guess I should've mentioned that in the patch log).
> 
> I see. I hadn't looked at the rest of the series yet because I ran out
> of time, but now that I'm skimming them, I see quite a few places that
> use non-EPROTO, but I wonder which of them actually should be
> reconnected. So far all I saw were presumably persistent errors where a
> retry won't help. Can you give me some examples?

E.g. the particular case you mention earlier, -ECONNREFUSED, is not
unlikely to happen due to the vhost-user server restart for maintenance;
in this case retying looks like a reasonable thing to do, doesn't it?

Thanks,
Roman.