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Re: [Qemu-devel] [Qemu-block] Clean Block Driver Shutdown

From: Peter Lieven
Subject: Re: [Qemu-devel] [Qemu-block] Clean Block Driver Shutdown
Date: Tue, 7 Nov 2017 11:48:37 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0

Am 07.11.2017 um 11:22 schrieb Markus Armbruster:
Stefan Hajnoczi <address@hidden> writes:

On Tue, Oct 17, 2017 at 01:46:25PM +0200, Kevin Wolf wrote:
Am 17.10.2017 um 12:33 hat Peter Lieven geschrieben:
I noticed that Qemu quits at several points with an exit() if the
supplied parameters in the commandline are incorrect. This at some
stages happens after there have already been connections to storage
backends established.
Maybe we need to come to the conclusion that exit() is always wrong,
even during the initialisation.

These connections are not cleanly shut down in this case. For posix
file backends that doesn't matter, but for other backends this leads
to errors. E.g. iSCSI Targets log an aborted iSCSI connection due to
tcp reset.

I wonder what is the best way to fix this. A simply call to
bdrv_close_all() in an atexit handler seems to work.  But is this a
good solution? Maybe register this handler only until the VM starts.
Or do we need an atexit handler in each block driver that requires a
clean shutdown?
No, definitely not code in every single block driver. We need to make
sure to properly clean up what has been started.

An atexit handler is probably relatively easy. I think it would be
cleaner to have proper error paths even in main(), like in every other
function. I'm not sure if this would be reasonably easy to achieve,
I agree that converting from exit(3) to real error handling is cleanest.
Doing so would also be a good opportunity to consolidate ad-hoc
fprintf(stderr) and error_report() calls.
error_report() & exit() assume a certain context.  They're good enough
when the assumption obviously holds.

We also use them in code that can run when the assumption doesn't hold.
Sometimes because the code acquired new users, sometimes because the
code was always wrong.  Regardless, these are bugs in need of fixing.

We also use them in code that currently happens to run only when the
assumption holds.  Trap for the unwary, cleanup can make sense, but is
hardly a priority.

Of course, no priority, but how would you then handle block drivers
that need a clean shutdown? Register atexit handlers for them?


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