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Re: [PATCH v4 1/4] qapi: Add a 'coroutine' flag for commands


From: Markus Armbruster
Subject: Re: [PATCH v4 1/4] qapi: Add a 'coroutine' flag for commands
Date: Fri, 06 Mar 2020 13:38:18 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux)

Kevin Wolf <address@hidden> writes:

> Am 06.03.2020 um 08:25 hat Markus Armbruster geschrieben:
>> Kevin Wolf <address@hidden> writes:
>> 
>> > Am 05.03.2020 um 16:30 hat Markus Armbruster geschrieben:
>> >> Kevin Wolf <address@hidden> writes:
>> >> 
>> >> > Am 22.01.2020 um 07:32 hat Markus Armbruster geschrieben:
>> >> >> Kevin Wolf <address@hidden> writes:
>> >> >> 
>> >> >> > This patch adds a new 'coroutine' flag to QMP command definitions 
>> >> >> > that
>> >> >> > tells the QMP dispatcher that the command handler is safe to be run 
>> >> >> > in a
>> >> >> > coroutine.
>> >> >> 
>> >> >> I'm afraid I missed this question in my review of v3: when is a handler
>> >> >> *not* safe to be run in a coroutine?
>> >> >
>> >> > That's a hard one to answer fully.
>> >> >
>> >> > Basically, I think the biggest problem is with calling functions that
>> >> > change their behaviour if run in a coroutine compared to running them
>> >> > outside of coroutine context. In most cases the differences like having
>> >> > a nested event loop instead of yielding are just fine, but they are
>> >> > still subtly different.
>> >> >
>> >> > I know this is vague, but I can assure you that problematic cases exist.
>> >> > I hit one of them with my initial hack that just moved everything into a
>> >> > coroutine. It was related to graph modifications and bdrv_drain and
>> >> > resulted in a hang. For the specifics, I would have to try and reproduce
>> >> > the problem again.
>> >> 
>> >> I'm afraid it's even more complicated.
>> >> 
>> >> Monitors (HMP and QMP) run in the main loop.  Before this patch, HMP and
>> >> QMP commands run start to finish, one after the other.
>> >> 
>> >> After this patch, QMP commands may elect to yield.  QMP commands still
>> >> run one after the other (the shared dispatcher ensures this even when we
>> >> have multiple QMP monitors).
>> >> 
>> >> However, *HMP* commands can now run interleaved with a coroutine-enabled
>> >> QMP command, i.e. between a yield and its re-enter.
>> >
>> > I guess that's right. :-(
>> >
>> >> Consider an HMP screendump running in the middle of a coroutine-enabled
>> >> QMP screendump, using Marc-André's patch.  As far as I can tell, it
>> >> works, because:
>> >> 
>> >> 1. HMP does not run in a coroutine.  If it did, and both dumps dumped
>> >> the same @con, then it would overwrite con->screndump_co.  If we ever
>> >> decide to make HMP coroutine-capable so it doesn't block the main loop,
>> >> this will become unsafe in a nasty way.
>> >
>> > At the same time, switching HMP to coroutines would give us an easy way
>> > to fix the problem: Just use a CoMutex so that HMP and QMP commands
>> > never run in parallel. Unless we actually _want_ to run both at the same
>> > time for some commands, but I don't see why.
>> 
>> As long as running QMP commands from *all* monitors one after the other
>> is good enough, I can't see why running HMP commands interleaved is
>> worth the risk.
>
> There is one exception, actually: Obviously, human-monitor-command must
> allow HMP commands to run in parallel.

Yes.

>> > While we don't have this, maybe it's worth considering if there is
>> > another simple way to achieve the same thing. Could QMP just suspend all
>> > HMP monitors while it's executing a command? At first sight it seems
>> > that this would work only for "interactive" monitors.
>> 
>> I believe the non-"interactive" HMP code is used only by gdbstub.c.
>
> monitor_init_hmp() is called from (based on my block branch):
>
> * monitor_init(): This is interactive.
> * qemu_chr_new_noreplay(): Interactive, too.
> * gdbserver_start(): Non-interactive.
>
> There is also qmp_human_monitor_command(), which manually creates a
> MonitorHMP without going through monitor_init_hmp(). It does call
> monitor_data_init(), though. There are no additional callers of
> monitor_data_init() and I think I would have added it to every creation
> of a Monitor object when I did the QMP/HMP split of the struct.
>
> So GDB and human-monitor-command should be the two non-interactive
> cases.

Yes.

>> I keep forgetting our GDB server stub creates an "HMP" monitor.
>> Possibly because I don't understand why.  Alex, Philippe, can you
>> enlighten me?
>
> I think you can send HMP commands from within gdb with it:
>
> (gdb) tar rem:1234
> Remote debugging using :1234
> [...]
> (gdb) monitor info block
> ide1-cd0: [not inserted]
>     Attached to:      /machine/unattached/device[23]
>     Removable device: not locked, tray closed
>
> floppy0: [not inserted]
>     Attached to:      /machine/unattached/device[16]
>     Removable device: not locked, tray closed
>
> sd0: [not inserted]
>     Removable device: not locked, tray closed

Argh!

Any idea where we define GDB command "monitor"?

> So we do want stop it from processing requests while a QMP command is
> running.

Then a slow QMP command can interfere with debugging.

Perhaps we can synchronize just the "monitor" command.




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