Peter Xu <pet...@redhat.com> writes:
> On Fri, Sep 28, 2018 at 01:00:26PM +0400, Marc-André Lureau wrote:
>> Hi
>>
>> On Fri, Sep 28, 2018 at 12:02 PM Wolfgang Bumiller
>> <w.bumil...@proxmox.com> wrote:
>> >
>> > Commit d32749deb615 moved the call to monitor_init_globals()
>> > to before os_daemonize(), making it an unsuitable place to
>> > spawn the monitor iothread as it won't be inherited over the
>> > fork() in os_daemonize().
>> >
>> > We now spawn the thread the first time we instantiate a
>> > monitor which actually has use_io_thread == true. Therefore
>> > mon_iothread initialization is protected by monitor_lock.
>> >
>> > We still need to create the qmp_dispatcher_bh when not using
>> > iothreads, so this now still happens via
>> > monitor_init_globals().
>> >
>> > Signed-off-by: Wolfgang Bumiller <w.bumil...@proxmox.com>
>> > Fixes: d32749deb615 ("monitor: move init global earlier")
>> > ---
>> > Changes to v1:
>> > - move mon_iothread declaration down to monitor_lock's declaration
>> > (updating monitor_lock's coverage comment)
>> > - in monitor_data_init() assert() that mon_iothread is not NULL or
>> > not used instead of initializing it there, as its usage pattern is
>> > so that it is a initialized once before being used, or never used
>> > at all.
>> > - in monitor_iothread_init(), protect mon_iothread initialization
>> > with monitor_lock
>> > - in monitor_init(): run monitor_ithread_init() in the `use_oob`
>> > branch.
>> > Note that I currently also test for mon_iothread being NULL there,
>> > which we could leave this out as spawning new monitors isn't
>> > something that happens a lot, but I like the idea of avoiding
>> > taking a lock when not required.
>> > Otherwise, I can send a v3 with this removed.
>> >
>> > monitor.c | 49 ++++++++++++++++++++++++++++++-------------------
>> > 1 file changed, 30 insertions(+), 19 deletions(-)
>> >
>> > diff --git a/monitor.c b/monitor.c
>> > index d47e4259fd..870584a548 100644
>> > --- a/monitor.c
>> > +++ b/monitor.c
>> > @@ -239,9 +239,6 @@ struct Monitor {
>> > int mux_out;
>> > };
>> >
>> > -/* Shared monitor I/O thread */
>> > -IOThread *mon_iothread;
>> > -
>> > /* Bottom half to dispatch the requests received from I/O thread */
>> > QEMUBH *qmp_dispatcher_bh;
>> >
>> > @@ -262,10 +259,11 @@ typedef struct QMPRequest QMPRequest;
>> > /* QMP checker flags */
>> > #define QMP_ACCEPT_UNKNOWNS 1
>> >
>> > -/* Protects mon_list, monitor_qapi_event_state. */
>> > +/* Protects mon_list, monitor_qapi_event_state and mon_iothread. */
>> > static QemuMutex monitor_lock;
>> > static GHashTable *monitor_qapi_event_state;
>> > static QTAILQ_HEAD(mon_list, Monitor) mon_list;
>> > +IOThread *mon_iothread; /* Shared monitor I/O thread */
>> >
>> > /* Protects mon_fdsets */
>> > static QemuMutex mon_fdsets_lock;
>> > @@ -710,6 +708,7 @@ static void handle_hmp_command(Monitor *mon, const
>> > char *cmdline);
>> > static void monitor_data_init(Monitor *mon, bool skip_flush,
>> > bool use_io_thread)
>> > {
>> > + assert(!use_io_thread || mon_iothread);
>> > memset(mon, 0, sizeof(Monitor));
>> > qemu_mutex_init(&mon->mon_lock);
>> > qemu_mutex_init(&mon->qmp.qmp_queue_lock);
>> > @@ -4453,16 +4452,11 @@ static AioContext *monitor_get_aio_context(void)
>> >
>> > static void monitor_iothread_init(void)
>> > {
>> > - mon_iothread = iothread_create("mon_iothread", &error_abort);
>> > -
>> > - /*
>> > - * The dispatcher BH must run in the main loop thread, since we
>> > - * have commands assuming that context. It would be nice to get
>> > - * rid of those assumptions.
>> > - */
>> > - qmp_dispatcher_bh = aio_bh_new(iohandler_get_aio_context(),
>> > - monitor_qmp_bh_dispatcher,
>> > - NULL);
>> > + qemu_mutex_lock(&monitor_lock);
>> > + if (!mon_iothread) {
>> > + mon_iothread = iothread_create("mon_iothread", &error_abort);
>> > + }
>> > + qemu_mutex_unlock(&monitor_lock);
>> > }
>> >
>> > void monitor_init_globals(void)
>> > @@ -4472,7 +4466,15 @@ void monitor_init_globals(void)
>> > sortcmdlist();
>> > qemu_mutex_init(&monitor_lock);
>> > qemu_mutex_init(&mon_fdsets_lock);
>> > - monitor_iothread_init();
>> > +
>> > + /*
>> > + * The dispatcher BH must run in the main loop thread, since we
>> > + * have commands assuming that context. It would be nice to get
>> > + * rid of those assumptions.
>> > + */
>> > + qmp_dispatcher_bh = aio_bh_new(iohandler_get_aio_context(),
>> > + monitor_qmp_bh_dispatcher,
>> > + NULL);
>> > }
>> >
>> > /* These functions just adapt the readline interface in a typesafe way.
>> > We
>> > @@ -4535,6 +4537,9 @@ static void monitor_qmp_setup_handlers_bh(void
>> > *opaque)
>> > monitor_list_append(mon);
>> > }
>> >
>> > +/*
>> > + * This expects to be run in the main thread.
>> > + */
>>
>> I read that Markus suggested that comment, but I don't really get why.
>>
>> It means that callers (chardev new) should also be restricted to main thread.
>
> My understanding is that Markus mentioned about uncertainty on the
> chardev creation paths. Though AFAIU if we're with the lock then we
> don't need this comment at all, do we?
The conversation (Message-ID: <87d0sz86f8....@dusky.pond.sub.org>) was:
Peter Xu <pet...@redhat.com> writes:
> On Thu, Sep 27, 2018 at 10:46:34AM +0200, Markus Armbruster wrote:
[...]
>> Should we put @mon_iothread under @monitor_lock?
>
> IMHO we can when we create the thread. I guess we don't need that
> lock when reading @mon_iothread, after all it's a very special
> variable in that:
>
> - it is only set once, or never
>
> - when reading @mon_iothread only, we must have it set or it should
> be a programming error, so it's more like an assert(mon_iothread)
> not a contention
>
>>
>> Could we accept this patch without doing that, on the theory that it
>> doesn't make things worse than they already are?
>
> If this bothers us that much, how about we just choose the option that
> Wolfgang offered at [1] above to create the iothread after daemonize
> (so we pick that out from monitor_global_init)?
I'd prefer this patch's approach, because it keeps the interface
simpler.
v2 uses this approach.
I can accept this patch as is, or with my incremental patch squashed
in. A comment explaining monitor_init() expects to run in the main
thread would be nice.
Acceptable alternative 1, with a few optional variations.
The comment makes sense because if monitor_init can run in other
threads, the creation of @iothread is racy. Acceptable since it's
really no worse than before (see the full message for why).
I'd also accept a patch that wraps
if (!mon_iothread) {
monitor_iothread_init();
}
in a critical section. Using @monitor_lock is fine. A new lock feels
unnecessarily fine-grained. If using @monitor_lock, move the definition
of @mon_iothread next to @monitor_lock, and update the comment there.
Acceptable alternative 2.
v2 appears to combine both alternatives. Not what I asked for. I
figure the comment still makes sense, since @iothread creation is just
one of the issues, and protecting it with a lock leaves the other issues
unaddressed.
If we actually run it in other threads, the comment needs to be
augmented with a suitable FIXME stating the problem.
Marc-André, does this make sense?
>>
>> > void monitor_init(Chardev *chr, int flags)
>> > {
>> > Monitor *mon = g_malloc(sizeof(*mon));
>> > @@ -4551,6 +4556,9 @@ void monitor_init(Chardev *chr, int flags)
>> > error_report("Monitor out-of-band is only supported by QMP");
>> > exit(1);
>> > }
>> > + if (!mon_iothread) {
>> > + monitor_iothread_init();
>> > + }
>>
>> I would call it unconditonnally, to avoid TOCTOU.
>
> Yeh agree that the "if" could be omitted; though there shouldn't be
> toctou since the function will check it again.
Really?
[...]
>> > }
>> >
>> > monitor_data_init(mon, false, use_oob);
>> > @@ -4607,7 +4615,9 @@ void monitor_cleanup(void)
>> > * we need to unregister from chardev below in
>> > * monitor_data_destroy(), and chardev is not thread-safe yet
>> > */
>> > - iothread_stop(mon_iothread);
>> > + if (mon_iothread) {
>> > + iothread_stop(mon_iothread);
>> > + }
>> >
>>
>> here the monitor_lock isn't taken, is there a reason worth a comment?
I don't know. What I know is that locking something only some of the
times (not counting a single-threaded initial stretch of initialization
code) is usually wrong.
>> > /* Flush output buffers and destroy monitors */
>> > qemu_mutex_lock(&monitor_lock);
[...]
Since the bug is inconveniencing people, should I merge v1 for now? We
can then figure out how to improve on it.
