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Re: [Qemu-devel] [PATCH v6 27/28] migration/qmp: add command migrate-pau
From: |
Peter Xu |
Subject: |
Re: [Qemu-devel] [PATCH v6 27/28] migration/qmp: add command migrate-pause |
Date: |
Tue, 13 Mar 2018 17:13:18 +0800 |
User-agent: |
Mutt/1.9.1 (2017-09-22) |
On Wed, Feb 28, 2018 at 08:14:19PM +0000, Dr. David Alan Gilbert wrote:
> * Peter Xu (address@hidden) wrote:
> > On Wed, Feb 14, 2018 at 06:56:59PM +0000, Dr. David Alan Gilbert wrote:
> > > * Peter Xu (address@hidden) wrote:
> > > > On Tue, Feb 13, 2018 at 08:11:00PM +0000, Dr. David Alan Gilbert wrote:
> > > > > * Peter Xu (address@hidden) wrote:
> > > > > > It pauses an ongoing migration. Currently it only supports
> > > > > > postcopy.
> > > > > > Note that this command will work on either side of the migration.
> > > > > > Basically when we trigger this on one side, it'll interrupt the
> > > > > > other
> > > > > > side as well since the other side will get notified on the
> > > > > > disconnect
> > > > > > event.
> > > > > >
> > > > > > However, it's still possible that the other side is not notified,
> > > > > > for
> > > > > > example, when the network is totally broken, or due to some firewall
> > > > > > configuration changes. In that case, we will also need to run the
> > > > > > same
> > > > > > command on the other side so both sides will go into the paused
> > > > > > state.
> > > > > >
> > > > > > Signed-off-by: Peter Xu <address@hidden>
> > > > > > ---
> > > > > > migration/migration.c | 27 +++++++++++++++++++++++++++
> > > > > > qapi/migration.json | 16 ++++++++++++++++
> > > > > > 2 files changed, 43 insertions(+)
> > > > > >
> > > > > > diff --git a/migration/migration.c b/migration/migration.c
> > > > > > index bb57ed9ade..139abec0c3 100644
> > > > > > --- a/migration/migration.c
> > > > > > +++ b/migration/migration.c
> > > > > > @@ -1448,6 +1448,33 @@ void qmp_migrate_recover(const char *uri,
> > > > > > Error **errp)
> > > > > > qemu_start_incoming_migration(uri, errp);
> > > > > > }
> > > > > >
> > > > > > +void qmp_migrate_pause(Error **errp)
> > > > > > +{
> > > > > > + MigrationState *ms = migrate_get_current();
> > > > > > + MigrationIncomingState *mis = migration_incoming_get_current();
> > > > > > + int ret;
> > > > > > +
> > > > > > + if (ms->state == MIGRATION_STATUS_POSTCOPY_ACTIVE) {
> > > > > > + /* Source side, during postcopy */
> > > > > > + ret = qemu_file_shutdown(ms->to_dst_file);
> > > > >
> > > > > This doesn't feel thread safe; although I'm not sure how to make it
> > > > > so.
> > > > > If the migration finishes just after we check the state but before the
> > > > > shutdown we end up using a bogus QEMUFile*
> > > > > Making all the places that close a QEMUFile* set hte pointer Null
> > > > > before
> > > > > they do the close doesn't help because you still race with that.
> > > > >
> > > > > (The race is small, but still)
> > > >
> > > > IMHO we can fix it by adding a migration lock for management code. If
> > > > you see my previous migrate cleanup series, it's in my todo. ;)
> > > >
> > > > The basic idea is that we take the lock for critical paths (but not
> > > > during most of the migration process). E.g., we may need the lock
> > > > for:
> > > >
> > > > - very beginning of migration, during setup
> > > > - reaching the end of migration
> > > > - every single migration QMP command (since HMP calls them so HMP will
> > > > also acquire the lock)
> > > > - anywhere else I didn't mention that may necessary, e.g., when we
> > > > change migrate state, meanwhile we do something else - basically
> > > > that should be an "atomic operation", and we need the lock to make
> > > > sure of that.
> > >
> > > But then we couldn't take that lock in an OOB command, you'd have to
> > > audit all of those places that took it to make sure it didn't do any of
> > > the things OOB commands aren't allowed to do.
> >
> > Yeah OOB commands will be special - my plan is that they just never
> > take the lock. E.g., they only touches FDs, and FDs are naturally
> > thread safe (like this command).
> >
> > And some major migration commands (like "migrate" itself) should never
> > be an OOB command.
>
> OK; I'm not sure what makes FDs naturally thread safe though; but
> lets see the code you have in mind.
I think I was wrong... it should need a lock.
>
> > >
> > > > For the recovery series, I would prefer that we ignore this issue for
> > > > now - since this problem is there for quite a long time AFAICT in the
> > > > whole migration code rather than this series only, and we need to
> > > > solve it once and for all.
> > >
> > > I don't think those problems happen (much) in the existing code, because
> > > everything is done in the main thread.
> >
> > But migration is running in its own thread (migration_thread)?
> >
> > For example: What if we send migration commands during the end of
> > migration or a failing migration? Could there be risk in old code
> > too since both main thread and migration thread may be manipulating
> > MigrationState object?
>
> Maybe; although migrate_set_state uses atomic_cmpxchg to ensure
> that it's safe, and starting a migration can't happen unless there
> isn't a migration in progress - and that's run under lock.
Yes I think if without OOB we should be fine since even the cleanup is
running with the BQL.
Now I don't have good idea to solve this problem except introducing a
lock. How about I add a patch to introduce the mgmt_lock, which
currently only protect the QEMUFile? Like:
----------------------------------
diff --git a/migration/migration.c b/migration/migration.c
index f31fcbb0d5..00c630326d 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -1195,8 +1195,10 @@ static void migrate_fd_cleanup(void *opaque)
if (multifd_save_cleanup(&local_err) != 0) {
error_report_err(local_err);
}
+ qemu_mutex_lock(&s->mgmt_lock);
qemu_fclose(s->to_dst_file);
s->to_dst_file = NULL;
+ qemu_mutex_unlock(&s->mgmt_lock);
}
assert((s->state != MIGRATION_STATUS_ACTIVE) &&
@@ -2493,8 +2495,10 @@ static MigThrError postcopy_pause(MigrationState *s)
/* Current channel is possibly broken. Release it. */
assert(s->to_dst_file);
+ qemu_mutex_lock(&s->mgmt_lock);
qemu_file_shutdown(s->to_dst_file);
qemu_fclose(s->to_dst_file);
s->to_dst_file = NULL;
+ qemu_mutex_unlock(&s->mgmt_lock);
error_report("Detected IO failure for postcopy. "
@@ -2970,6 +2974,7 @@ static void migration_instance_finalize(Object *obj)
qemu_sem_destroy(&ms->postcopy_pause_sem);
qemu_sem_destroy(&ms->postcopy_pause_rp_sem);
qemu_sem_destroy(&ms->rp_state.rp_sem);
+ qemu_mutex_destroy(&ms->mgmt_lock);
}
static void migration_instance_init(Object *obj)
@@ -3002,6 +3007,7 @@ static void migration_instance_init(Object *obj)
qemu_sem_init(&ms->postcopy_pause_sem, 0);
qemu_sem_init(&ms->postcopy_pause_rp_sem, 0);
qemu_sem_init(&ms->rp_state.rp_sem, 0);
+ qemu_mutex_init(&ms->mgmt_lock);
}
/*
diff --git a/migration/migration.h b/migration/migration.h
index c549859cc3..7fcb841978 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -98,6 +98,11 @@ struct MigrationState
QemuThread thread;
QEMUBH *cleanup_bh;
QEMUFile *to_dst_file;
+ /*
+ * Currently it only protects to_dst_file. We need to hold this
+ * lock when we want to modify in/out QEMUFiles.
+ */
+ QemuMutex mgmt_lock;
/* bytes already send at the beggining of current interation */
uint64_t iteration_initial_bytes;
----------------------------------
Then I take this lock in the OOB postcopy-pause handler. Since the
lock holding scenarios are always extremely fast, I assume it's pretty
safe. Would that be okay with you?
Thanks,
--
Peter Xu
- Re: [Qemu-devel] [PATCH v6 27/28] migration/qmp: add command migrate-pause,
Peter Xu <=