When a multifd sender thread hit errors, it always needs to kick the main
thread by kicking all the semaphores that it can be waiting upon.
Provide a helper for it and deduplicate the code.
Signed-off-by: Peter Xu <peterx@redhat.com>
---
migration/multifd.c | 21 +++++++++++++++------
1 file changed, 15 insertions(+), 6 deletions(-)
diff --git a/migration/multifd.c b/migration/multifd.c
index 4afdd88602..33fb21d0e4 100644
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -374,6 +374,18 @@ struct {
MultiFDMethods *ops;
} *multifd_send_state;
+/*
+ * The migration thread can wait on either of the two semaphores. This
+ * function can be used to kick the main thread out of waiting on either of
+ * them. Should mostly only be called when something wrong happened with
+ * the current multifd send thread.
+ */
+static void multifd_send_kick_main(MultiFDSendParams *p)
+{
+ qemu_sem_post(&p->sem_sync);
+ qemu_sem_post(&multifd_send_state->channels_ready);
+}
+
/*
* How we use multifd_send_state->pages and channel->pages?
*
@@ -746,8 +758,7 @@ out:
assert(local_err);
trace_multifd_send_error(p->id);
multifd_send_terminate_threads(local_err);
- qemu_sem_post(&p->sem_sync);
- qemu_sem_post(&multifd_send_state->channels_ready);
+ multifd_send_kick_main(p);
error_free(local_err);
}
@@ -787,8 +798,7 @@ static void multifd_tls_outgoing_handshake(QIOTask *task,
* is not created, and then tell who pay attention to me.
*/
p->quit = true;
- qemu_sem_post(&multifd_send_state->channels_ready);
- qemu_sem_post(&p->sem_sync);
+ multifd_send_kick_main(p);
error_free(err);
}
@@ -859,8 +869,7 @@ static void multifd_new_send_channel_cleanup(MultiFDSendParams *p,
{
migrate_set_error(migrate_get_current(), err);
/* Error happen, we need to tell who pay attention to me */
- qemu_sem_post(&multifd_send_state->channels_ready);
- qemu_sem_post(&p->sem_sync);
+ multifd_send_kick_main(p);
/*
* Although multifd_send_thread is not created, but main migration
* thread need to judge whether it is running, so we need to mark
--
2.41.0Peter Xu <peterx@redhat.com> writes:
> When a multifd sender thread hit errors, it always needs to kick the main
> thread by kicking all the semaphores that it can be waiting upon.
>
> Provide a helper for it and deduplicate the code.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> migration/multifd.c | 21 +++++++++++++++------
> 1 file changed, 15 insertions(+), 6 deletions(-)
>
> diff --git a/migration/multifd.c b/migration/multifd.c
> index 4afdd88602..33fb21d0e4 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -374,6 +374,18 @@ struct {
> MultiFDMethods *ops;
> } *multifd_send_state;
>
> +/*
> + * The migration thread can wait on either of the two semaphores. This
> + * function can be used to kick the main thread out of waiting on either of
> + * them. Should mostly only be called when something wrong happened with
> + * the current multifd send thread.
> + */
> +static void multifd_send_kick_main(MultiFDSendParams *p)
> +{
> + qemu_sem_post(&p->sem_sync);
> + qemu_sem_post(&multifd_send_state->channels_ready);
> +}
> +
> /*
> * How we use multifd_send_state->pages and channel->pages?
> *
> @@ -746,8 +758,7 @@ out:
> assert(local_err);
> trace_multifd_send_error(p->id);
> multifd_send_terminate_threads(local_err);
> - qemu_sem_post(&p->sem_sync);
> - qemu_sem_post(&multifd_send_state->channels_ready);
> + multifd_send_kick_main(p);
> error_free(local_err);
> }
>
> @@ -787,8 +798,7 @@ static void multifd_tls_outgoing_handshake(QIOTask *task,
> * is not created, and then tell who pay attention to me.
> */
> p->quit = true;
> - qemu_sem_post(&multifd_send_state->channels_ready);
> - qemu_sem_post(&p->sem_sync);
> + multifd_send_kick_main(p);
There's a bug here in the original code:
It's not really safe to call any of these outside of the channel lock
because multifd_save_cleanup() could execute at the same time and call
qemu_sem_destroy() -> qemu_mutex_destroy(), which can assert because we
might be holding the sem_lock.
It seems the reason we get away with this today is merely due to
timing. A subset of this problem was already encountered here:
[PATCH] migrate/multifd: fix coredump when the multifd thread cleanup
https://lore.kernel.org/r/20230621081826.3203053-1-zhangjianguo18@huawei.com
We could probably release the semaphores for all channels at once inside
multifd_save_cleanup() in the main thread. We'd have a
multifd_send_kick_main() in each channel when it fails and this:
void multifd_save_cleanup(void)
{
...
for (i = 0; i < migrate_multifd_channels(); i++) {
MultiFDSendParams *p = &multifd_send_state->params[i];
if (p->running) {
qemu_thread_join(&p->thread);
} else {
multifd_send_kick_main(p);
}
}
for (i = 0; i < migrate_multifd_channels(); i++) {
qemu_sem_destroy, etc...
}
...
}
On Wed, Nov 08, 2023 at 07:49:53PM -0300, Fabiano Rosas wrote:
> Peter Xu <peterx@redhat.com> writes:
>
> > When a multifd sender thread hit errors, it always needs to kick the main
> > thread by kicking all the semaphores that it can be waiting upon.
> >
> > Provide a helper for it and deduplicate the code.
> >
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> > migration/multifd.c | 21 +++++++++++++++------
> > 1 file changed, 15 insertions(+), 6 deletions(-)
> >
> > diff --git a/migration/multifd.c b/migration/multifd.c
> > index 4afdd88602..33fb21d0e4 100644
> > --- a/migration/multifd.c
> > +++ b/migration/multifd.c
> > @@ -374,6 +374,18 @@ struct {
> > MultiFDMethods *ops;
> > } *multifd_send_state;
> >
> > +/*
> > + * The migration thread can wait on either of the two semaphores. This
> > + * function can be used to kick the main thread out of waiting on either of
> > + * them. Should mostly only be called when something wrong happened with
> > + * the current multifd send thread.
> > + */
> > +static void multifd_send_kick_main(MultiFDSendParams *p)
> > +{
> > + qemu_sem_post(&p->sem_sync);
> > + qemu_sem_post(&multifd_send_state->channels_ready);
> > +}
> > +
> > /*
> > * How we use multifd_send_state->pages and channel->pages?
> > *
> > @@ -746,8 +758,7 @@ out:
> > assert(local_err);
> > trace_multifd_send_error(p->id);
> > multifd_send_terminate_threads(local_err);
> > - qemu_sem_post(&p->sem_sync);
> > - qemu_sem_post(&multifd_send_state->channels_ready);
> > + multifd_send_kick_main(p);
> > error_free(local_err);
> > }
> >
> > @@ -787,8 +798,7 @@ static void multifd_tls_outgoing_handshake(QIOTask *task,
> > * is not created, and then tell who pay attention to me.
> > */
> > p->quit = true;
> > - qemu_sem_post(&multifd_send_state->channels_ready);
> > - qemu_sem_post(&p->sem_sync);
> > + multifd_send_kick_main(p);
>
> There's a bug here in the original code:
>
> It's not really safe to call any of these outside of the channel lock
> because multifd_save_cleanup() could execute at the same time and call
> qemu_sem_destroy() -> qemu_mutex_destroy(), which can assert because we
> might be holding the sem_lock.
If you meant "p->mutex" as the "channel lock", IIUC even holding that won't
work? Because it'll also be freed in multifd_save_cleanup().
>
> It seems the reason we get away with this today is merely due to
> timing. A subset of this problem was already encountered here:
>
> [PATCH] migrate/multifd: fix coredump when the multifd thread cleanup
> https://lore.kernel.org/r/20230621081826.3203053-1-zhangjianguo18@huawei.com
>
> We could probably release the semaphores for all channels at once inside
> multifd_save_cleanup() in the main thread. We'd have a
> multifd_send_kick_main() in each channel when it fails and this:
>
> void multifd_save_cleanup(void)
> {
> ...
> for (i = 0; i < migrate_multifd_channels(); i++) {
> MultiFDSendParams *p = &multifd_send_state->params[i];
>
> if (p->running) {
> qemu_thread_join(&p->thread);
> } else {
> multifd_send_kick_main(p);
> }
> }
> for (i = 0; i < migrate_multifd_channels(); i++) {
> qemu_sem_destroy, etc...
> }
> ...
> }
>
--
Peter Xu
Peter Xu <peterx@redhat.com> writes:
> On Wed, Nov 08, 2023 at 07:49:53PM -0300, Fabiano Rosas wrote:
>> Peter Xu <peterx@redhat.com> writes:
>>
>> > When a multifd sender thread hit errors, it always needs to kick the main
>> > thread by kicking all the semaphores that it can be waiting upon.
>> >
>> > Provide a helper for it and deduplicate the code.
>> >
>> > Signed-off-by: Peter Xu <peterx@redhat.com>
>> > ---
>> > migration/multifd.c | 21 +++++++++++++++------
>> > 1 file changed, 15 insertions(+), 6 deletions(-)
>> >
>> > diff --git a/migration/multifd.c b/migration/multifd.c
>> > index 4afdd88602..33fb21d0e4 100644
>> > --- a/migration/multifd.c
>> > +++ b/migration/multifd.c
>> > @@ -374,6 +374,18 @@ struct {
>> > MultiFDMethods *ops;
>> > } *multifd_send_state;
>> >
>> > +/*
>> > + * The migration thread can wait on either of the two semaphores. This
>> > + * function can be used to kick the main thread out of waiting on either of
>> > + * them. Should mostly only be called when something wrong happened with
>> > + * the current multifd send thread.
>> > + */
>> > +static void multifd_send_kick_main(MultiFDSendParams *p)
>> > +{
>> > + qemu_sem_post(&p->sem_sync);
>> > + qemu_sem_post(&multifd_send_state->channels_ready);
>> > +}
>> > +
>> > /*
>> > * How we use multifd_send_state->pages and channel->pages?
>> > *
>> > @@ -746,8 +758,7 @@ out:
>> > assert(local_err);
>> > trace_multifd_send_error(p->id);
>> > multifd_send_terminate_threads(local_err);
>> > - qemu_sem_post(&p->sem_sync);
>> > - qemu_sem_post(&multifd_send_state->channels_ready);
>> > + multifd_send_kick_main(p);
>> > error_free(local_err);
>> > }
>> >
>> > @@ -787,8 +798,7 @@ static void multifd_tls_outgoing_handshake(QIOTask *task,
>> > * is not created, and then tell who pay attention to me.
>> > */
>> > p->quit = true;
>> > - qemu_sem_post(&multifd_send_state->channels_ready);
>> > - qemu_sem_post(&p->sem_sync);
>> > + multifd_send_kick_main(p);
>>
>> There's a bug here in the original code:
>>
>> It's not really safe to call any of these outside of the channel lock
>> because multifd_save_cleanup() could execute at the same time and call
>> qemu_sem_destroy() -> qemu_mutex_destroy(), which can assert because we
>> might be holding the sem_lock.
>
> If you meant "p->mutex" as the "channel lock", IIUC even holding that won't
> work? Because it'll also be freed in multifd_save_cleanup().
>
You're right, I just sent an RFC about this, please take a look.
Peter Xu <peterx@redhat.com> writes: > When a multifd sender thread hit errors, it always needs to kick the main > thread by kicking all the semaphores that it can be waiting upon. > > Provide a helper for it and deduplicate the code. > > Signed-off-by: Peter Xu <peterx@redhat.com> Reviewed-by: Fabiano Rosas <farosas@suse.de>
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