Re: [Qemu-devel] [PATCH v13 11/12] migration: Remove not needed semaphore and quit

[Juan Quintela]

"Dr. David Alan Gilbert" <address@hidden> wrote:
> * Juan Quintela (address@hidden) wrote:
>> "Dr. David Alan Gilbert" <address@hidden> wrote:
>> > * Juan Quintela (address@hidden) wrote:
>> >> "Dr. David Alan Gilbert" <address@hidden> wrote:
>> >> > * Juan Quintela (address@hidden) wrote:
>> >> >> "Dr. David Alan Gilbert" <address@hidden> wrote:
>> >> >> > * Juan Quintela (address@hidden) wrote:
>> >> >> >> We know quit closing the QIO.
>> 
>> ...
>> >> > Two questions from that then:
>> >> >   a) Are you sure it's safe to close the qio_channel while another
>> >> > thread is in qio_channel_read_all_eof?  Is it really defined that it
>> >> > causes the other thread to exit with an error;  close() in some stuff
>> >> > frees data structures that the other thread is still reading; that's
>> >> > why I've used shutdown(2) in the past rather than close on fd's
>> >> 
>> >> Dunno if it is safe (I think it is), but I agree that shutdown will also
>> >> get what I need.
>> >> 
>> >> >   b) I don't think your answer explains why it's an object_unref?
>> >> 
>> >> That is the standard way to closing qios to not have to take into
>> >> account who have it oppened.  See previous paragraph, it is better to
>> >> use shutdown, done.
>> >
>> > OK, great;  I suspect it's unsafe because as soon as you do the unref
>> > it could free the object; actually you should have a ref from each of
>> > the threads to sotp it being freed while they use it.
>> >
>> > Dave
>> >
>> >> Thanks, Juan.
>> >> 
>> >> > Dave
>> >> >
>> >> >> Later, Juan.
>> >> > --
>> >> > Dr. David Alan Gilbert / address@hidden / Manchester, UK
>> > --
>> > Dr. David Alan Gilbert / address@hidden / Manchester, UK
>> 
>> What do you think about this, to avoid me resend the whole series?
>> 
>> From e03d77de1ca179fa0168cead7c23cfeae57f1787 Mon Sep 17 00:00:00 2001
>> From: Juan Quintela <address@hidden>
>> Date: Wed, 18 Apr 2018 00:49:19 +0200
>> Subject: [PATCH 17/18] migration: Remove not needed semaphore and quit
>> 
>> We know quit with shutdwon in the QIO.
>> 
>> Signed-off-by: Juan Quintela <address@hidden>
>> --
>> Add comment
>> Use shutdown() instead of unref()
>> ---
>>  migration/ram.c | 14 +++++---------
>>  1 file changed, 5 insertions(+), 9 deletions(-)
>> 
>> diff --git a/migration/ram.c b/migration/ram.c
>> index 2c3a452a7d..be5d26f4cb 100644
>> --- a/migration/ram.c
>> +++ b/migration/ram.c
>> @@ -594,14 +594,10 @@ typedef struct {
>>      QemuThread thread;
>>      /* communication channel */
>>      QIOChannel *c;
>> -    /* sem where to wait for more work */
>> -    QemuSemaphore sem;
>>      /* this mutex protects the following parameters */
>>      QemuMutex mutex;
>>      /* is this channel thread running */
>>      bool running;
>> -    /* should this thread finish */
>> -    bool quit;
>>      /* array of pages to receive */
>>      MultiFDPages_t *pages;
>>      /* packet allocated len */
>> @@ -1152,8 +1148,11 @@ static void multifd_recv_terminate_threads(Error *err)
>>          MultiFDRecvParams *p = &multifd_recv_state->params[i];
>>  
>>          qemu_mutex_lock(&p->mutex);
>> -        p->quit = true;
>> -        qemu_sem_post(&p->sem);
>> +        /* We could arrive here for two reasons:
>> +           - normal quit, i.e. everything went fine, just finished
>> +           - error quit: We close the channels so the channel threads
>> +             finish the qio_channel_read_all_eof() */
>> +        qio_channel_shutdown(p->c, QIO_CHANNEL_SHUTDOWN_BOTH, NULL);
>
> OK, so with any luck all the threads now exit;  do we still have a
> close/unref once we're sure they have all exited?

Yeap.

static void multifd_recv_terminate_threads(Error *err)
{
    [...]
    for (i = 0; i < migrate_multifd_channels(); i++) {
        MultiFDRecvParams *p = &multifd_recv_state->params[i];

        qemu_mutex_lock(&p->mutex);
        /* We could arrive here for two reasons:
           - normal quit, i.e. everything went fine, just finished
           - error quit: We close the channels so the channel threads
             finish the qio_channel_read_all_eof() */
        qio_channel_shutdown(p->c, QIO_CHANNEL_SHUTDOWN_BOTH, NULL);
        qemu_mutex_unlock(&p->mutex);
    }
}

int multifd_load_cleanup(Error **errp)
{
    int i;
    int ret = 0;

    if (!migrate_use_multifd()) {
        return 0;
    }
    multifd_recv_terminate_threads(NULL);
    for (i = 0; i < migrate_multifd_channels(); i++) {
        MultiFDRecvParams *p = &multifd_recv_state->params[i];

        if (p->running) {
            qemu_thread_join(&p->thread);
        }
        object_unref(OBJECT(p->c));
        p->c = NULL;
        [...]
    }
    [...]
}

Omited the things that we don't care about for this.

Thanks, Juan.