Re: [Qemu-devel] [PATCH] block/backup: install notifier during creation

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

09.08.2019 23:13, John Snow wrote:
> Backup jobs may yield prior to installing their handler, because of the
> job_co_entry shim which guarantees that a job won't begin work until
> we are ready to start an entire transaction.
> 
> Unfortunately, this makes proving correctness about transactional
> points-in-time for backup hard to reason about. Make it explicitly clear
> by moving the handler registration to creation time, and changing the
> write notifier to a no-op until the job is started.
> 
> Reported-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>   block/backup.c     | 32 +++++++++++++++++++++++---------
>   include/qemu/job.h |  5 +++++
>   job.c              |  2 +-
>   3 files changed, 29 insertions(+), 10 deletions(-)
> 
> diff --git a/block/backup.c b/block/backup.c
> index 07d751aea4..4df5b95415 100644
> --- a/block/backup.c
> +++ b/block/backup.c
> @@ -344,6 +344,13 @@ static int coroutine_fn backup_before_write_notify(
>       assert(QEMU_IS_ALIGNED(req->offset, BDRV_SECTOR_SIZE));
>       assert(QEMU_IS_ALIGNED(req->bytes, BDRV_SECTOR_SIZE));
>   
> +    /* The handler is installed at creation time; the actual point-in-time
> +     * starts at job_start(). Transactions guarantee those two points are
> +     * the same point in time. */
> +    if (!job_started(&job->common.job)) {
> +        return 0;
> +    }

Hmm, sorry if it is a stupid question, I'm not good in multiprocessing and in
Qemu iothreads..

job_started just reads job->co. If bs runs in iothread, and therefore write-notifier
is in iothread, when job_start is called from main thread.. Is it guaranteed that
write-notifier will see job->co variable change early enough to not miss guest write?
Should not job->co be volatile for example or something like this?

If not think about this patch looks good for me.

> +
>       return backup_do_cow(job, req->offset, req->bytes, NULL, true);
>   }
>   
> @@ -398,6 +405,12 @@ static void backup_clean(Job *job)
>       BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
>       BlockDriverState *bs = blk_bs(s->common.blk);
>   
> +    /* cancelled before job_start: remove write_notifier */
> +    if (s->before_write.notify) {
> +        notifier_with_return_remove(&s->before_write);
> +        s->before_write.notify = NULL;
> +    }
> +
>       if (s->copy_bitmap) {
>           bdrv_release_dirty_bitmap(bs, s->copy_bitmap);
>           s->copy_bitmap = NULL;
> @@ -527,17 +540,8 @@ static void backup_init_copy_bitmap(BackupBlockJob *job)
>   static int coroutine_fn backup_run(Job *job, Error **errp)
>   {
>       BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
> -    BlockDriverState *bs = blk_bs(s->common.blk);
>       int ret = 0;
>   
> -    QLIST_INIT(&s->inflight_reqs);
> -    qemu_co_rwlock_init(&s->flush_rwlock);
> -
> -    backup_init_copy_bitmap(s);
> -
> -    s->before_write.notify = backup_before_write_notify;
> -    bdrv_add_before_write_notifier(bs, &s->before_write);
> -
>       if (s->sync_mode == MIRROR_SYNC_MODE_TOP) {
>           int64_t offset = 0;
>           int64_t count;
> @@ -572,6 +576,7 @@ static int coroutine_fn backup_run(Job *job, Error **errp)
>   
>    out:
>       notifier_with_return_remove(&s->before_write);
> +    s->before_write.notify = NULL;
>   
>       /* wait until pending backup_do_cow() calls have completed */
>       qemu_co_rwlock_wrlock(&s->flush_rwlock);
> @@ -767,6 +772,15 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
>                          &error_abort);
>       job->len = len;
>   
> +    /* Finally, install a write notifier that takes effect after job_start() */
> +    backup_init_copy_bitmap(job);
> +
> +    QLIST_INIT(&job->inflight_reqs);
> +    qemu_co_rwlock_init(&job->flush_rwlock);
> +
> +    job->before_write.notify = backup_before_write_notify;
> +    bdrv_add_before_write_notifier(bs, &job->before_write);
> +
>       return &job->common;
>   
>    error:
> diff --git a/include/qemu/job.h b/include/qemu/job.h
> index 9e7cd1e4a0..733afb696b 100644
> --- a/include/qemu/job.h
> +++ b/include/qemu/job.h
> @@ -394,6 +394,11 @@ void job_enter_cond(Job *job, bool(*fn)(Job *job));
>    */
>   void job_start(Job *job);
>   
> +/**
> + * job_started returns true if the @job has started.
> + */
> +bool job_started(Job *job);
> +
>   /**
>    * @job: The job to enter.
>    *
> diff --git a/job.c b/job.c
> index 28dd48f8a5..745af659ff 100644
> --- a/job.c
> +++ b/job.c
> @@ -243,7 +243,7 @@ bool job_is_completed(Job *job)
>       return false;
>   }
>   
> -static bool job_started(Job *job)
> +bool job_started(Job *job)
>   {
>       return job->co;
>   }
> 


-- 
Best regards,
Vladimir