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
> Signed-off-by: John Snow
> ---
> 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
