Re: [PATCH v10 14/21] jobs: protect job.aio_context with BQL and job_mutex

[Kevin Wolf]

Am 25.07.2022 um 09:38 hat Emanuele Giuseppe Esposito geschrieben:
> In order to make it thread safe, implement a "fake rwlock",
> where we allow reads under BQL *or* job_mutex held, but
> writes only under BQL *and* job_mutex.

Oh, so the "or BQL" part is only for job.aio_context? Okay.

> The only write we have is in child_job_set_aio_ctx, which always
> happens under drain (so the job is paused).
> For this reason, introduce job_set_aio_context and make sure that
> the context is set under BQL, job_mutex and drain.
> Also make sure all other places where the aiocontext is read
> are protected.
> 
> Note: at this stage, job_{lock/unlock} and job lock guard macros
> are *nop*.
> 
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  block/replication.c |  6 ++++--
>  blockjob.c          |  3 ++-
>  include/qemu/job.h  | 19 ++++++++++++++++++-
>  job.c               | 12 ++++++++++++
>  4 files changed, 36 insertions(+), 4 deletions(-)
> 
> diff --git a/block/replication.c b/block/replication.c
> index 55c8f894aa..2189863df1 100644
> --- a/block/replication.c
> +++ b/block/replication.c
> @@ -148,8 +148,10 @@ static void replication_close(BlockDriverState *bs)
>      }
>      if (s->stage == BLOCK_REPLICATION_FAILOVER) {
>          commit_job = &s->commit_job->job;
> -        assert(commit_job->aio_context == qemu_get_current_aio_context());
> -        job_cancel_sync(commit_job, false);
> +        WITH_JOB_LOCK_GUARD() {
> +            assert(commit_job->aio_context == 
> qemu_get_current_aio_context());
> +            job_cancel_sync_locked(commit_job, false);
> +        }
>      }

.bdrv_close runs under the BQL, so why is this needed? Maybe a
GLOBAL_STATE_CODE() annotation would be helpful, though.

>      if (s->mode == REPLICATION_MODE_SECONDARY) {
> diff --git a/blockjob.c b/blockjob.c
> index 96fb9d9f73..9ff2727025 100644
> --- a/blockjob.c
> +++ b/blockjob.c
> @@ -162,12 +162,13 @@ static void child_job_set_aio_ctx(BdrvChild *c, 
> AioContext *ctx,
>          bdrv_set_aio_context_ignore(sibling->bs, ctx, ignore);
>      }
>  
> -    job->job.aio_context = ctx;
> +    job_set_aio_context(&job->job, ctx);
>  }
>  
>  static AioContext *child_job_get_parent_aio_context(BdrvChild *c)
>  {
>      BlockJob *job = c->opaque;
> +    assert(qemu_in_main_thread());

Any reason not to use GLOBAL_STATE_CODE()?

>      return job->job.aio_context;
>  }
> diff --git a/include/qemu/job.h b/include/qemu/job.h
> index 5709e8d4a8..c144aabefc 100644
> --- a/include/qemu/job.h
> +++ b/include/qemu/job.h
> @@ -77,7 +77,12 @@ typedef struct Job {
>  
>      /** Protected by AioContext lock */

I think this section comment should move down below aio_context now.

> -    /** AioContext to run the job coroutine in */
> +    /**
> +     * AioContext to run the job coroutine in.
> +     * This field can be read when holding either the BQL (so we are in
> +     * the main loop) or the job_mutex.
> +     * It can be only written when we hold *both* BQL and job_mutex.
> +     */
>      AioContext *aio_context;
>  
>      /** Reference count of the block job */
> @@ -741,4 +746,16 @@ int job_finish_sync(Job *job, void (*finish)(Job *, 
> Error **errp),
>  int job_finish_sync_locked(Job *job, void (*finish)(Job *, Error **errp),
>                             Error **errp);
>  
> +/**
> + * Sets the @job->aio_context.
> + * Called with job_mutex *not* held.
> + *
> + * This function must run in the main thread to protect against
> + * concurrent read in job_finish_sync_locked(),

Odd line break here in the middle of a sentence.

> + * takes the job_mutex lock to protect against the read in
> + * job_do_yield_locked(), and must be called when the coroutine
> + * is quiescent.
> + */
> +void job_set_aio_context(Job *job, AioContext *ctx);
> +
>  #endif
> diff --git a/job.c b/job.c
> index ecec66b44e..0a857b1468 100644
> --- a/job.c
> +++ b/job.c
> @@ -394,6 +394,17 @@ Job *job_get(const char *id)
>      return job_get_locked(id);
>  }
>  
> +void job_set_aio_context(Job *job, AioContext *ctx)
> +{
> +    /* protect against read in job_finish_sync_locked and job_start */
> +    assert(qemu_in_main_thread());

Same question about GLOBAL_STATE_CODE().

> +    /* protect against read in job_do_yield_locked */
> +    JOB_LOCK_GUARD();
> +    /* ensure the coroutine is quiescent while the AioContext is changed */
> +    assert(job->pause_count > 0);

job->pause_count only shows that pausing was requested. The coroutine is
only really quiescent if job->busy == false, too.

Or maybe job->paused is actually the one you want here.

> +    job->aio_context = ctx;
> +}
> +
>  /* Called with job_mutex *not* held. */
>  static void job_sleep_timer_cb(void *opaque)
>  {
> @@ -1376,6 +1387,7 @@ int job_finish_sync_locked(Job *job,
>  {
>      Error *local_err = NULL;
>      int ret;
> +    assert(qemu_in_main_thread());
>  
>      job_ref_locked(job);

Another GLOBAL_STATE_CODE()?

Kevin