Re: [Qemu-stable] [Qemu-devel] [PATCH] job: Fix nested aio_poll() hanging in job_txn_apply

[Eric Blake]

On 08/21/2018 01:45 AM, Fam Zheng wrote:
> All callers have acquired ctx already. Doing that again results in
> aio_poll() hang. This fixes the problem that a BDRV_POLL_WHILE() in the
> callback cannot make progress because ctx is recursively locked, for
> example, when drive-backup finishes.
>
> Cc: address@hidden
> Reported-by: Gu Nini <address@hidden>
> Signed-off-by: Fam Zheng <address@hidden>
> ---
>   job.c | 18 +++++-------------
>   1 file changed, 5 insertions(+), 13 deletions(-)
>
> diff --git a/job.c b/job.c
> index fa671b431a..b07c5d0ffc 100644
> --- a/job.c
> +++ b/job.c
> @@ -136,21 +136,13 @@ static void job_txn_del_job(Job *job)
>       }
>   }
>   
> -static int job_txn_apply(JobTxn *txn, int fn(Job *), bool lock)
> +static int job_txn_apply(JobTxn *txn, int fn(Job *))

Is it worth adding a comment here that callers must have already 
acquired the job context?

However, I was unable to quickly audit whether all callers really did 
have the lock (it balloons into whether all callers of job_finalize() 
have the lock), so I'm reluctant to give R-b.

> @@ -857,10 +849,10 @@ static void job_completed_txn_success(Job *job)
>           assert(other_job->ret == 0);
>       }
>   
> -    job_txn_apply(txn, job_transition_to_pending, false);
> +    job_txn_apply(txn, job_transition_to_pending);
>   
>       /* If no jobs need manual finalization, automatically do so */
> -    if (job_txn_apply(txn, job_needs_finalize, false) == 0) {
> +    if (job_txn_apply(txn, job_needs_finalize) == 0) {
>           job_do_finalize(job);

That said, the change makes sense here: since the first direct call to 
job_txn_apply() did not need to lock, why should the second indirect 
call through job_do_finalize() need it?

Or, is the fix to have job_do_finalize() gain a bool parameter, where 
job_completed_txn_success() would pass false to that parameter, while 
job_finalize() would pass true (again, going back to auditing whether 
all callers of job_finalize() have already acquired the context).

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org