Re: [pve-devel] [PATCH v2 qemu] fix #4101: acquire job's aio context before calling job_unref

[Wolfgang Bumiller <w.bumiller@proxmox.com>]

On Thu, Jun 09, 2022 at 02:31:13PM +0200, Fabian Ebner wrote:
> Otherwise, we might run into an abort via bdrv_co_yield_to_drain()
> (can at least happen when a disk with iothread is used):
> > #0  0x00007fef4f5dece1 __GI_raise (libc.so.6 + 0x3bce1)
> > #1  0x00007fef4f5c8537 __GI_abort (libc.so.6 + 0x25537)
> > #2  0x00005641bce3c71f error_exit (qemu-system-x86_64 + 0x80371f)
> > #3  0x00005641bce3d02b qemu_mutex_unlock_impl (qemu-system-x86_64 + 0x80402b)
> > #4  0x00005641bcd51655 bdrv_co_yield_to_drain (qemu-system-x86_64 + 0x718655)
> > #5  0x00005641bcd52de8 bdrv_do_drained_begin (qemu-system-x86_64 + 0x719de8)
> > #6  0x00005641bcd47e07 blk_drain (qemu-system-x86_64 + 0x70ee07)
> > #7  0x00005641bcd498cd blk_unref (qemu-system-x86_64 + 0x7108cd)
> > #8  0x00005641bcd31e6f block_job_free (qemu-system-x86_64 + 0x6f8e6f)
> > #9  0x00005641bcd32d65 job_unref (qemu-system-x86_64 + 0x6f9d65)
> > #10 0x00005641bcd93b3d pvebackup_co_complete_stream (qemu-system-x86_64 + 0x75ab3d)
> > #11 0x00005641bce4e353 coroutine_trampoline (qemu-system-x86_64 + 0x815353)
> 
> Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>

Acked-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

> ---
> 
> Changes from v1:
>     * Fix commit message.
>     * Do di->job = NULL before releasing the context.
>     * Also keep context before job_ref.
>     * Avoid temporarily releasing/re-acquiring aio context in error
>       scenario.
> 
> Sent as a direct patch for reviewability. Intended to be squashed into
> 0055-PVE-Backup-ensure-jobs-in-di_list-are-referenced.patch
> 
>  pve-backup.c | 20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
> 
> diff --git a/pve-backup.c b/pve-backup.c
> index be21027dad..2e22030eec 100644
> --- a/pve-backup.c
> +++ b/pve-backup.c
> @@ -317,8 +317,11 @@ static void coroutine_fn pvebackup_co_complete_stream(void *opaque)
>      }
>  
>      if (di->job) {
> +        AioContext *ctx = di->job->job.aio_context;
> +        aio_context_acquire(ctx);
>          job_unref(&di->job->job);
>          di->job = NULL;
> +        aio_context_release(ctx);
>      }
>  
>      // remove self from job list
> @@ -513,13 +516,13 @@ static void create_backup_jobs_bh(void *opaque) {
>              bitmap_mode, false, NULL, &perf, BLOCKDEV_ON_ERROR_REPORT, BLOCKDEV_ON_ERROR_REPORT,
>              JOB_DEFAULT, pvebackup_complete_cb, di, backup_state.txn, &local_err);
>  
> -        aio_context_release(aio_context);
> -
>          di->job = job;
>          if (job) {
>              job_ref(&job->job);
>          }
>  
> +        aio_context_release(aio_context);
> +
>          if (!job || local_err) {
>              error_setg(errp, "backup_job_create failed: %s",
>                         local_err ? error_get_pretty(local_err) : "null");
> @@ -546,17 +549,16 @@ static void create_backup_jobs_bh(void *opaque) {
>                  di->target = NULL;
>              }
>  
> -            if (!canceled && di->job) {
> +            if (di->job) {
>                  AioContext *ctx = di->job->job.aio_context;
>                  aio_context_acquire(ctx);
> -                job_cancel_sync(&di->job->job, true);
> -                aio_context_release(ctx);
> -                canceled = true;
> -            }
> -
> -            if (di->job) {
> +                if (!canceled) {
> +                    job_cancel_sync(&di->job->job, true);
> +                    canceled = true;
> +                }
>                  job_unref(&di->job->job);
>                  di->job = NULL;
> +                aio_context_release(ctx);
>              }
>          }
>      }
> -- 
> 2.30.2