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

[Fabian Ebner]

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 at 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