block: acquire AioContext in generic blockjob QMP commands

block-job-set-speed, block-job-cancel, block-job-pause,
block-job-resume, and block-job-complete must acquire the
BlockDriverState AioContext so that it is safe to access bs.

At the moment bs->job is always NULL when dataplane is active because op
blockers prevent blockjobs from starting.  Once the rest of the blockjob
API has been made aware of AioContext we can drop the op blocker.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Message-id: 1413889440-32577-2-git-send-email-stefanha@redhat.com

blockdev.c

@@ -2375,20 +2375,35 @@ void qmp_drive_mirror(const char *device, const char *target,
     }
 }
 
-static BlockJob *find_block_job(const char *device)
+/* Get the block job for a given device name and acquire its AioContext */
+static BlockJob *find_block_job(const char *device, AioContext **aio_context)
 {
     BlockDriverState *bs;
 
     bs = bdrv_find(device);
-    if (!bs || !bs->job) {
-        return NULL;
+    if (!bs) {
+        goto notfound;
+    }
+
+    *aio_context = bdrv_get_aio_context(bs);
+    aio_context_acquire(*aio_context);
+
+    if (!bs->job) {
+        aio_context_release(*aio_context);
+        goto notfound;
     }
+
     return bs->job;
+
+notfound:
+    *aio_context = NULL;
+    return NULL;
 }
 
 void qmp_block_job_set_speed(const char *device, int64_t speed, Error **errp)
 {
-    BlockJob *job = find_block_job(device);
+    AioContext *aio_context;
+    BlockJob *job = find_block_job(device, &aio_context);
 
     if (!job) {
         error_set(errp, QERR_BLOCK_JOB_NOT_ACTIVE, device);
@@ -2396,34 +2411,40 @@ void qmp_block_job_set_speed(const char *device, int64_t speed, Error **errp)
     }
 
     block_job_set_speed(job, speed, errp);
+    aio_context_release(aio_context);
 }
 
 void qmp_block_job_cancel(const char *device,
                           bool has_force, bool force, Error **errp)
 {
-    BlockJob *job = find_block_job(device);
-
-    if (!has_force) {
-        force = false;
-    }
+    AioContext *aio_context;
+    BlockJob *job = find_block_job(device, &aio_context);
 
     if (!job) {
         error_set(errp, QERR_BLOCK_JOB_NOT_ACTIVE, device);
         return;
     }
+
+    if (!has_force) {
+        force = false;
+    }
+
     if (job->paused && !force) {
         error_setg(errp, "The block job for device '%s' is currently paused",
                    device);
-        return;
+        goto out;
     }
 
     trace_qmp_block_job_cancel(job);
     block_job_cancel(job);
+out:
+    aio_context_release(aio_context);
 }
 
 void qmp_block_job_pause(const char *device, Error **errp)
 {
-    BlockJob *job = find_block_job(device);
+    AioContext *aio_context;
+    BlockJob *job = find_block_job(device, &aio_context);
 
     if (!job) {
         error_set(errp, QERR_BLOCK_JOB_NOT_ACTIVE, device);
@@ -2432,11 +2453,13 @@ void qmp_block_job_pause(const char *device, Error **errp)
 
     trace_qmp_block_job_pause(job);
     block_job_pause(job);
+    aio_context_release(aio_context);
 }
 
 void qmp_block_job_resume(const char *device, Error **errp)
 {
-    BlockJob *job = find_block_job(device);
+    AioContext *aio_context;
+    BlockJob *job = find_block_job(device, &aio_context);
 
     if (!job) {
         error_set(errp, QERR_BLOCK_JOB_NOT_ACTIVE, device);
@@ -2445,11 +2468,13 @@ void qmp_block_job_resume(const char *device, Error **errp)
 
     trace_qmp_block_job_resume(job);
     block_job_resume(job);
+    aio_context_release(aio_context);
 }
 
 void qmp_block_job_complete(const char *device, Error **errp)
 {
-    BlockJob *job = find_block_job(device);
+    AioContext *aio_context;
+    BlockJob *job = find_block_job(device, &aio_context);
 
     if (!job) {
         error_set(errp, QERR_BLOCK_JOB_NOT_ACTIVE, device);
@@ -2458,6 +2483,7 @@ void qmp_block_job_complete(const char *device, Error **errp)
 
     trace_qmp_block_job_complete(job);
     block_job_complete(job, errp);
+    aio_context_release(aio_context);
 }
 
 void qmp_change_backing_file(const char *device,