[Qemu-block] [PATCH v10 01/16] block: Pause all jobs during bdrv_reopen_multiple()

[Alberto Garcia]

When a BlockDriverState is about to be reopened it can trigger certain
operations that need to write to disk. During this process a different
block job can be woken up. If that block job completes and also needs
to call bdrv_reopen() it can happen that it needs to do it on the same
BlockDriverState that is still in the process of being reopened.

This can have fatal consequences, like in this example:

  1) Block job A starts and sleeps after a while.
  2) Block job B starts and tries to reopen node1 (a qcow2 file).
  3) Reopening node1 means flushing and replacing its qcow2 cache.
  4) While the qcow2 cache is being flushed, job A wakes up.
  5) Job A completes and reopens node1, replacing its cache.
  6) Job B resumes, but the cache that was being flushed no longer
     exists.

This patch pauses all block jobs during bdrv_reopen_multiple(), so
that step 4 can never happen and the operation is safe.

Note that this scenario can only happen if both bdrv_reopen() calls
are made by block jobs on the same backing chain. Otherwise there's no
chance that the same BlockDriverState appears in both reopen queues.

Signed-off-by: Alberto Garcia <address@hidden>
---
 block.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/block.c b/block.c
index bb1f1ec..c80b528 100644
--- a/block.c
+++ b/block.c
@@ -2087,9 +2087,19 @@ int bdrv_reopen_multiple(BlockReopenQueue *bs_queue, 
Error **errp)
     int ret = -1;
     BlockReopenQueueEntry *bs_entry, *next;
     Error *local_err = NULL;
+    BlockJob *job = NULL;
 
     assert(bs_queue != NULL);
 
+    /* Pause all block jobs */
+    while ((job = block_job_next(job))) {
+        AioContext *aio_context = blk_get_aio_context(job->blk);
+
+        aio_context_acquire(aio_context);
+        block_job_pause(job);
+        aio_context_release(aio_context);
+    }
+
     bdrv_drain_all();
 
     QSIMPLEQ_FOREACH(bs_entry, bs_queue, entry) {
@@ -2120,6 +2130,17 @@ cleanup:
         g_free(bs_entry);
     }
     g_free(bs_queue);
+
+    /* Resume all block jobs */
+    job = NULL;
+    while ((job = block_job_next(job))) {
+        AioContext *aio_context = blk_get_aio_context(job->blk);
+
+        aio_context_acquire(aio_context);
+        block_job_resume(job);
+        aio_context_release(aio_context);
+    }
+
     return ret;
 }
 
-- 
2.9.3