blockjob: do not allow coroutine double entry or entry-after-completion

When block_job_sleep_ns() is called, the co-routine is scheduled for
future execution.  If we allow the job to be re-entered prior to the
scheduled time, we present a race condition in which a coroutine can be
entered recursively, or even entered after the coroutine is deleted.

The job->busy flag is used by blockjobs when a coroutine is busy
executing. The function 'block_job_enter()' obeys the busy flag,
and will not enter a coroutine if set.  If we sleep a job, we need to
leave the busy flag set, so that subsequent calls to block_job_enter()
are prevented.

This changes the prior behavior of block_job_cancel() being able to
immediately wake up and cancel a job; in practice, this should not be an
issue, as the coroutine sleep times are generally very small, and the
cancel will occur the next time the coroutine wakes up.

This fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1508708

Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 5 insertions, 2 deletions

diff --git a/blockjob.c b/blockjob.c
index 3a0c491..ff9a614 100644
--- a/blockjob.c
+++ b/blockjob.c
@@ -797,11 +797,14 @@ void block_job_sleep_ns(BlockJob *job, QEMUClockType type, int64_t ns)
         return;
     }
 
-    job->busy = false;
+    /* We need to leave job->busy set here, because when we have
+     * put a coroutine to 'sleep', we have scheduled it to run in
+     * the future.  We cannot enter that same coroutine again before
+     * it wakes and runs, otherwise we risk double-entry or entry after
+     * completion. */
     if (!block_job_should_pause(job)) {
         co_aio_sleep_ns(blk_get_aio_context(job->blk), type, ns);
     }
-    job->busy = true;
 
     block_job_pause_point(job);
 }