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[Qemu-devel] [PULL 1/4] blockjob: do not allow coroutine double entry or

From: Jeff Cody
Subject: [Qemu-devel] [PULL 1/4] blockjob: do not allow coroutine double entry or entry-after-completion
Date: Tue, 21 Nov 2017 12:03:47 -0500

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 <address@hidden>
Reviewed-by: Stefan Hajnoczi <address@hidden>
 blockjob.c                   | 7 +++++--
 include/block/blockjob_int.h | 3 ++-
 2 files changed, 7 insertions(+), 3 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)
-    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;
diff --git a/include/block/blockjob_int.h b/include/block/blockjob_int.h
index f13ad05..43f3be2 100644
--- a/include/block/blockjob_int.h
+++ b/include/block/blockjob_int.h
@@ -143,7 +143,8 @@ void *block_job_create(const char *job_id, const 
BlockJobDriver *driver,
  * @ns: How many nanoseconds to stop for.
  * Put the job to sleep (assuming that it wasn't canceled) for @ns
- * nanoseconds.  Canceling the job will interrupt the wait immediately.
+ * nanoseconds.  Canceling the job will not interrupt the wait, so the
+ * cancel will not process until the coroutine wakes up.
 void block_job_sleep_ns(BlockJob *job, QEMUClockType type, int64_t ns);

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