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[Qemu-devel] [PATCH 1/5] blockjob: do not allow coroutine double entry o

From: Jeff Cody
Subject: [Qemu-devel] [PATCH 1/5] blockjob: do not allow coroutine double entry or entry-after-completion
Date: Sun, 19 Nov 2017 21:46:42 -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 fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1508708

Also, in block_job_start(), set the relevant job flags (.busy, .paused)
before creating the coroutine, not just before executing it.

Signed-off-by: Jeff Cody <address@hidden>
 blockjob.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/blockjob.c b/blockjob.c
index 3a0c491..e181295 100644
--- a/blockjob.c
+++ b/blockjob.c
@@ -291,10 +291,10 @@ void block_job_start(BlockJob *job)
     assert(job && !block_job_started(job) && job->paused &&
            job->driver && job->driver->start);
-    job->co = qemu_coroutine_create(block_job_co_entry, job);
     job->busy = true;
     job->paused = false;
+    job->co = qemu_coroutine_create(block_job_co_entry, job);
     bdrv_coroutine_enter(blk_bs(job->blk), job->co);
@@ -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;

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