workqueue: improve destroy_workqueue() debuggability

Now that the worklist is global, having works pending after wq
destruction can easily lead to oops and destroy_workqueue() have
several BUG_ON()s to catch these cases.  Unfortunately, BUG_ON()
doesn't tell much about how the work became pending after the final
flush_workqueue().

This patch adds WQ_DYING which is set before the final flush begins.
If a work is requested to be queued on a dying workqueue,
WARN_ON_ONCE() is triggered and the request is ignored.  This clearly
indicates which caller is trying to queue a work on a dying workqueue
and keeps the system working in most cases.

Locking rule comment is updated such that the 'I' rule includes
modifying the field from destruction path.

Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index 4f9d277bcd9a..c959666eafca 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -241,6 +241,8 @@ enum {
         WQ_HIGHPRI              = 1 << 4, /* high priority */
         WQ_CPU_INTENSIVE        = 1 << 5, /* cpu instensive workqueue */
 
+        WQ_DYING                = 1 << 6, /* internal: workqueue is dying */
+
         WQ_MAX_ACTIVE           = 512,    /* I like 512, better ideas? */
         WQ_MAX_UNBOUND_PER_CPU  = 4,      /* 4 * #cpus for unbound wq */
         WQ_DFL_ACTIVE           = WQ_MAX_ACTIVE / 2,