Synchronous requet_module() from an async worker can lead to deadlock
because module init path may invoke async_synchronize_full(). The
async worker waits for request_module() to complete and the module
loading waits for the async task to finish. This bug happened in the
block layer because of default elevator auto-loading.
Block layer has been updated not to do default elevator auto-loading
and it has been decided to disallow synchronous request_module() from
async workers.
Trigger WARN_ON_ONCE() on synchronous request_module() from async
workers.
For more details, please refer to the following thread.
http://thread.gmane.org/gmane.linux.kernel/1420814
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Alex Riesen <raa.lkml@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
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