On Wed, Jan 16, 2013 at 10:52 AM, Tejun Heo <tj@xxxxxxxxxx> wrote: > If the default iosched is built as module, the kernel may deadlock > while trying to load the iosched module on device probe if the probing > was running off async. This is because async_synchronize_full() at > the end of module init ends up waiting for the async job which > initiated the module loading. > > async A modprobe > > 1. finds a device > 2. registers the block device > 3. request_module(default iosched) > 4. modprobe in userland > 5. load and init module > 6. async_synchronize_full() > > Async A waits for modprobe to finish in request_module() and modprobe > waits for async A to finish in async_synchronize_full(). > > Because there's no easy to track dependency once control goes out to > userland, implementing properly nested flushing is difficult. For > now, make module init perform async_synchronize_full() iff module init > has queued async jobs as suggested by Linus. > > This avoids the described deadlock because iosched module doesn't use > async and thus wouldn't invoke async_synchronize_full(). This is > hacky and incomplete. It will deadlock if async module loading nests; > however, this works around the known problem case and seems to be the > best of bad options. > > For more details, please refer to the following thread. > > http://thread.gmane.org/gmane.linux.kernel/1420814 > > Signed-off-by: Tejun Heo <tj@xxxxxxxxxx> > Reported-by: Alex Riesen <raa.lkml@xxxxxxxxx> > Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > --- Looks it does fix the deadlock problem on my Pandaboard, also the scsi disk device node(/dev/sdX) comes just after loading module of 'sd_mod'. Tested-by: Ming Lei <ming.lei@xxxxxxxxxxxxx> Thanks, -- Ming Lei -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
