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Date: Wed, 16 Jan 2013 11:30:46 +0800
Message-ID: <CACVXFVM5Nr9YdN80TkotAcKff3=cZVG-R2sCiwXMJPv1gx38dg@mail.gmail.com>
Subject: Re: [PATCH] module, async: async_synchronize_full() on module init
 iff async is used
From: Ming Lei <ming.lei@canonical.com>
To: Tejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Alex Riesen <raa.lkml@gmail.com>,
        Alan Stern <stern@rowland.harvard.edu>, Jens Axboe <axboe@kernel.dk>,
        USB list <linux-usb@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Rusty Russell <rusty@rustcorp.com.au>
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On Wed, Jan 16, 2013 at 10:52 AM, Tejun Heo <tj@kernel.org> 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@kernel.org>
> Reported-by: Alex Riesen <raa.lkml@gmail.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> ---

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@canonical.com>

Thanks,
--
Ming Lei