linux-kernel - Re: loop: fix deadlock when sysfs and LOOP_CLR

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Date:	Sun, 31 Jul 2011 22:20:24 +0200
From:	Jens Axboe <jaxboe@...ionio.com>
To:	Kay Sievers <kay.sievers@...y.org>
CC:	Milan Broz <mbroz@...hat.com>, Tejun Heo <tj@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: loop: fix deadlock when sysfs and LOOP_CLR_FD race against each
  other

On 2011-07-30 21:19, Kay Sievers wrote:
> From: Kay Sievers <kay.sievers@...y.org>
> Subject: loop: fix deadlock when sysfs and LOOP_CLR_FD race against each other
> 
> LOOP_CLR_FD takes lo->lo_ctl_mutex and tries to remove the loop sysfs
> files. Sysfs calls show() and waits for lo->lo_ctl_mutex. LOOP_CLR_FD
> waits for show() to finish to remove the sysfs file.
> 
>   cat /sys/class/block/loop0/loop/backing_file
>     mutex_lock_nested+0x176/0x350
>     ? loop_attr_do_show_backing_file+0x2f/0xd0 [loop]
>     ? loop_attr_do_show_backing_file+0x2f/0xd0 [loop]
>     loop_attr_do_show_backing_file+0x2f/0xd0 [loop]
>     dev_attr_show+0x1b/0x60
>     ? sysfs_read_file+0x86/0x1a0
>     ? __get_free_pages+0x12/0x50
>     sysfs_read_file+0xaf/0x1a0
> 
>   ioctl(LOOP_CLR_FD):
>     wait_for_common+0x12c/0x180
>     ? try_to_wake_up+0x2a0/0x2a0
>     wait_for_completion+0x18/0x20
>     sysfs_deactivate+0x178/0x180
>     ? sysfs_addrm_finish+0x43/0x70
>     ? sysfs_addrm_start+0x1d/0x20
>     sysfs_addrm_finish+0x43/0x70
>     sysfs_hash_and_remove+0x85/0xa0
>     sysfs_remove_group+0x59/0x100
>     loop_clr_fd+0x1dc/0x3f0 [loop]
>     lo_ioctl+0x223/0x7a0 [loop]
> 
> Instead of taking the lo_ctl_mutex from sysfs code, take the inner
> lo->lo_lock, to protect the access to the backing_file data.
> 
> Thanks to Tejun for help debugging and finding a solution.

Looks good, looks like something that should have a stable tag as well?

-- 
Jens Axboe

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