linux-kernel - Re: [PATCH] block: make gendisk hold a reference to its queue

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Date:	Mon, 17 Oct 2011 13:44:09 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Tejun Heo <tj@...nel.org>
CC:	stable@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: make gendisk hold a reference to its queue

On 2011-10-17 04:43, Tejun Heo wrote:
> The following command sequence triggers an oops.
> 
> # mount /dev/sdb1 /mnt
> # echo 1 > /sys/class/scsi_device/0\:0\:1\:0/device/delete
> # umount /mnt
> 
>  general protection fault: 0000 [#1] PREEMPT SMP 
>  CPU 2 
>  Modules linked in:
>  
>  Pid: 791, comm: umount Not tainted 3.1.0-rc3-work+ #8 Bochs Bochs
>  RIP: 0010:[<ffffffff810d0879>]  [<ffffffff810d0879>] __lock_acquire+0x389/0x1d60
> ...
>  Call Trace:
>   [<ffffffff810d2845>] lock_acquire+0x95/0x140
>   [<ffffffff81aed87b>] _raw_spin_lock+0x3b/0x50
>   [<ffffffff811573bc>] bdi_lock_two+0x5c/0x70
>   [<ffffffff811c2f6c>] bdev_inode_switch_bdi+0x4c/0xf0
>   [<ffffffff811c3fcb>] __blkdev_put+0x11b/0x1d0
>   [<ffffffff811c4010>] __blkdev_put+0x160/0x1d0
>   [<ffffffff811c40df>] blkdev_put+0x5f/0x190
>   [<ffffffff8118f18d>] kill_block_super+0x4d/0x80
>   [<ffffffff8118f4a5>] deactivate_locked_super+0x45/0x70
>   [<ffffffff8119003a>] deactivate_super+0x4a/0x70
>   [<ffffffff811ac4ad>] mntput_no_expire+0xed/0x130
>   [<ffffffff811acf2e>] sys_umount+0x7e/0x3a0
>   [<ffffffff81aeeeab>] system_call_fastpath+0x16/0x1b
> 
> This is because bdev holds on to disk but disk doesn't pin the
> associated queue.  If a SCSI device is removed while the device is
> still open, the sdev puts the base reference to the queue on release.
> When the bdev is finally released, the associated queue is already
> gone along with the bdi and bdev_inode_switch_bdi() ends up
> dereferencing already freed bdi.
> 
> Even if it were not for this bug, disk not holding onto the associated
> queue is very unusual and error-prone.
> 
> Fix it by making add_disk() take an extra reference to its queue and
> put it on disk_release() and ensuring that disk and its fops owner are
> put in that order after all accesses to the disk and queue are
> complete.

Thanks, applied.

-- 
Jens Axboe

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