Re: livelock during MD device open

From: Nicolas Schichan
Date: Wed Jan 15 2014 - 08:11:36 EST

On 01/15/2014 02:57 AM, NeilBrown wrote:
[...]
That's a very small race you are consistently losing - if I understand
correctly.

In __blkdev_get:

restart:

ret = -ENXIO;
disk = get_gendisk(bdev->bd_dev, &partno);
if (!disk)
goto out;
owner = disk->fops->owner;

disk_block_events(disk);
mutex_lock_nested(&bdev->bd_mutex, for_part);


The "get_gendisk" calls into md_alloc (via md_probe) and then add_disk(),
which generates a uevent which is handled by udev.
And before the above code gets to the mutex_lock_nexted(), the process run by
udev must have opened the device (executing all that code above and more) and
issued the ioctl.

I guess it is possible, but happening every time to produce a live-lock
suggests that the scheduler must be encouraging that behaviour. Presumably
this is a virtual machine with just one CPU ??

add_disk() will call schedule via call_usermode_helper() (for /sbin/hotplug), the scheduler will, I think, almost always choose to schedule the "udev" process instead of the process which did the first open to the md device, once the usermode helper has run.

SysRq + 'w' consistently showed a process that had called schedule() from md_alloc() -> add_disk() ... -> call_usermodehelper():


[ 57.932671] test D 805ac41c 0 748 745 0x00000000
[ 57.939075] [<805ac41c>] (__schedule+0x33c/0x3b0) from [<805aa8f0>] (schedule_timeout+0x18/0x168)
[ 57.947984] [<805aa8f0>] (schedule_timeout+0x18/0x168) from [<805abfa4>] (wait_for_common+0xf0/0x198)
[ 57.957245] [<805abfa4>] (wait_for_common+0xf0/0x198) from [<80029e30>] (call_usermodehelper_exec+0xf8/0x15c)
[ 57.967205] [<80029e30>] (call_usermodehelper_exec+0xf8/0x15c) from [<8028abd0>] (kobject_uevent_env+0x37c/0x3e8)
[ 57.977515] [<8028abd0>] (kobject_uevent_env+0x37c/0x3e8) from [<8027cff8>] (add_disk+0x29c/0x400)
[ 57.986520] [<8027cff8>] (add_disk+0x29c/0x400) from [<803bbcac>] (md_alloc+0x1cc/0x2cc)
[ 57.994645] [<803bbcac>] (md_alloc+0x1cc/0x2cc) from [<803bbe38>] (md_probe+0xc/0x14)
[ 58.002511] [<803bbe38>] (md_probe+0xc/0x14) from [<802ede10>] (kobj_lookup+0xd8/0x110)
[ 58.010550] [<802ede10>] (kobj_lookup+0xd8/0x110) from [<8027cacc>] (get_gendisk+0x2c/0xe0)
[ 58.018942] [<8027cacc>] (get_gendisk+0x2c/0xe0) from [<800c0598>] (__blkdev_get+0x28/0x364)
[ 58.027416] [<800c0598>] (__blkdev_get+0x28/0x364) from [<800c0ab0>] (blkdev_get+0x1dc/0x318)
[ 58.035983] [<800c0ab0>] (blkdev_get+0x1dc/0x318) from [<800918bc>] (do_dentry_open.isra.15+0x184/0x248)
[ 58.045510] [<800918bc>] (do_dentry_open.isra.15+0x184/0x248) from [<800919a4>] (finish_open+0x24/0x38)
[ 58.054945] [<800919a4>] (finish_open+0x24/0x38) from [<8009fb40>] (do_last+0x9a8/0xc08)
[ 58.063071] [<8009fb40>] (do_last+0x9a8/0xc08) from [<8009ffdc>] (path_openat+0x23c/0x5c8)
[ 58.071371] [<8009ffdc>] (path_openat+0x23c/0x5c8) from [<800a0620>] (do_filp_open+0x2c/0x78)
[ 58.079935] [<800a0620>] (do_filp_open+0x2c/0x78) from [<800929b4>] (do_sys_open+0x124/0x1c4)
[ 58.088498] [<800929b4>] (do_sys_open+0x124/0x1c4) from [<80009060>] (ret_fast_syscall+0x0/0x2c)

The system showing the livelock is a Marvell 88F6282 CPU (single core processor).

It took me some time start to suspect the problem was due to the md_ioctl() code :)

I suppose the best fix is, as you suggest, to avoid clearing hold_active for
invalid ioctls. It feels a bit like papering over a bug, but I think the
only way to really fix it is to add extra locking to the above code sequence
and I don't want to do that.

Of your two suggestions I much prefer the second. It will be more code, but
it is also more obviously correct. The current code is rather messy with
respect to invalid ioctl commands.

I would be happy to accept a patch which aborted md_ioctl if the cmd wasn't
one of those known to md.

I'll send a patch for that then.

Regards,


--
Nicolas Schichan
Freebox SAS
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