All of lore.kernel.org
 help / color / mirror / Atom feed
From: David Sterba <dsterba@suse.cz>
To: Josef Bacik <josef@toxicpanda.com>
Cc: dsterba@suse.cz, linux-btrfs@vger.kernel.org, kernel-team@fb.com
Subject: Re: [PATCH v2 2/7] btrfs: do not take the uuid_mutex in btrfs_rm_device
Date: Fri, 17 Sep 2021 16:33:17 +0200	[thread overview]
Message-ID: <20210917143317.GU9286@twin.jikos.cz> (raw)
In-Reply-To: <bfd5a7be-da05-62de-997e-2e513c606915@toxicpanda.com>

On Thu, Sep 02, 2021 at 10:10:04AM -0400, Josef Bacik wrote:
> > This is a bit hand wavy but the critical part of the correctness proof,
> > and it's not explaining it enough IMO. The important piece happens in
> > device_list_add, the fs_devices lookup and EBUSY, but all that is now
> > excluded completely by the uuid_mutex from running in parallel with any
> > part of rm_device.
> > 
> > This means that the state of the device is seen complete by each (scan,
> > rm device). Without the uuid mutex the scaning can find the signature,
> > then try to lookup the device in the list, while in parallel the rm
> > device changes the signature or manipulates the list. But not everything
> > is covered by the device list mutex so there are combinations of both
> > tasks with some in-progress state.  Also count in the RCU protection.
> > 
> >  From high level it is what you say about ordering scan/scratch, but
> > otherwise I'm not convinced that the change is not subtly breaking
> > something.
> > 
> 
> Yeah this is far from ideal, we really need to rework our entire device 
> liftetime handling and locking, however this isn't going to break 
> anything.  We are worried about rm and scan racing with each other, 
> before this change we'll zero the device out under the UUID mutex so 
> when scan does run it'll make sure that it can go through the whole 
> device scan thing without rm messing with us.
> 
> We aren't worried if the scratch happens first, because the result is we 
> don't think this is a btrfs device and we bail out.
> 
> The only case we are concerned with is we scratch _after_ scan is able 
> to read the superblock and gets a seemingly valid super block, so lets 
> consider this case.
> 
> Scan will call device_list_add() with the device we're removing.  We'll 
> call find_fsid_with_metadata_uuid() and get our fs_devices for this 
> UUID.  At this point we lock the fs_devices->device_list_mutex.  This is 
> what protects us in this case, but we have two cases here.
> 
> 1. We aren't to the device removal part of the RM.  We found our device, 
> and device name matches our path, we go down and we set total_devices to 
> our super number of devices, which doesn't affect anything because we 
> haven't done the remove yet.
> 
> 2. We are past the device removal part, which is protected by the 
> device_list_mutex.  Scan doesn't find the device, it goes down and does the
> 
> if (fs_devices->opened)
> 	return -EBUSY;
> 
> check and we bail out.
> 
> Nothing about this situation is ideal, but the lockdep splat is real, 
> and the fix is safe, tho admittedly a bit scary looking.  Thanks,

Thanks, reading the code a few more times I tend to agree, I've added
this another explanation to the changelog.

  reply	other threads:[~2021-09-17 14:33 UTC|newest]

Thread overview: 39+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-27 21:01 [PATCH v2 0/7] Josef Bacik
2021-07-27 21:01 ` [PATCH v2 1/7] btrfs: do not call close_fs_devices in btrfs_rm_device Josef Bacik
2021-09-01  8:13   ` Anand Jain
2021-07-27 21:01 ` [PATCH v2 2/7] btrfs: do not take the uuid_mutex " Josef Bacik
2021-09-01 12:01   ` Anand Jain
2021-09-01 17:08     ` David Sterba
2021-09-01 17:10     ` Josef Bacik
2021-09-01 19:49       ` Anand Jain
2021-09-02 12:58   ` David Sterba
2021-09-02 14:10     ` Josef Bacik
2021-09-17 14:33       ` David Sterba [this message]
2021-09-20  7:45   ` Anand Jain
2021-09-20  8:26     ` David Sterba
2021-09-20  9:41       ` Anand Jain
2021-09-23  4:33         ` Anand Jain
2021-09-21 11:59   ` Filipe Manana
2021-09-21 12:17     ` Filipe Manana
2021-09-22 15:33       ` Filipe Manana
2021-09-23  4:15         ` Anand Jain
2021-09-23  3:58   ` [PATCH] btrfs: drop lockdep assert in close_fs_devices() Anand Jain
2021-09-23  4:04     ` Anand Jain
2021-07-27 21:01 ` [PATCH v2 3/7] btrfs: do not read super look for a device path Josef Bacik
2021-08-25  2:00   ` Anand Jain
2021-09-27 15:32     ` Josef Bacik
2021-09-28 11:50       ` Anand Jain
2021-07-27 21:01 ` [PATCH v2 4/7] btrfs: update the bdev time directly when closing Josef Bacik
2021-08-25  0:35   ` Anand Jain
2021-09-02 12:16   ` David Sterba
2021-07-27 21:01 ` [PATCH v2 5/7] btrfs: delay blkdev_put until after the device remove Josef Bacik
2021-08-25  1:00   ` Anand Jain
2021-09-02 12:16   ` David Sterba
2021-07-27 21:01 ` [PATCH v2 6/7] btrfs: unify common code for the v1 and v2 versions of " Josef Bacik
2021-08-25  1:19   ` Anand Jain
2021-09-01 14:05   ` Nikolay Borisov
2021-07-27 21:01 ` [PATCH v2 7/7] btrfs: do not take the device_list_mutex in clone_fs_devices Josef Bacik
2021-08-24 22:08   ` Anand Jain
2021-09-01 13:35   ` Nikolay Borisov
2021-09-02 12:59   ` David Sterba
2021-09-17 15:06 ` [PATCH v2 0/7] David Sterba

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20210917143317.GU9286@twin.jikos.cz \
    --to=dsterba@suse.cz \
    --cc=josef@toxicpanda.com \
    --cc=kernel-team@fb.com \
    --cc=linux-btrfs@vger.kernel.org \
    --subject='Re: [PATCH v2 2/7] btrfs: do not take the uuid_mutex in btrfs_rm_device' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.