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Thu, 9 Oct 2025 15:58:44 +0100 (BST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=harmstone.com; s=mail; t=1760021924; bh=xYThC8cj/xCCOy6wVjLm9mf3iVTO58ekPonFxrGiFsg=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=pVZS1RtZ03b65q4kEz5UWfQPRzK9cUzTQEHYNznbaoq14fcWRYj9C8sJonSXgBC2C TdNnVtldKUtKOqcyCkaoAa2jGM1DOt2d+fGfNr7Hief94mhBwVJSbzAzcSsNp8aPqU C+guJxS1wOWp2u72AUWl1B8E2QJskd0i9j+PvieI= Message-ID: <9e72962e-3d4b-4e1a-b206-512904d701ff@harmstone.com> Date: Thu, 9 Oct 2025 15:58:45 +0100 Precedence: bulk X-Mailing-List: linux-btrfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Subject: Re: [PATCH v3 09/17] btrfs: release BG lock before calling btrfs_link_bg_list() To: Filipe Manana Cc: linux-btrfs@vger.kernel.org References: <20251009112814.13942-1-mark@harmstone.com> <20251009112814.13942-10-mark@harmstone.com> Content-Language: en-US From: Mark Harmstone Autocrypt: addr=mark@harmstone.com; keydata= xsBNBFp/GMsBCACtFsuHZqHWpHtHuFkNZhMpiZMChyou4X8Ueur3XyF8KM2j6TKkZ5M/72qT EycEM0iU1TYVN/Rb39gBGtRclLFVY1bx4i+aUCzh/4naRxqHgzM2SeeLWHD0qva0gIwjvoRs FP333bWrFKPh5xUmmSXBtBCVqrW+LYX4404tDKUf5wUQ9bQd2ItFRM2mU/l6TUHVY2iMql6I s94Bz5/Zh4BVvs64CbgdyYyQuI4r2tk/Z9Z8M4IjEzQsjSOfArEmb4nj27R3GOauZTO2aKlM 8821rvBjcsMk6iE/NV4SPsfCZ1jvL2UC3CnWYshsGGnfd8m2v0aLFSHZlNd+vedQOTgnABEB AAHNI01hcmsgSGFybXN0b25lIDxtYXJrQGhhcm1zdG9uZS5jb20+wsCRBBMBCAA7AhsvBQsJ CAcCBhUICQoLAgQWAgMBAh4BAheAFiEEG2JgKYgV0WRwIJAqbKyhHeAWK+0FAmRQOkICGQEA CgkQbKyhHeAWK+22wgf/dBOJ0pHdkDi5fNmWynlxteBsy3VCo0qC25DQzGItL1vEY95EV4uX re3+6eVRBy9gCKHBdFWk/rtLWKceWVZ86XfTMHgy+ZnIUkrD3XZa3oIV6+bzHgQ15rXXckiE A5N+6JeY/7hAQpSh/nOqqkNMmRkHAZ1ZA/8KzQITe1AEULOn+DphERBFD5S/EURvC8jJ5hEr lQj8Tt5BvA57sLNBmQCE19+IGFmq36EWRCRJuH0RU05p/MXPTZB78UN/oGT69UAIJAEzUzVe sN3jiXuUWBDvZz701dubdq3dEdwyrCiP+dmlvQcxVQqbGnqrVARsGCyhueRLnN7SCY1s5OHK ls7ATQRafxjLAQgAvkcSlqYuzsqLwPzuzoMzIiAwfvEW3AnZxmZn9bQ+ashB9WnkAy2FZCiI /BPwiiUjqgloaVS2dIrVFAYbynqSbjqhki+uwMliz7/jEporTDmxx7VGzdbcKSCe6rkE/72o 6t7KG0r55cmWnkdOWQ965aRnRAFY7Zzd+WLqlzeoseYsNj36RMaqNR7aL7x+kDWnwbw+jgiX tgNBcnKtqmJc04z/sQTa+sUX53syht1Iv4wkATN1W+ZvQySxHNXK1r4NkcDA9ZyFA3NeeIE6 ejiO7RyC0llKXk78t0VQPdGS6HspVhYGJJt21c5vwSzIeZaneKULaxXGwzgYFTroHD9n+QAR AQABwsGsBBgBCAAgFiEEG2JgKYgV0WRwIJAqbKyhHeAWK+0FAlp/GMsCGy4BQAkQbKyhHeAW K+3AdCAEGQEIAB0WIQR6bEAu0hwk2Q9ibSlt5UHXRQtUiwUCWn8YywAKCRBt5UHXRQtUiwdE B/9OpyjmrshY40kwpmPwUfode2Azufd3QRdthnNPAY8Tv9erwsMS3sMh+M9EP+iYJh+AIRO7 fDN/u0AWIqZhHFzCndqZp8JRYULnspXSKPmVSVRIagylKew406XcAVFpEjloUtDhziBN7ykk srAMoLASaBHZpAfp8UAGDrr8Fx1on46rDxsWbh1K1h4LEmkkVooDELjsbN9jvxr8ym8Bkt54 FcpypTOd8jkt/lJRvnKXoL3rZ83HFiUFtp/ZkveZKi53ANUaqy5/U5v0Q0Ppz9ujcRA9I/V3 B66DKMg1UjiigJG6espeIPjXjw0n9BCa9jqGICyJTIZhnbEs1yEpsM87eUIH/0UFLv0b8IZe pL/3QfiFoYSqMEAwCVDFkCt4uUVFZczKTDXTFkwm7zflvRHdy5QyVFDWMyGnTN+Bq48Gwn1M uRT/Sg37LIjAUmKRJPDkVr/DQDbyL6rTvNbA3hTBu392v0CXFsvpgRNYaT8oz7DDBUUWj2Ny 6bZCBtwr/O+CwVVqWRzKDQgVo4t1xk2ts1F0R1uHHLsX7mIgfXBYdo/y4UgFBAJH5NYUcBR+ QQcOgUUZeF2MC9i0oUaHJOIuuN2q+m9eMpnJdxVKAUQcZxDDvNjZwZh+ejsgG4Ejd2XR/T0y XFoR/dLFIhf2zxRylN1xq27M9P2t1xfQFocuYToPsVk= In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 09/10/2025 12.56 pm, Filipe Manana wrote: > On Thu, Oct 9, 2025 at 12:29 PM Mark Harmstone wrote: >> >> Release block_group->lock before calling btrfs_link_bg_list() in >> btrfs_delete_unused_bgs(), as this was causing lockdep issues. > > I believe this was asked before: > > What issues? > Do we have for example any other place where we have a different > locking order and can cause a deadlock? > > Can you please paste the lockdep splat? I didn't take a copy the first time, and I've not been able to replicate it since. But you can see the issue in patch 4, "btrfs: remove remapped block groups from the free-space tree". In btrfs_discard_punt_unused_bgs_list() we acquire unused_bgs_lock to loop through the unused_bgs list, then take the individual BG lock so we can check its flags. In btrfs_delete_unused_bgs() we're acquiring the unused_bgs lock through btrfs_link_bg_list(), while still unnecessarily holding the BG lock. The reason it's in this patchset is that a minor existing bug (holding a spinlock longer than we strictly need to) becomes a potential deadlock because of patch 4. > >> >> This lock isn't held in any other place that we call btrfs_link_bg_list(), as >> the block group lists are manipulated while holding fs_info->unused_bgs_lock. >> >> Signed-off-by: Mark Harmstone >> Fixes: 0497dfba98c0 ("btrfs: codify pattern for adding block_group to bg_list") > > Also as told before, this doesn't seem related to the rest of the > patchset (the new remap tree feature). > So instead of dragging this along in every new version of the > patchset, can you please make it a standalone patch and remove it from > future versions of the patchset? > > Thanks. > >> --- >> fs/btrfs/block-group.c | 3 ++- >> 1 file changed, 2 insertions(+), 1 deletion(-) >> >> diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c >> index d3433a5b169f..a3c984f905fc 100644 >> --- a/fs/btrfs/block-group.c >> +++ b/fs/btrfs/block-group.c >> @@ -1620,6 +1620,8 @@ void btrfs_delete_unused_bgs(struct btrfs_fs_info *fs_info) >> if ((space_info->total_bytes - block_group->length < used && >> block_group->zone_unusable < block_group->length) || >> has_unwritten_metadata(block_group)) { >> + spin_unlock(&block_group->lock); >> + >> /* >> * Add a reference for the list, compensate for the ref >> * drop under the "next" label for the >> @@ -1628,7 +1630,6 @@ void btrfs_delete_unused_bgs(struct btrfs_fs_info *fs_info) >> btrfs_link_bg_list(block_group, &retry_list); >> >> trace_btrfs_skip_unused_block_group(block_group); >> - spin_unlock(&block_group->lock); >> spin_unlock(&space_info->lock); >> up_write(&space_info->groups_sem); >> goto next; >> -- >> 2.49.1 >> >>