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Wed, 14 Jan 2026 19:29:45 +0000 Received: from SA1PR15MB5819.namprd15.prod.outlook.com ([fe80::920c:d2ba:5432:b539]) by SA1PR15MB5819.namprd15.prod.outlook.com ([fe80::920c:d2ba:5432:b539%4]) with mapi id 15.20.9520.003; Wed, 14 Jan 2026 19:29:45 +0000 From: Viacheslav Dubeyko To: "wangjinchao600@gmail.com" CC: "glaubitz@physik.fu-berlin.de" , "frank.li@vivo.com" , "slava@dubeyko.com" , "linux-kernel@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "syzbot+1e3ff4b07c16ca0f6fe2@syzkaller.appspotmail.com" Thread-Topic: [EXTERNAL] Re: [RFC PATCH] fs/hfs: fix ABBA deadlock in hfs_mdb_commit Thread-Index: AQHchQJlsj80quEwJ0uCe1jfsMW4jLVSDkAA Date: Wed, 14 Jan 2026 19:29:45 +0000 Message-ID: References: <68b0240f.a00a0220.1337b0.0006.GAE@google.com> <20260113081952.2431735-1-wangjinchao600@gmail.com> In-Reply-To: Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-publictraffictype: Email x-ms-traffictypediagnostic: SA1PR15MB5819:EE_|PH0PR15MB4527:EE_ x-ms-office365-filtering-correlation-id: 62a2dea0-ce46-42f8-5354-08de53a34651 x-ms-exchange-senderadcheck: 1 x-ms-exchange-antispam-relay: 0 x-microsoft-antispam: BCL:0;ARA:13230040|376014|10070799003|366016|1800799024|38070700021; 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charset="utf-8" Content-ID: <7C2F37950050494C96B05C449BDCFCC0@namprd15.prod.outlook.com> Content-Transfer-Encoding: quoted-printable Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: RE: [RFC PATCH] fs/hfs: fix ABBA deadlock in hfs_mdb_commit X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1121,Hydra:6.1.9,FMLib:17.12.100.49 definitions=2026-01-14_06,2026-01-14_01,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 suspectscore=0 bulkscore=0 spamscore=0 impostorscore=0 adultscore=0 malwarescore=0 phishscore=0 clxscore=1015 lowpriorityscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=2 engine=8.19.0-2512120000 definitions=main-2601140160 On Wed, 2026-01-14 at 11:03 +0800, Jinchao Wang wrote: > On Tue, Jan 13, 2026 at 08:52:45PM +0000, Viacheslav Dubeyko wrote: > > On Tue, 2026-01-13 at 16:19 +0800, Jinchao Wang wrote: > > > syzbot reported a hung task in hfs_mdb_commit where a deadlock occurs > > > between the MDB buffer lock and the folio lock. > > >=20 > > > The deadlock happens because hfs_mdb_commit() holds the mdb_bh > > > lock while calling sb_bread(), which attempts to acquire the lock > > > on the same folio. > >=20 > > I don't quite to follow to your logic. We have only one sb_bread() [1] = in > > hfs_mdb_commit(). This read is trying to extract the volume bitmap. How= is it > > possible that superblock and volume bitmap is located at the same folio= ? Are you > > sure? Which size of the folio do you imply here? > >=20 > > Also, it your logic is correct, then we never could be able to mount/un= mount or > > run any operations on HFS volumes because of likewise deadlock. However= , I can > > run xfstests on HFS volume. > >=20 > > [1] https://elixir.bootlin.com/linux/v6.19-rc5/source/fs/hfs/mdb.c#L324= =20 >=20 > Hi Viacheslav, >=20 > After reviewing your feedback, I realized that my previous RFC was not in > the correct format. It was not intended to be a final, merge-ready patch, > but rather a record of the analysis and trial fixes conducted so far. > I apologize for the confusion caused by my previous email. >=20 > The details are reorganized as follows: >=20 > - Observation > - Analysis > - Verification > - Conclusion >=20 > Observation > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D >=20 > Syzbot report: https://syzkaller.appspot.com/bug?extid=3D1e3ff4b07c16ca0f= 6fe2 =20 >=20 > For this version: > > time | kernel | Commit | Syzkaller | > > 2025/12/20 17:03 | linux-next | cc3aa43b44bd | d6526ea3 | >=20 > Crash log: https://syzkaller.appspot.com/text?tag=3DCrashLog&x=3D12909b1a= 580000 =20 >=20 > The report indicates hung tasks within the hfs context. >=20 > Analysis > =3D=3D=3D=3D=3D=3D=3D=3D > In the crash log, the lockdep information requires adjustment based on th= e call stack. > After adjustment, a deadlock is identified: >=20 > task syz.1.1902:8009 > - held &disk->open_mutex > - held foio lock > - wait lock_buffer(bh) > Partial call trace: > ->blkdev_writepages() > ->writeback_iter() > ->writeback_get_folio() > ->folio_lock(folio) > ->block_write_full_folio() > __block_write_full_folio() > ->lock_buffer(bh) >=20 > task syz.0.1904:8010 > - held &type->s_umount_key#66 down_read > - held lock_buffer(HFS_SB(sb)->mdb_bh); > - wait folio > Partial call trace: > hfs_mdb_commit > ->lock_buffer(HFS_SB(sb)->mdb_bh); > ->bh =3D sb_bread(sb, block); > ...->folio_lock(folio) >=20 >=20 > Other hung tasks are secondary effects of this deadlock. The issue > is reproducible in my local environment usuing the syz-reproducer. >=20 > Verification > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D >=20 > Two patches are verified against the syz-reproducer. > Neither reproduce the deadlock. >=20 > Option 1: Removing `un/lock_buffer(HFS_SB(sb)->mdb_bh)` > ------------------------------------------------------ >=20 > diff --git a/fs/hfs/mdb.c b/fs/hfs/mdb.c > index 53f3fae60217..c641adb94e6f 100644 > --- a/fs/hfs/mdb.c > +++ b/fs/hfs/mdb.c > @@ -268,7 +268,6 @@ void hfs_mdb_commit(struct super_block *sb) > if (sb_rdonly(sb)) > return; >=20 > - lock_buffer(HFS_SB(sb)->mdb_bh); > if (test_and_clear_bit(HFS_FLG_MDB_DIRTY, &HFS_SB(sb)->flags)) { > /* These parameters may have been modified, so write them= back */ > mdb->drLsMod =3D hfs_mtime(); > @@ -340,7 +339,6 @@ void hfs_mdb_commit(struct super_block *sb) > size -=3D len; > } > } > - unlock_buffer(HFS_SB(sb)->mdb_bh); > } >=20 >=20 > Options 2: Moving `unlock_buffer(HFS_SB(sb)->mdb_bh)` > -------------------------------------------------------- >=20 > diff --git a/fs/hfs/mdb.c b/fs/hfs/mdb.c > index 53f3fae60217..ec534c630c7e 100644 > --- a/fs/hfs/mdb.c > +++ b/fs/hfs/mdb.c > @@ -309,6 +309,7 @@ void hfs_mdb_commit(struct super_block *sb) > sync_dirty_buffer(HFS_SB(sb)->alt_mdb_bh); > } > =20 > + unlock_buffer(HFS_SB(sb)->mdb_bh); > if (test_and_clear_bit(HFS_FLG_BITMAP_DIRTY, &HFS_SB(sb)->flags))= { > struct buffer_head *bh; > sector_t block; > @@ -340,7 +341,6 @@ void hfs_mdb_commit(struct super_block *sb) > size -=3D len; > } > } > - unlock_buffer(HFS_SB(sb)->mdb_bh); > } >=20 > Conclusion > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D >=20 > The analysis and verification confirms that the hung tasks are caused by > the deadlock between `lock_buffer(HFS_SB(sb)->mdb_bh)` and `sb_bread(sb, = block)`. First of all, we need to answer this question: How is it possible that superblock and volume bitmap is located at the same folio or logical block? In normal case, the superblock and volume bitmap should not = be located in the same logical block. It sounds to me that you have corrupted volume and this is why this logic [1] finally overlap with superblock locat= ion: block =3D be16_to_cpu(HFS_SB(sb)->mdb->drVBMSt) + HFS_SB(sb)->part_start; off =3D (block << HFS_SECTOR_SIZE_BITS) & (sb->s_blocksize - 1); block >>=3D sb->s_blocksize_bits - HFS_SECTOR_SIZE_BITS; I assume that superblock is corrupted and the mdb->drVBMSt [2] has incorrect metadata. As a result, we have this deadlock situation. The fix should be n= ot here but we need to add some sanity check of mdb->drVBMSt somewhere in hfs_fill_super() workflow. Could you please check my vision? Thanks, Slava. [1] https://elixir.bootlin.com/linux/v6.19-rc5/source/fs/hfs/mdb.c#L318 [2] https://elixir.bootlin.com/linux/v6.19-rc5/source/include/linux/hfs_common.= h#L196