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([2405:201:682f:3094:6bb3:b65d:f7d0:cab4]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-27ed6700cb6sm73743805ad.34.2025.09.27.01.25.09 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 27 Sep 2025 01:25:13 -0700 (PDT) From: Deepanshu Kartikey To: Kent Overstreet Cc: linux-bcachefs@vger.kernel.org, linux-kernel@vger.kernel.org, Deepanshu Kartikey , syzbot+cb91f22d8a581fc19edf@syzkaller.appspotmail.com, Deepanshu Kartikey Subject: [PATCH] bcachefs: Fix deadlocks between fallocate and readahead Date: Sat, 27 Sep 2025 13:55:03 +0530 Message-ID: <20250927082503.40951-1-kartikey406@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" There are ABBA deadlocks between fallocate and readahead operations at two locations in __bchfs_fallocate(): Thread 1 (fallocate): bch2_fallocate_dispatch inode_lock(&inode->v) bch2_pagecache_block_get(inode) // Acquires two_state_lock __bchfs_fallocate bch2_clamp_data_hole (or bch2_mark_pagecache_reserved) bch2_seek_pagecache_hole __filemap_get_folio folio_lock() // BLOCKS - Thread 2 holds it Thread 2 (readahead via copy_file_range): bch2_readahead folio_lock() // Holds page lock __bch2_two_state_lock(&pagecache_lock) // BLOCKS - Thread 1 holds it The issue is that drop_locks_do() only releases btree transaction locks, but Thread 2 is blocked waiting for the two_state_lock (pagecache_block) held by bch2_pagecache_block_get(). Fix by explicitly releasing and re-acquiring the pagecache_block lock around both blocking operations (bch2_clamp_data_hole and bch2_mark_pagecache_reserved), following the same pattern used in bch2_page_fault(). Force a transaction restart after lock release to ensure consistency. Reported-by: syzbot+cb91f22d8a581fc19edf@syzkaller.appspotmail.com Tested-by: syzbot+cb91f22d8a581fc19edf@syzkaller.appspotmail.com Link: https://syzkaller.appspot.com/bug?extid=3Dcb91f22d8a581fc19edf Signed-off-by: Deepanshu Kartikey --- fs/bcachefs/fs-io.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/fs/bcachefs/fs-io.c b/fs/bcachefs/fs-io.c index a233f45875e9..66a60e5f03fc 100644 --- a/fs/bcachefs/fs-io.c +++ b/fs/bcachefs/fs-io.c @@ -694,13 +694,19 @@ static noinline int __bchfs_fallocate(struct bch_inod= e_info *inode, int mode, &hole_start, &hole_end, opts.data_replicas, true)) { + /* Release pagecache_block to prevent deadlock with readahead */ + bch2_pagecache_block_put(inode); ret =3D drop_locks_do(trans, (bch2_clamp_data_hole(&inode->v, &hole_start, &hole_end, opts.data_replicas, false), 0)); + bch2_pagecache_block_get(inode); if (ret) goto bkey_err; + /* Force transaction restart to revalidate state */ + ret =3D -BCH_ERR_transaction_restart; + goto bkey_err; } bch2_btree_iter_set_pos(trans, &iter, POS(iter.pos.inode, hole_start)); =20 @@ -730,11 +736,17 @@ static noinline int __bchfs_fallocate(struct bch_inod= e_info *inode, int mode, =20 if (bch2_mark_pagecache_reserved(inode, &hole_start, iter.pos.offset, true)) { + /* Release pagecache_block to prevent deadlock */ + bch2_pagecache_block_put(inode); + ret =3D drop_locks_do(trans, bch2_mark_pagecache_reserved(inode, &hole_start, iter.pos.offset, false)); + bch2_pagecache_block_get(inode); if (ret) goto bkey_err; + ret =3D -BCH_ERR_transaction_restart; + goto bkey_err; } bkey_err: bch2_quota_reservation_put(c, inode, "a_res); --=20 2.43.0