[PATCH] bcachefs: Fix deadlocks between fallocate and readahead

[Deepanshu Kartikey <kartikey406@gmail.com>]

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=cb91f22d8a581fc19edf
Signed-off-by: Deepanshu Kartikey <Kartikey406@gmail.com>
---
 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_inode_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 = 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 = -BCH_ERR_transaction_restart;
+				goto bkey_err;
 			}
 			bch2_btree_iter_set_pos(trans, &iter, POS(iter.pos.inode, hole_start));
 
@@ -730,11 +736,17 @@ static noinline int __bchfs_fallocate(struct bch_inode_info *inode, int mode,
 
 		if (bch2_mark_pagecache_reserved(inode, &hole_start,
 						 iter.pos.offset, true)) {
+			/* Release pagecache_block to prevent deadlock */
+			bch2_pagecache_block_put(inode);
+
 			ret = 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 = -BCH_ERR_transaction_restart;
+			goto bkey_err;
 		}
 bkey_err:
 		bch2_quota_reservation_put(c, inode, &quota_res);
-- 
2.43.0