On Tue, Sep 10, 2019 at 03:26:49PM +0100, fdman...@kernel.org wrote: > From: Filipe Manana <fdman...@suse.com> > > Sometimes when fsync'ing a file we need to log that other inodes exist and > when we need to do that we acquire a reference on the inodes and then drop > that reference using iput() after logging them. > > That generally is not a problem except if we end up doing the final iput() > (dropping the last reference) on the inode and that inode has a link count > of 0, which can happen in a very short time window if the logging path > gets a reference on the inode while it's being unlinked. > > In that case we end up getting the eviction callback, btrfs_evict_inode(), > invoked through the iput() call chain which needs to drop all of the > inode's items from its subvolume btree, and in order to do that, it needs > to join a transaction at the helper function evict_refill_and_join(). > However because the task previously started a transaction at the fsync > handler, btrfs_sync_file(), it has current->journal_info already pointing > to a transaction handle and therefore evict_refill_and_join() will get > that transaction handle from btrfs_join_transaction(). From this point on, > two different problems can happen: > > 1) evict_refill_and_join() will often change the transaction handle's > block reserve (->block_rsv) and set its ->bytes_reserved field to a > value greater than 0. If evict_refill_and_join() never commits the > transaction, the eviction handler ends up decreasing the reference > count (->use_count) of the transaction handle through the call to > btrfs_end_transaction(), and after that point we have a transaction > handle with a NULL ->block_rsv (which is the value prior to the > transaction join from evict_refill_and_join()) and a ->bytes_reserved > value greater than 0. If after the eviction/iput completes the inode > logging path hits an error or it decides that it must fallback to a > transaction commit, the btrfs fsync handle, btrfs_sync_file(), gets a > non-zero value from btrfs_log_dentry_safe(), and because of that > non-zero value it tries to commit the transaction using a handle with > a NULL ->block_rsv and a non-zero ->bytes_reserved value. This makes > the transaction commit hit an assertion failure at > btrfs_trans_release_metadata() because ->bytes_reserved is not zero but > the ->block_rsv is NULL. The produced stack trace for that is like the > following: > > [192922.917158] assertion failed: !trans->bytes_reserved, file: > fs/btrfs/transaction.c, line: 816 > [192922.917553] ------------[ cut here ]------------ > [192922.917922] kernel BUG at fs/btrfs/ctree.h:3532! > [192922.918310] invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC PTI > [192922.918666] CPU: 2 PID: 883 Comm: fsstress Tainted: G W > 5.1.4-btrfs-next-47 #1 > [192922.919035] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), > BIOS rel-1.11.2-0-gf9626ccb91-prebuilt.qemu-project.org 04/01/2014 > [192922.919801] RIP: 0010:assfail.constprop.25+0x18/0x1a [btrfs] > (...) > [192922.920925] RSP: 0018:ffffaebdc8a27da8 EFLAGS: 00010286 > [192922.921315] RAX: 0000000000000051 RBX: ffff95c9c16a41c0 RCX: > 0000000000000000 > [192922.921692] RDX: 0000000000000000 RSI: ffff95cab6b16838 RDI: > ffff95cab6b16838 > [192922.922066] RBP: ffff95c9c16a41c0 R08: 0000000000000000 R09: > 0000000000000000 > [192922.922442] R10: ffffaebdc8a27e70 R11: 0000000000000000 R12: > ffff95ca731a0980 > [192922.922820] R13: 0000000000000000 R14: ffff95ca84c73338 R15: > ffff95ca731a0ea8 > [192922.923200] FS: 00007f337eda4e80(0000) GS:ffff95cab6b00000(0000) > knlGS:0000000000000000 > [192922.923579] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > [192922.923948] CR2: 00007f337edad000 CR3: 00000001e00f6002 CR4: > 00000000003606e0 > [192922.924329] DR0: 0000000000000000 DR1: 0000000000000000 DR2: > 0000000000000000 > [192922.924711] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: > 0000000000000400 > [192922.925105] Call Trace: > [192922.925505] btrfs_trans_release_metadata+0x10c/0x170 [btrfs] > [192922.925911] btrfs_commit_transaction+0x3e/0xaf0 [btrfs] > [192922.926324] btrfs_sync_file+0x44c/0x490 [btrfs] > [192922.926731] do_fsync+0x38/0x60 > [192922.927138] __x64_sys_fdatasync+0x13/0x20 > [192922.927543] do_syscall_64+0x60/0x1c0 > [192922.927939] entry_SYSCALL_64_after_hwframe+0x49/0xbe > (...) > [192922.934077] ---[ end trace f00808b12068168f ]--- > > 2) If evict_refill_and_join() decides to commit the transaction, it will > be able to do it, since the nested transaction join only increments the > transaction handle's ->use_count reference counter and it does not > prevent the transaction from getting committed. This means that after
This brings up a good point, we should probably not allow the commit in this case, or add an ASSERT(use_count == 1) or something, cause this would be bad. Thanks, Josef
