Re: [PATCH] Btrfs: fix crash when starting transaction

[Miao Xie <miaox@cn.fujitsu.com>]

CC Josef

On Mon, 23 Jun 2014 12:58:59 +0100, Filipe David Borba Manana wrote:
> Often when starting a transaction we commit the currently running transaction,
> which can end up writing block group caches when the current process has its
> journal_info set to NULL (and not to a transaction). This makes our assertion
> at btrfs_check_data_free_space() (current_journal != NULL) fail, resulting
> in a crash/hang. Therefore fix it by setting journal_info.
> 
> Two different traces of this issue follow below.
> 
> 1)
> 
>     [51502.241936] BTRFS: assertion failed: current->journal_info, file: fs/btrfs/extent-tree.c, line: 3670
>     [51502.242213] ------------[ cut here ]------------
>     [51502.242493] kernel BUG at fs/btrfs/ctree.h:3964!
>     [51502.242669] invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC
>     (...)
>     [51502.244010] Call Trace:
>     [51502.244010]  [<ffffffffa02bc025>] btrfs_check_data_free_space+0x395/0x3a0 [btrfs]
>     [51502.244010]  [<ffffffffa02c3bdc>] btrfs_write_dirty_block_groups+0x4ac/0x640 [btrfs]
>     [51502.244010]  [<ffffffffa0357a6a>] commit_cowonly_roots+0x164/0x226 [btrfs]
>     [51502.244010]  [<ffffffffa02d53cd>] btrfs_commit_transaction+0x4ed/0xab0 [btrfs]
>     [51502.244010]  [<ffffffff8168ec7b>] ? _raw_spin_unlock+0x2b/0x40
>     [51502.244010]  [<ffffffffa02d6259>] start_transaction+0x459/0x620 [btrfs]
>     [51502.244010]  [<ffffffffa02d67ab>] btrfs_start_transaction+0x1b/0x20 [btrfs]
>     [51502.244010]  [<ffffffffa02d73e1>] __unlink_start_trans+0x31/0xe0 [btrfs]
>     [51502.244010]  [<ffffffffa02dea67>] btrfs_unlink+0x37/0xc0 [btrfs]
>     [51502.244010]  [<ffffffff811bb054>] ? do_unlinkat+0x114/0x2a0
>     [51502.244010]  [<ffffffff811baebc>] vfs_unlink+0xcc/0x150
>     [51502.244010]  [<ffffffff811bb1a0>] do_unlinkat+0x260/0x2a0
>     [51502.244010]  [<ffffffff811a9ef4>] ? filp_close+0x64/0x90
>     [51502.244010]  [<ffffffff810aaea6>] ? trace_hardirqs_on_caller+0x16/0x1e0
>     [51502.244010]  [<ffffffff81349cab>] ? trace_hardirqs_on_thunk+0x3a/0x3f
>     [51502.244010]  [<ffffffff811be9eb>] SyS_unlinkat+0x1b/0x40
>     [51502.244010]  [<ffffffff81698452>] system_call_fastpath+0x16/0x1b
>     [51502.244010] Code: 0b 55 48 89 e5 0f 0b 55 48 89 e5 0f 0b 55 89 f1 48 c7 c2 71 13 36 a0 48 89 fe 31 c0 48 c7 c7 b8 43 36 a0 48 89 e5 e8 5d b0 32 e1 <0f> 0b 0f 1f 44 00 00 55 b9 11 00 00 00 48 89 e5 41 55 49 89 f5
>     [51502.244010] RIP  [<ffffffffa03575da>] assfail.constprop.88+0x1e/0x20 [btrfs]
> 
> 2)
> 
>     [25405.097230] BTRFS: assertion failed: current->journal_info, file: fs/btrfs/extent-tree.c, line: 3670
>     [25405.097488] ------------[ cut here ]------------
>     [25405.097767] kernel BUG at fs/btrfs/ctree.h:3964!
>     [25405.097940] invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC
>     (...)
>     [25405.100008] Call Trace:
>     [25405.100008]  [<ffffffffa02bc025>] btrfs_check_data_free_space+0x395/0x3a0 [btrfs]
>     [25405.100008]  [<ffffffffa02c3bdc>] btrfs_write_dirty_block_groups+0x4ac/0x640 [btrfs]
>     [25405.100008]  [<ffffffffa035755a>] commit_cowonly_roots+0x164/0x226 [btrfs]
>     [25405.100008]  [<ffffffffa02d53cd>] btrfs_commit_transaction+0x4ed/0xab0 [btrfs]
>     [25405.100008]  [<ffffffff8109c170>] ? bit_waitqueue+0xc0/0xc0
>     [25405.100008]  [<ffffffffa02d6259>] start_transaction+0x459/0x620 [btrfs]
>     [25405.100008]  [<ffffffffa02d67ab>] btrfs_start_transaction+0x1b/0x20 [btrfs]
>     [25405.100008]  [<ffffffffa02e3407>] btrfs_create+0x47/0x210 [btrfs]
>     [25405.100008]  [<ffffffffa02d74cc>] ? btrfs_permission+0x3c/0x80 [btrfs]
>     [25405.100008]  [<ffffffff811bc63b>] vfs_create+0x9b/0x130
>     [25405.100008]  [<ffffffff811bcf19>] do_last+0x849/0xe20
>     [25405.100008]  [<ffffffff811b9409>] ? link_path_walk+0x79/0x820
>     [25405.100008]  [<ffffffff811bd5b5>] path_openat+0xc5/0x690
>     [25405.100008]  [<ffffffff810ab07d>] ? trace_hardirqs_on+0xd/0x10
>     [25405.100008]  [<ffffffff811cdcd2>] ? __alloc_fd+0x32/0x1d0
>     [25405.100008]  [<ffffffff811be2a3>] do_filp_open+0x43/0xa0
>     [25405.100008]  [<ffffffff811cddf1>] ? __alloc_fd+0x151/0x1d0
>     [25405.100008]  [<ffffffff811abcfc>] do_sys_open+0x13c/0x230
>     [25405.100008]  [<ffffffff810aaea6>] ? trace_hardirqs_on_caller+0x16/0x1e0
>     [25405.100008]  [<ffffffff811abe12>] SyS_open+0x22/0x30
>     [25405.100008]  [<ffffffff81698452>] system_call_fastpath+0x16/0x1b
>     [25405.100008] Code: 0b 55 48 89 e5 0f 0b 55 48 89 e5 0f 0b 55 89 f1 48 c7 c2 51 13 36 a0 48 89 fe 31 c0 48 c7 c7 d0 43 36 a0 48 89 e5 e8 6d b5 32 e1 <0f> 0b 0f 1f 44 00 00 55 b9 11 00 00 00 48 89 e5 41 55 49 89 f5
>     [25405.100008] RIP  [<ffffffffa03570ca>] assfail.constprop.88+0x1e/0x20 [btrfs]
> 
> Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
> ---
>  fs/btrfs/transaction.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
> index ac984a3..fe4abe9 100644
> --- a/fs/btrfs/transaction.c
> +++ b/fs/btrfs/transaction.c
> @@ -491,7 +491,11 @@ again:
>  	smp_mb();
>  	if (cur_trans->state >= TRANS_STATE_BLOCKED &&
>  	    may_wait_transaction(root, type)) {
> +		void *journal_info = current->journal_info;
> +	        if (!journal_info)
> +			current->journal_info = h;
>  		btrfs_commit_transaction(h, root);
> +		current->journal_info = journal_info;
>  		goto again;

It seems it is impossible that the task who set BTRFS_SEND_TRANS_STUB would start a transaction,
that is if current->journal_info is not NULL, we are sure there is a transaction handle in it,
so we just join that handle and then go out. In other words, current->journal_info here must be NULL,
so the if sentence is unnecessary, 
 	smp_mb();
 	if (cur_trans->state >= TRANS_STATE_BLOCKED &&
 	    may_wait_transaction(root, type)) {
+		current->journal_info = h;
 		btrfs_commit_transaction(h, root);
 		goto again;

is OK.

Maybe there is something wrong in what I said, if so, please correct me.

Thanks
Miao