Re: [PATCH] Btrfs: fix list_add corruption and soft lockups in fsync

[Liu Bo <bo.li.liu@oracle.com>]

On Mon, Nov 20, 2017 at 06:34:34PM +0100, David Sterba wrote:
> On Mon, Nov 13, 2017 at 10:57:24AM -0700, Liu Bo wrote:
> > Xfstests btrfs/146 revealed this corruption,
> > 
> > [   58.138831] Buffer I/O error on dev dm-0, logical block 2621424, async page read
> > [   58.151233] BTRFS error (device sdf): bdev /dev/mapper/error-test errs: wr 1, rd 0, flush 0, corrupt 0, gen 0
> > [   58.152403] list_add corruption. prev->next should be next (ffff88005e6775d8), but was ffffc9000189be88. (prev=ffffc9000189be88).
> > [   58.153518] ------------[ cut here ]------------
> > [   58.153892] WARNING: CPU: 1 PID: 1287 at lib/list_debug.c:31 __list_add_valid+0x169/0x1f0
> > ...
> > [   58.157379] RIP: 0010:__list_add_valid+0x169/0x1f0
> > ...
> > [   58.161956] Call Trace:
> > [   58.162264]  btrfs_log_inode_parent+0x5bd/0xfb0 [btrfs]
> > [   58.163583]  btrfs_log_dentry_safe+0x60/0x80 [btrfs]
> > [   58.164003]  btrfs_sync_file+0x4c2/0x6f0 [btrfs]
> > [   58.164393]  vfs_fsync_range+0x5f/0xd0
> > [   58.164898]  do_fsync+0x5a/0x90
> > [   58.165170]  SyS_fsync+0x10/0x20
> > [   58.165395]  entry_SYSCALL_64_fastpath+0x1f/0xbe
> > ...
> > 
> > It turns out that we could record btrfs_log_ctx:io_err in
> > log_one_extents when IO fails, but make log_one_extents() return '0'
> > instead of -EIO, so the IO error is not acknowledged by the callers,
> > i.e.  btrfs_log_inode_parent(), which would remove btrfs_log_ctx:list
> > from list head 'root->log_ctxs'.  Since btrfs_log_ctx is allocated
> > from stack memory, it'd get freed with a object alive on the
> > list. then a future list_add will throw the above warning.
> > 
> > This returns the correct error in the above case.
> > 
> > Jeff also reported this while testing against his fsync error
> > patch set[1].
> > 
> > [1]: https://www.spinics.net/lists/linux-btrfs/msg65308.html
> > "btrfs list corruption and soft lockups while testing writeback error handling"
> > 
> 
> Fixes: 8407f553268a4611f254 ("Btrfs: fix data corruption after fast fsync and writeback error")
> 
> you can also add the commit author to CC.

Hmm, somehow I thought ctx->list hasn't been added at that time, but
looks like you're right.

> 
> > Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
> > ---
> >  fs/btrfs/file.c     | 1 +
> >  fs/btrfs/tree-log.c | 2 +-
> >  2 files changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> > index 063180b..db70eaa 100644
> > --- a/fs/btrfs/file.c
> > +++ b/fs/btrfs/file.c
> > @@ -2241,6 +2241,7 @@ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
> >  	if (ctx.io_err) {
> >  		btrfs_end_transaction(trans);
> >  		ret = ctx.io_err;
> > +		ASSERT(list_empty(&ctx.list));
> 
> Please move that to the label 'out', so all exit paths can catch the
> problem.
>

'out' can't be used as we can goto 'out' before ctx is initialized,
I'll add a new label 'out_ctx' in a separate patch (Feel free to fold
them into one if you prefer).

Thanks,

-liubo
> >  		goto out;
> >  	}
> >  
> > diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
> > index c800d06..d300284 100644
> > --- a/fs/btrfs/tree-log.c
> > +++ b/fs/btrfs/tree-log.c
> > @@ -4100,7 +4100,7 @@ static int log_one_extent(struct btrfs_trans_handle *trans,
> >  
> >  	if (ordered_io_err) {
> >  		ctx->io_err = -EIO;
> > -		return 0;
> > +		return ctx->io_err;
> >  	}
> >  
> >  	btrfs_init_map_token(&token);
> > -- 
> > 2.9.4
> > 
> > --
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