Re: [PATCH 2/9] vfs: export do_splice_direct() to modules

[Al Viro]

On Mon, Mar 18, 2013 at 04:39:36PM +0100, Jan Kara wrote:
>   IMO the deadlock is real. In freeze_super() we wait for all writers to
> the filesystem to finish while blocking beginning of any further writes. So
> we have a deadlock scenario like:
> 
>   THREAD1		THREAD2				THREAD3
> mnt_want_write()	mutex_lock(&inode->i_mutex);
> ...							freeze_super()
> block on mutex_lock(&inode->i_mutex)
> 							  sb_wait_write(sb, SB_FREEZE_WRITE);
> 			block in sb_start_write()

The bug is on fsfreeze side and this is not the only problem related to it.
I've missed the implications when I applied "fs: Add freezing handling
to mnt_want_write() / mnt_drop_write()" last June ;-/

The thing is, until then mnt_want_write() used to be a counter; it could be
nested.  Now any such nesting is a deadlock you've just described.  This
is seriously wrong, IMO.

BTW, having sb_start_write() buried in individual ->splice_write() is
asking for trouble; could you describe the rules for that?  E.g. where
does it nest wrt filesystem-private locks?  XFS iolock, for example...
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