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

From: Jan Kara
Date: Mon Mar 18 2013 - 11:39:44 EST

On Sun 17-03-13 13:06:59, David Howells wrote:
> Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> > Export do_splice_direct() to modules. Needed by overlay filesystem.
> Apparently you cannot call this from any function that is holding an i_mutex
> if the target of the splice uses generic_file_splice_write().
> The problem is a potential deadlock situation:
> We have places already that do:
> mnt_want_write()
> mutex_lock()
> This can be found in do_last() for example.
> However, mnt_want_write() calls sb_start_write() as does
> generic_file_splice_write(). So now in ovl_copy_up_locked() you're adding:
> mutex_lock()
> sb_start_write()
> which lockdep reports as a potential ABBA deadlock.
> Now, looking at __sb_start_write(), I'm not entirely sure how the deadlock
> might operate, so it's possible that this is a false alarm. Maybe Jan Kara can
> illuminate further, so I've added him to the cc list.
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:

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()

Jan Kara <jack@xxxxxxx>
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