Re: xfs_file_splice_read: possible circular locking dependency detected

[Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>]

On Thu, Sep 8, 2016 at 11:12 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> So the problem really is that the vfs layer seems to simply not allow
> the filesystem to do any locking around the generic page cache helper
> functions. And XFS really wants to do that.

Hmm.

I wonder if we could just take the pipe lock *much* earlier at the
splice() layer? Do it before any callbacks to the low-level
filesystems, not inside the "generic" splice helpers at all?

That would clean up a ton of crap.

The *one* reason that seems impossible right now seems to be that we
use "pipe_wait()" in our splice ops. And "pipe_wait()" drops and
retakes the pipe lock over the waiting.

BUT.

What if we got rid of all the pipe-wait crap entirely, and just made
all the splice routines return EAGAIN instead of waiting? And then do
the pipe_wait() at the higher level, outside the filesystem callback
code, and outside the low-level generic helpers?

Maybe that pipe_wait() movement doesn't really work for some reason
that I didn't look at, but that would really help make the locking
enormously simpler. And then the pipe lock would *obviously* be the
outermost lock, and we'd get rid of all the issues with filesystem
lock ordering.

               Linus