Re: [PATCH 13/18] io_uring: add file set registration

On 2/5/19 5:56 PM, Al Viro wrote:

On Tue, Feb 05, 2019 at 12:08:25PM -0700, Jens Axboe wrote:

quoted

Proof is in the pudding, here's the main commit introducing io_uring
and now wiring it up to the AF_UNIX garbage collection:

http://git.kernel.dk/cgit/linux-block/commit/?h=io_uring&id=158e6f42b67d0abe9ee84886b96ca8c4b3d3dfd5

How does that look?

In a word - wrong.  Some theory: garbage collector assumes that there is
a subset of file references such that
	* for all files with such references there's an associated unix_sock.
	* all such references are stored in SCM_RIGHTS datagrams that can be
found by the garbage collector (currently: for data-bearing AF_UNIX sockets -
queued SCM_RIGHTS datagrams, for listeners - SCM_RIGHTS datagrams sent via
yet-to-be-accepted connections).
	* there is an efficient way to count those references for given file
(->inflight of the corresponding unix_sock).
	* removal of those references would render the graph acyclic.
	* file can _NOT_ be subject to syscalls unless there are references
to it outside of that subset.

IOW, we cannot use fget() for registering files, and we still need fget/fput
in the fast path to retain safe use of the file. If I'm understanding you
correctly?

Just trying to ensure that I understand what you're saying here, as it seems
to refer to the file registration part, not the main patch (which did get
reworked, though).

-- 
Jens Axboe