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

[Jens Axboe <axboe@kernel.dk>]

On 2/5/19 5:56 PM, Al Viro wrote:
> On Tue, Feb 05, 2019 at 12:08:25PM -0700, Jens Axboe wrote:
>> 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