Re: [PATCHSET 0/4] Add support for no-lock sockets

Eric Dumazet
2022-04-13T02:32:54Z

On Tue, Apr 12, 2022 at 7:27 PM Jens Axboe <axboe@kernel.dk> wrote:
>
> On 4/12/22 8:19 PM, Eric Dumazet wrote:
> > On Tue, Apr 12, 2022 at 7:12 PM Jens Axboe <axboe@kernel.dk> wrote:
> >>
> >> On 4/12/22 8:05 PM, Eric Dumazet wrote:
> >>> On Tue, Apr 12, 2022 at 7:01 PM Jens Axboe <axboe@kernel.dk> wrote:
> >>>>
> >>>> On 4/12/22 7:54 PM, Eric Dumazet wrote:
> >>>>> On Tue, Apr 12, 2022 at 6:26 PM Jens Axboe <axboe@kernel.dk> wrote:
> >>>>>>
> >>>>>> On 4/12/22 6:40 PM, Eric Dumazet wrote:
> >>>>>>>
> >>>>>>> On 4/12/22 13:26, Jens Axboe wrote:
> >>>>>>>> Hi,
> >>>>>>>>
> >>>>>>>> If we accept a connection directly, eg without installing a file
> >>>>>>>> descriptor for it, or if we use IORING_OP_SOCKET in direct mode, then
> >>>>>>>> we have a socket for recv/send that we can fully serialize access to.
> >>>>>>>>
> >>>>>>>> With that in mind, we can feasibly skip locking on the socket for TCP
> >>>>>>>> in that case. Some of the testing I've done has shown as much as 15%
> >>>>>>>> of overhead in the lock_sock/release_sock part, with this change then
> >>>>>>>> we see none.
> >>>>>>>>
> >>>>>>>> Comments welcome!
> >>>>>>>>
> >>>>>>> How BH handlers (including TCP timers) and io_uring are going to run
> >>>>>>> safely ? Even if a tcp socket had one user, (private fd opened by a
> >>>>>>> non multi-threaded program), we would still to use the spinlock.
> >>>>>>
> >>>>>> But we don't even hold the spinlock over lock_sock() and release_sock(),
> >>>>>> just the mutex. And we do check for running eg the backlog on release,
> >>>>>> which I believe is done safely and similarly in other places too.
> >>>>>
> >>>>> So lets say TCP stack receives a packet in BH handler... it proceeds
> >>>>> using many tcp sock fields.
> >>>>>
> >>>>> Then io_uring wants to read/write stuff from another cpu, while BH
> >>>>> handler(s) is(are) not done yet,
> >>>>> and will happily read/change many of the same fields
> >>>>
> >>>> But how is that currently protected?
> >>>
> >>> It is protected by current code.
> >>>
> >>> What you wrote would break TCP stack quite badly.
> >>
> >> No offense, but your explanations are severely lacking. By "current
> >> code"? So what you're saying is that it's protected by how the code
> >> currently works? From how that it currently is? Yeah, that surely
> >> explains it.
> >>
> >>> I suggest you setup/run a syzbot server/farm, then you will have a
> >>> hundred reports quite easily.
> >>
> >> Nowhere am I claiming this is currently perfect, and it should have had
> >> an RFC on it. Was hoping for some constructive criticism on how to move
> >> this forward, as high frequency TCP currently _sucks_ in the stack.
> >> Instead I get useless replies, not very encouraging.
> >>
> >> I've run this quite extensively on just basic send/receive over sockets,
> >> so it's not like it hasn't been run at all. And it's been fine so far,
> >> no ill effects observed. If we need to tighten down the locking, perhaps
> >> a valid use would be to simply skip the mutex and retain the bh lock for
> >> setting owner. As far as I can tell, should still be safe to skip on
> >> release, except if we need to process the backlog. And it'd serialize
> >> the owner setting with the BH, which seems to be your main objection in.
> >> Mostly guessing here, based on the in-depth replies.
> >>
> >> But it'd be nice if we could have a more constructive dialogue about
> >> this, rather than the weird dismisiveness.
> >>
> >>
> >
> > Sure. It would be nice that I have not received such a patch series
> > the day I am sick.
>
> I'm sorry that you are sick - but if you are not in a state to reply,
> then please just don't. It sets a bad example. It was sent to the list,
> not to you personally.

I tried to be as constructive as possible, and Jakub pinged me about
this series,
so I really thought Jakub was okay with it.

So I am a bit concerned.

>
> Don't check email then, putting the blame on ME for posting a patchset
> while you are sick is uncalled for and rude. If I had a crystal ball, I
> would not be spending my time working on the kernel. You know what
> would've been a better idea? Replying that you are sick and that you are
> sorry for being an ass on the mailing list.

Wow.

>
> > Jakub, David, Paolo, please provide details to Jens, thanks.
>
> There's no rush here fwiw - I'm heading out on PTO rest of the week,
> so we can pick this back up when I get back. I'll check in on emails,
> but activity will be sparse.
>
> --
> Jens Axboe
>