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

Eric Dumazet
2022-04-13T02:19:40Z

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.

Jakub, David, Paolo, please provide details to Jens, thanks.