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Date:   Tue, 12 Apr 2022 19:26:07 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Eric Dumazet <eric.dumazet@...il.com>, io-uring@...r.kernel.org,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCHSET 0/4] Add support for no-lock sockets

On Tue, Apr 12, 2022 at 7:19 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Tue, Apr 12, 2022 at 7:12 PM Jens Axboe <axboe@...nel.dk> wrote:
> >
> > On 4/12/22 8:05 PM, Eric Dumazet wrote:
> > > On Tue, Apr 12, 2022 at 7:01 PM Jens Axboe <axboe@...nel.dk> wrote:
> > >>
> > >> On 4/12/22 7:54 PM, Eric Dumazet wrote:
> > >>> On Tue, Apr 12, 2022 at 6:26 PM Jens Axboe <axboe@...nel.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.

FYI, include/net/sock.h has this comment, which has been served for
20+ years just fine.

/* Used by processes to "lock" a socket state, so that
 * interrupts and bottom half handlers won't change it
 * from under us. It essentially blocks any incoming
 * packets, so that we won't get any new data or any
 * packets that change the state of the socket.
 *
 * While locked, BH processing will add new packets to
 * the backlog queue.  This queue is processed by the
 * owner of the socket lock right before it is released.
 *
 * Since ~2.3.5 it is also exclusive sleep lock serializing
 * accesses from user process context.
 */

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