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