Re: [PATCH] net: Do not break out of sk_stream_wait_memory() with TIF_NOTIFY_SIGNAL

[Sascha Hauer]

On Tue, Mar 19, 2024 at 01:55:21PM +0000, Pavel Begunkov wrote:
> On 3/19/24 10:50, Sascha Hauer wrote:
> > On Mon, Mar 18, 2024 at 01:19:19PM +0000, Pavel Begunkov wrote:
> > > On 3/18/24 12:10, Sascha Hauer wrote:
> > > > On Fri, Mar 15, 2024 at 05:02:05PM +0000, Pavel Begunkov wrote:
> > > > > On 3/15/24 10:01, Sascha Hauer wrote:
> > > > > > It can happen that a socket sends the remaining data at close() time.
> > > > > > With io_uring and KTLS it can happen that sk_stream_wait_memory() bails
> > > > > > out with -512 (-ERESTARTSYS) because TIF_NOTIFY_SIGNAL is set for the
> > > > > > current task. This flag has been set in io_req_normal_work_add() by
> > > > > > calling task_work_add().
> > > > > 
> > > > > The entire idea of task_work is to interrupt syscalls and let io_uring
> > > > > do its job, otherwise it wouldn't free resources it might be holding,
> > > > > and even potentially forever block the syscall.
> > > > > 
> > > > > I'm not that sure about connect / close (are they not restartable?),
> > > > > but it doesn't seem to be a good idea for sk_stream_wait_memory(),
> > > > > which is the normal TCP blocking send path. I'm thinking of some kinds
> > > > > of cases with a local TCP socket pair, the tx queue is full as well
> > > > > and the rx queue of the other end, and io_uring has to run to receive
> > > > > the data.
> > > 
> > > There is another case, let's say the IO is done via io-wq
> > > (io_uring's worker thread pool) and hits the waiting. Now the
> > > request can't get cancelled, which is done by interrupting the
> > > task with TIF_NOTIFY_SIGNAL. User requested request cancellations
> > > is one thing, but we'd need to check if io_uring can ever be closed
> > > in this case.
> > > 
> > > 
> > > > > If interruptions are not welcome you can use different io_uring flags,
> > > > > see IORING_SETUP_COOP_TASKRUN and/or IORING_SETUP_DEFER_TASKRUN.
> > > > 
> > > > I tried with different combinations of these flags. For example
> > > > IORING_SETUP_TASKRUN_FLAG | IORING_SETUP_SINGLE_ISSUER | IORING_SETUP_DEFER_TASKRUN
> > > > makes the issue less likely, but nevertheless it still happens.
> > > > 
> > > > However, reading the documentation of these flags, they shall provide
> > > > hints to the kernel for optimizations, but it should work without these
> > > > flags, right?
> > > 
> > > That's true, and I guess there are other cases as well, like
> > > io-wq and perhaps even a stray fput.
> > > 
> > > 
> > > > > Maybe I'm missing something, why not restart your syscall?
> > > > 
> > > > The problem comes with TLS. Normally with synchronous encryption all
> > > > data on a socket is written during write(). When asynchronous
> > > > encryption comes into play, then not all data is written during write(),
> > > > but instead the remaining data is written at close() time.
> > > 
> > > Was it considered to do the final cleanup in workqueue
> > > and only then finalising the release?
> > 
> > No, but I don't really understand what you mean here. Could you
> > elaborate?
> 
> The suggestion is instead of executing the release and that final
> flush off of the context you're in, namely userspace task,
> you can spin up a kernel task (they're not getting any signals)
> and execute it from there.
> 
> void deferred_release_fn(struct work_struct *work)
> {
> 	do_release();
> 	...
> }
> 
> struct work_struct work;
> INIT_WORK(&work, deferred_release_fn);
> queue_work(system_unbound_wq, &work);
> 
> 
> There is a catch. Even though close() is not obliged to close
> the file / socket immediately, but it still not nice when you
> drop the final ref but port and other bits are not released
> until some time after. So, you might want to wait for that
> deferred release to complete before returning to the
> userspace.
> 
> I'm assuming it's fine to run it by a kernel task since
> IIRC fput might delay release to it anyway, but it's better
> to ask net maintainers. In theory it shouldn't need
> mm,fs,etc that user task would hold.

Ok, I'll have a look into it. Thanks for your input.

Sascha

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