netdev - Re: [PATCH] net: Do not break out of sk_stream_wait_memory() with TIF_NOTIFY

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Message-ID: <Zflt3EVf744LOA6i@pengutronix.de>
Date: Tue, 19 Mar 2024 11:50:04 +0100
From: Sascha Hauer <s.hauer@...gutronix.de>
To: Pavel Begunkov <asml.silence@...il.com>
Cc: netdev@...r.kernel.org, kernel@...gutronix.de,
	linux-kernel@...r.kernel.org, Paolo Abeni <pabeni@...hat.com>,
	Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org
Subject: Re: [PATCH] net: Do not break out of sk_stream_wait_memory() with
 TIF_NOTIFY_SIGNAL

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?

Sascha

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