Re: [PATCH net-next v5 6/6] tls: Flush backlog before waiting for a new record

2026-03-24, 15:07:00 -0400, Chuck Lever wrote:
> 
> 
> On Tue, Mar 24, 2026, at 12:18 PM, Sabrina Dubroca wrote:
> > 2026-03-24, 08:53:28 -0400, Chuck Lever wrote:
> >> From: Chuck Lever <[email protected]>
> >> 
> >> While lock_sock is held, incoming TCP segments land on
> >> sk->sk_backlog rather than sk->sk_receive_queue.
> >> tls_rx_rec_wait() inspects only sk_receive_queue, so
> >> backlog data remains invisible. For non-blocking callers
> >> (read_sock, and recvmsg or splice_read with MSG_DONTWAIT)
> >> this causes a spurious -EAGAIN. For blocking callers it
> >> forces an unnecessary sleep/wakeup cycle.
> >> 
> >> Flush the backlog inside tls_rx_rec_wait() before checking
> >> sk_receive_queue so the strparser can parse newly-arrived
> >> segments immediately.
> >> 
> >> Fixes: 20ffc7adf53a ("net/tls: missing received data after fast remote close")
> >
> > How did you pick that Fixes tag? That commit mentions FIN/connection
> > closing, which doesn't seem related to the local backlog.
> 
> 20ffc7adf53a introduced the sk_receive_queue check inside the
> wait loop (then called tls_wait_data(), later refactored into
> tls_rx_rec_wait()).
> 
> When lock_sock is held, incoming TCP will segments land on
> sk->sk_backlog, not sk->sk_receive_queue. The sk_receive_queue
> check introduced by 20ffc7adf53a doesn't see backlog data.

But without this check, we'd go straight to the EAGAIN/sleep cycle, so
things were even worse before?

(btw, you don't need a Fixes tag for net-next patches. if you think
this is a bug, the patch should be extracted from this series and
submitted separately for the "net" tree, with the appropriate Fixes
tag)


> >> diff --git a/net/tls/tls_sw.c b/net/tls/tls_sw.c
> >> index 8fb2f2a93846..84c4ae0330d1 100644
> >> --- a/net/tls/tls_sw.c
> >> +++ b/net/tls/tls_sw.c
> >> @@ -1372,6 +1372,7 @@ tls_rx_rec_wait(struct sock *sk, struct sk_psock *psock, bool nonblock,
> >>  		if (ret < 0)
> >>  			return ret;
> >>  
> >> +		sk_flush_backlog(sk);
> >
> > Do we need to update released when this returns true, like callers of
> > tls_read_flush_backlog() do?
> 
> Good catch. v6 will do that.
> 
> 
> > I also wonder if we'd want to update the
> > caller's flushed_at to avoid bypassing the "smart checks" in
> > tls_read_flush_backlog().
> 
> The flush in tls_rx_rec_wait() only fires when the loop finds
> no ready message, which is the cold path.

Right.

> The redundant flush
> from tls_read_flush_backlog() on the next iteration is wasteful
> but harmless. I'm not sure the additional complexity would be
> worth it, but if you believe it will add some value, let me
> know and I will add it.

No, that sounds ok. But probably worth a quick mention in the commit
message about this wasteful sk_flush_backlog() when we've already gone
on the cold path, and we can revisit in the future if needed.

-- 
Sabrina