Re: [PATCH 3/4] tcp: Set EPOLLET when when reading from a socket fails with EAGAIN

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Jan 16, 2025 at 09:32:49PM +0100, Stefano Brivio wrote:
> Before SO_PEEK_OFF support was introduced by commit e63d281871ef
> ("tcp: leverage support of SO_PEEK_OFF socket option when available"),
> we would peek data from sockets using a "discard" buffer as first
> iovec element, so that, unless we had no pending data at all, we would
> always get a positive return code from recvmsg() (except for closing
> connections or errors).
> 
> If we couldn't send more data to the guest, in the window, we would
> set the STALLED flag (causing the epoll descriptor to switch to
> edge-triggered mode), and return early from tcp_data_from_sock().
> 
> With SO_PEEK_OFF, we don't have a discard buffer, and if there's data
> on the socket, but nothing beyond our current peeking offset, we'll
> get EAGAIN instead of our current "discard" length. In that case, we
> return even earlier, and we don't set EPOLLET on the socket as a
> result.
> 
> As reported by Asahi Lina, this causes event loops where the kernel is
> signalling socket readiness, because there's data we didn't dequeue
> yet (waiting for the guest to acknowledge it), but we won't actually
> peek anything new, and return early without setting EPOLLET.
> 
> This is the original report, mentioning the originally proposed fix:

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>


-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
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http://www.ozlabs.org/~dgibson

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