Re: [PATCH] tty: n_tty: order lockless input availability checks

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, May 04, 2026 at 03:23:21PM +0800, Cen Zhang wrote:
> The N_TTY read buffer uses release/acquire ordering for its
> lockless ring indices. Input producers release-publish canon_head and
> commit_head after updating the buffer and delimiter flags, and readers
> acquire those heads before copying data. Readers also release-publish
> read_tail before producers use it to calculate room.
> 
> chars_in_buffer() and input_available_p() sample the same indices
> for availability and flow-control decisions, but use plain loads. That
> can miss the ordering used by the data-copy paths and can also let
> poll() observe termios-synthesized availability with weaker ordering
> than normal receive-side publication.
> 
> Use acquire loads for the lockless head/tail samples in those
> helpers. When n_tty_set_termios() updates canonical/noncanonical
> availability, publish the updated heads with release stores as well.
> Keep the cached icanon bit as an intentionally lockless mode snapshot
> and annotate that access.
> 
> Fixes: 70aca71f92ca ("n_tty: Fix unordered accesses to lockless read buffer")
> Signed-off-by: Cen Zhang <zzzccc427@gmail.com>

What tests show that this is needed?  That commit was a long time ago,
and surely we would have had some bug reports since then, right?

thanks,

greg k-h