[LTP] [PATCH v4 1/2] pty04: Use guarded buffers for transmission

[Richard Palethorpe <rpalethorpe@suse.de>]

Hello,

Petr Vorel <pvorel@suse.cz> writes:

> Hi Richard,
>
>> Signed-off-by: Richard Palethorpe <rpalethorpe@suse.com>
>> ---
>
> Reviewed-by: Petr Vorel <pvorel@suse.cz>
>
> BTW Every second run with this patch it blocks after pty04.c:214: PASS: Read netdev 1
> and then:
> tst_checkpoint.c:147: BROK: pty04.c:249: tst_checkpoint_wait(0, 10000): ETIMEDOUT (110)
> tst_test.c:373: BROK: Reported by child (26650)
> safe_macros.c:258: BROK: pty04.c:215: read(5,0x7efebc306001,8191) failed, returned -1: ENETDOWN (100)
> pty04.c:139: PASS: Writing to PTY interrupted by hangup
> tst_test.c:373: WARN: Reported by child (26648)
>
> Tested on 5.7.0-rc3 in Tumbleweed.
> But it looks this is not caused by this change, but was here before, because the
> same behavior I see when testing pty04 *without* this patch on various kernels
> (5.3.7, 5.6.0-rc5) and some of the never SLES (4.12 based).
>
> Kind regards,
> Petr

This looks similar to the issue reported by Jan:

https://github.com/linux-test-project/ltp/issues/674

Is this the full output?

Thinking aloud: the following (probably) happens when writing to the PTY

write() -> PTY -> SLIP/SLCAN -> netdev -> read()

Writing to the PTY causes the PTY to write to the line discipline. What
I found was that when the line discipline receive buffer got full and the PTY
send buffer got full. The write would go to sleep and never wake up
because the line discipline drained the receive buffer, but doesn't
signal it is ready for more data (with tty_unthrottle). So I used
nonblocking writes which just retry writing.

From Jan's errors it looks like it might just be reading that is failing
in one case and that writing is also failing in the other until we
cancel the read. I doubt this is anything to do with the netdev code
because it is generic networking code AFAICT and should work correctly
with blocking reads...

-- 
Thank you,
Richard.