On 11/26/2013 04:53 PM, Jason Gunthorpe wrote:
Hello Peter,
I have been testing 3.13-rc1 and I noticed a change in behavior in the
tty/serial layer. Specifically I have a login running on serial that
presents the usual login:/password: prompt.
3.10.12 does this:
login: admin
password:
prompt>
While 3.13-rc1 does this:
login: admin
password: prompt>
That is to say, a \n got lost. I have also noticed while using the
serial console that '\n's are occasionally lost, and the lossage is
highly repeatable and seemingly based on number of chars being output
in a burst, eg entering 'foo bar' at a shell prompt might loose the \n,
while entering 'foo bar ' will not.
I ran a full git bisect search and determined that the attached patch
introduced the problem.
This seems like a bug - at least the commit comment seems like it
should not cause any change in behaviour. I can reproduce it and help
debug, if you can give some guidance, I am not a tty layer expert :)
Hi Jason,
Thanks for the bisect: this is definitely a bug and not designed behavior.
But just to be clear here, the error you're seeing is strictly wrt.
echo output; normal output remains uncorrupted?
Also, you're not seeing other echo drop-outs?
Although there are some fixes due for 3.13-rc2 concerning echoes, I
think this may be a smp weak memory ordering problem.
I have seen this bug on ppc with an out-of-tree serial driver, and on
ARM kirkwood with the serial8250 driver. I did the bisection search on
PPC.
Ah, the games processors play. I assume the ppc is the host machine; the
serial terminal remained unchanged, yes?
Would you please test with the patch below which uses ftrace to
capture the output? [The trace only tracks the echo buffer indices, not
any of the data.]
When you see the lost '\n', copy /sys/kernel/debug/tracing/trace
to a file and send it to me (compressed if too big for the lists). Or
you can send it off-list, if you'd prefer.
[Apologies if the patch is white-space mangled; my mailer [Thunderbird]
is dain-bramaged. I think git am will still do the right thing.]
--- >% ----
Subject: [PATCH] n_tty: Add trace support for echo output
Debug patch.
Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
---
drivers/tty/n_tty.c | 24 ++++++++++++++++++++++++
1 file changed, 24 insertions(+)
diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 7cdd1eb..b055838 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -79,6 +79,12 @@
#define ECHO_BLOCK 256
#define ECHO_DISCARD_WATERMARK N_TTY_BUF_SIZE - (ECHO_BLOCK + 32)
+#define trace_facility trace_printk
+#define _n_tty_trace(tty, f, args...) \
+ do { \
+ char buf[64]; \
+ trace_facility("[%s] " f, tty_name(tty, buf), ##args); \
+ } while (0)
#undef N_TTY_TRACE
#ifdef N_TTY_TRACE
@@ -87,6 +93,15 @@
# define n_tty_trace(f, args...)
#endif
+#undef N_TTY_TRACE_ECHOES
+#define N_TTY_TRACE_ECHOES 1
+#ifdef N_TTY_TRACE_ECHOES
+# define n_tty_trace_echoes(tty, f, args...) _n_tty_trace(tty, f, args)
+#else
+# define n_tty_trace_echoes(tty, f, args...) do { } while (0)
+#endif
+
+
struct n_tty_data {
/* producer-published */
size_t read_head;
@@ -763,10 +778,19 @@ static size_t __process_echoes(struct tty_struct *tty)
}
}
+ n_tty_trace_echoes(tty, "echoes:%zu,%zu commit:%zu head:%zu\n",
+ ldata->echo_tail, tail, ldata->echo_commit,
+ ldata->echo_head);
+
/* If the echo buffer is nearly full (so that the possibility exists
* of echo overrun before the next commit), then discard enough
* data at the tail to prevent a subsequent overrun */
while (ldata->echo_commit - tail >= ECHO_DISCARD_WATERMARK) {
+
+ n_tty_trace_echoes(tty, "discard echoes (%zu - %zu > %d)\n",
+ ldata->echo_commit, tail,
+ ECHO_DISCARD_WATERMARK);
+
if (echo_buf(ldata, tail) == ECHO_OP_START) {
if (echo_buf(ldata, tail) == ECHO_OP_ERASE_TAB)
tail += 3;
--
1.8.1.2
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