[PATCH] tty: n_tty: annotate lockless read of ldata->icanon in input_available_p()

[Ziyu Zhang <ziyuzhang201@gmail.com>]

n_tty_poll() calls input_available_p() without holding termios_rwsem to
check input readiness for select()/poll(). input_available_p() reads
ldata->icanon, which can be concurrently written by n_tty_set_termios()
under down_write(termios_rwsem).

This is a benign race: poll/select readiness is best-effort, and the
actual n_tty_read() path re-checks icanon under down_read(termios_rwsem).
A stale icanon value in poll only causes a transiently incorrect
readiness result, which is permitted by POSIX poll/select semantics.

Since icanon is a bitfield, READ_ONCE()/WRITE_ONCE() cannot be used.
Annotate the read with data_race() to document the intentional lockless
access and suppress data race detector warnings.

Signed-off-by: Ziyu Zhang <ziyuzhang201@gmail.com>
---
 drivers/tty/n_tty.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index e6a0f5b40..aa3c11623 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1909,7 +1909,8 @@ static inline int input_available_p(const struct tty_struct *tty, int poll)
 	const struct n_tty_data *ldata = tty->disc_data;
 	int amt = poll && !TIME_CHAR(tty) && MIN_CHAR(tty) ? MIN_CHAR(tty) : 1;
 
-	if (ldata->icanon && !L_EXTPROC(tty))
+	/* data_race: benign race, poll readiness is best-effort */
+	if (data_race(ldata->icanon) && !L_EXTPROC(tty))
 		return ldata->canon_head != ldata->read_tail;
 	else
 		return ldata->commit_head - ldata->read_tail >= amt;
-- 
2.39.5 (Apple Git-154)