3.13.11-ckt15 -stable review patch. If anyone has any objections, please let
me know.
------------------
From: Christian Riesch <christian.rie...@omicron.at>
commit 8bfbe2de769afda051c56aba5450391670e769fc upstream.
Commit 19e2ad6a09f0c06dbca19c98e5f4584269d913dd ("n_tty: Remove overflow
tests from receive_buf() path") moved the increment of read_head into
the arguments list of read_buf_addr(). Function calls represent a
sequence point in C. Therefore read_head is incremented before the
character c is placed in the buffer. Since the circular read buffer is
a lock-less design since commit 6d76bd2618535c581f1673047b8341fd291abc67
("n_tty: Make N_TTY ldisc receive path lockless"), this creates a race
condition that leads to communication errors.
This patch modifies the code to increment read_head _after_ the data
is placed in the buffer and thus fixes the race for non-SMP machines.
To fix the problem for SMP machines, memory barriers must be added in
a separate patch.
Signed-off-by: Christian Riesch <christian.rie...@omicron.at>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <ka...@canonical.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 86093e2..7bdaa14 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -319,7 +319,8 @@ static void n_tty_check_unthrottle(struct tty_struct *tty)
static inline void put_tty_queue(unsigned char c, struct n_tty_data *ldata)
{
- *read_buf_addr(ldata, ldata->read_head++) = c;
+ *read_buf_addr(ldata, ldata->read_head) = c;
+ ldata->read_head++;
}
/**
--
1.9.1
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
