This is a note to let you know that I've just added the patch titled
n_tty: Fix read_buf race condition, increment read_head after pushing data
to the 3.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
n_tty-fix-read_buf-race-condition-increment-read_head-after-pushing-data.patch
and it can be found in the queue-3.14 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <[email protected]> know about it.
>From 8bfbe2de769afda051c56aba5450391670e769fc Mon Sep 17 00:00:00 2001
From: Christian Riesch <[email protected]>
Date: Thu, 13 Nov 2014 05:53:26 +0100
Subject: n_tty: Fix read_buf race condition, increment read_head after pushing
data
From: Christian Riesch <[email protected]>
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 <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
drivers/tty/n_tty.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -321,7 +321,8 @@ static void n_tty_check_unthrottle(struc
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++;
}
/**
Patches currently in stable-queue which might be from
[email protected] are
queue-3.14/n_tty-fix-read_buf-race-condition-increment-read_head-after-pushing-data.patch
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