diff options
authorChristian Riesch <christian.riesch@omicron.at>2014-11-13 05:53:26 +0100
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2015-01-16 06:59:32 -0800
commit7810e6d8f1390a96e634e66072064c69a6f7d08d (patch)
parent24003a22ef28a69dd07fa5277ce7ef0c944e8a0e (diff)
n_tty: Fix read_buf race condition, increment read_head after pushing data
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.riesch@omicron.at> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
1 files changed, 2 insertions, 1 deletions
diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 28ac3f3b7ec3..d46b4ccec8cd 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -321,7 +321,8 @@ 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++;