From: Christian Riesch Date: Thu, 13 Nov 2014 04:53:26 +0000 (+0100) Subject: n_tty: Fix read_buf race condition, increment read_head after pushing data X-Git-Url: http://git.cdn.openwrt.org/?a=commitdiff_plain;h=8bfbe2de769afda051c56aba5450391670e769fc;p=openwrt%2Fstaging%2Fblogic.git n_tty: Fix read_buf race condition, increment read_head after pushing data 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 Cc: Signed-off-by: Greg Kroah-Hartman --- diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c index 8358daa865e5..9e3b21624c82 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++; } /**