Messages in this thread Patch in this message | | | From | Christian Riesch <> | Subject | [PATCH v3] n_tty: Fix read_buf race condition, increment read_head after pushing data | Date | Thu, 13 Nov 2014 05:53:26 +0100 |
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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> Cc: <stable@vger.kernel.org> ---
Changes for v3: - Removed unnecessary comment.
Changes for v2: - Rewrote commit message. Since I did not know better, I blamed the compiler in v1, but actually the code was wrong. See the discussion in [1]. - Removed memory barriers. For non-SMP machines they are not required, for SMP machines more brainwork and discussions are needed.
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 2e900a9..47ca0f3 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++; } /** -- 1.7.9.5
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