[PATCH v4 14/24] n_tty: Make N_TTY ldisc receive path lockless
From: Peter Hurley
Date: Sat Jun 15 2013 - 09:22:01 EST
n_tty has a single-producer/single-consumer input model;
use lockless publish instead.
Use termios_rwsem to exclude both consumer and producer while
changing or resetting buffer indices, eg., when flushing. Also,
claim exclusive termios_rwsem to safely retrieve the buffer
indices from a thread other than consumer or producer
(eg., TIOCINQ ioctl).
Note the read_tail is published _after_ clearing the newline
indicator in read_flags to avoid racing the producer.
Drop read_lock spinlock.
Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
---
drivers/tty/n_tty.c | 173 ++++++++++++++++++++++++++++------------------------
1 file changed, 92 insertions(+), 81 deletions(-)
diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 1098dd7..c7f71cb 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -110,7 +110,6 @@ struct n_tty_data {
struct mutex atomic_read_lock;
struct mutex output_lock;
struct mutex echo_lock;
- raw_spinlock_t read_lock;
};
static inline size_t read_cnt(struct n_tty_data *ldata)
@@ -168,7 +167,10 @@ static int receive_room(struct tty_struct *tty)
*
* Re-schedules the flip buffer work if space just became available.
*
- * Locks: Concurrent update is protected with read_lock
+ * Caller holds exclusive termios_rwsem
+ * or
+ * n_tty_read()/consumer path:
+ * holds non-exclusive termios_rwsem
*/
static void n_tty_set_room(struct tty_struct *tty)
@@ -191,34 +193,27 @@ static void n_tty_set_room(struct tty_struct *tty)
}
}
-static void put_tty_queue_nolock(unsigned char c, struct n_tty_data *ldata)
-{
- if (read_cnt(ldata) < N_TTY_BUF_SIZE) {
- *read_buf_addr(ldata, ldata->read_head) = c;
- ldata->read_head++;
- }
-}
-
/**
* put_tty_queue - add character to tty
* @c: character
* @ldata: n_tty data
*
- * Add a character to the tty read_buf queue. This is done under the
- * read_lock to serialize character addition and also to protect us
- * against parallel reads or flushes
+ * Add a character to the tty read_buf queue.
+ *
+ * n_tty_receive_buf()/producer path:
+ * caller holds non-exclusive termios_rwsem
+ * modifies read_head
+ *
+ * read_head is only considered 'published' if canonical mode is
+ * not active.
*/
static void put_tty_queue(unsigned char c, struct n_tty_data *ldata)
{
- unsigned long flags;
- /*
- * The problem of stomping on the buffers ends here.
- * Why didn't anyone see this one coming? --AJK
- */
- raw_spin_lock_irqsave(&ldata->read_lock, flags);
- put_tty_queue_nolock(c, ldata);
- raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
+ if (read_cnt(ldata) < N_TTY_BUF_SIZE) {
+ *read_buf_addr(ldata, ldata->read_head) = c;
+ ldata->read_head++;
+ }
}
/**
@@ -228,16 +223,13 @@ static void put_tty_queue(unsigned char c, struct n_tty_data *ldata)
* Reset the read buffer counters and clear the flags.
* Called from n_tty_open() and n_tty_flush_buffer().
*
- * Locking: tty_read_lock for read fields.
+ * Locking: caller holds exclusive termios_rwsem
+ * (or locking is not required)
*/
static void reset_buffer_flags(struct n_tty_data *ldata)
{
- unsigned long flags;
-
- raw_spin_lock_irqsave(&ldata->read_lock, flags);
ldata->read_head = ldata->canon_head = ldata->read_tail = 0;
- raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
mutex_lock(&ldata->echo_lock);
ldata->echo_pos = ldata->echo_cnt = ldata->echo_overrun = 0;
@@ -267,47 +259,55 @@ static void n_tty_packet_mode_flush(struct tty_struct *tty)
* buffer flushed (eg at hangup) or when the N_TTY line discipline
* internally has to clean the pending queue (for example some signals).
*
- * Locking: ctrl_lock, read_lock.
+ * Holds termios_rwsem to exclude producer/consumer while
+ * buffer indices are reset.
+ *
+ * Locking: ctrl_lock, exclusive termios_rwsem
*/
static void n_tty_flush_buffer(struct tty_struct *tty)
{
+ down_write(&tty->termios_rwsem);
reset_buffer_flags(tty->disc_data);
n_tty_set_room(tty);
if (tty->link)
n_tty_packet_mode_flush(tty);
+ up_write(&tty->termios_rwsem);
}
-/**
- * n_tty_chars_in_buffer - report available bytes
- * @tty: tty device
- *
- * Report the number of characters buffered to be delivered to user
- * at this instant in time.
- *
- * Locking: read_lock
- */
-
static ssize_t chars_in_buffer(struct tty_struct *tty)
{
struct n_tty_data *ldata = tty->disc_data;
- unsigned long flags;
ssize_t n = 0;
- raw_spin_lock_irqsave(&ldata->read_lock, flags);
if (!ldata->icanon)
n = read_cnt(ldata);
else
n = ldata->canon_head - ldata->read_tail;
- raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
return n;
}
+/**
+ * n_tty_chars_in_buffer - report available bytes
+ * @tty: tty device
+ *
+ * Report the number of characters buffered to be delivered to user
+ * at this instant in time.
+ *
+ * Locking: exclusive termios_rwsem
+ */
+
static ssize_t n_tty_chars_in_buffer(struct tty_struct *tty)
{
+ ssize_t n;
+
WARN_ONCE(1, "%s is deprecated and scheduled for removal.", __func__);
- return chars_in_buffer(tty);
+
+ down_write(&tty->termios_rwsem);
+ n = chars_in_buffer(tty);
+ up_write(&tty->termios_rwsem);
+ return n;
}
/**
@@ -915,7 +915,12 @@ static inline void finish_erasing(struct n_tty_data *ldata)
* present in the stream from the driver layer. Handles the complexities
* of UTF-8 multibyte symbols.
*
- * Locking: read_lock for tty buffers
+ * n_tty_receive_buf()/producer path:
+ * caller holds non-exclusive termios_rwsem
+ * modifies read_head
+ *
+ * Modifying the read_head is not considered a publish in this context
+ * because canonical mode is active -- only canon_head publishes
*/
static void eraser(unsigned char c, struct tty_struct *tty)
@@ -925,9 +930,7 @@ static void eraser(unsigned char c, struct tty_struct *tty)
size_t head;
size_t cnt;
int seen_alnums;
- unsigned long flags;
- /* FIXME: locking needed ? */
if (ldata->read_head == ldata->canon_head) {
/* process_output('\a', tty); */ /* what do you think? */
return;
@@ -938,15 +941,11 @@ static void eraser(unsigned char c, struct tty_struct *tty)
kill_type = WERASE;
else {
if (!L_ECHO(tty)) {
- raw_spin_lock_irqsave(&ldata->read_lock, flags);
ldata->read_head = ldata->canon_head;
- raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
return;
}
if (!L_ECHOK(tty) || !L_ECHOKE(tty) || !L_ECHOE(tty)) {
- raw_spin_lock_irqsave(&ldata->read_lock, flags);
ldata->read_head = ldata->canon_head;
- raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
finish_erasing(ldata);
echo_char(KILL_CHAR(tty), tty);
/* Add a newline if ECHOK is on and ECHOKE is off. */
@@ -958,7 +957,6 @@ static void eraser(unsigned char c, struct tty_struct *tty)
}
seen_alnums = 0;
- /* FIXME: Locking ?? */
while (ldata->read_head != ldata->canon_head) {
head = ldata->read_head;
@@ -980,9 +978,7 @@ static void eraser(unsigned char c, struct tty_struct *tty)
break;
}
cnt = ldata->read_head - head;
- raw_spin_lock_irqsave(&ldata->read_lock, flags);
ldata->read_head = head;
- raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
if (L_ECHO(tty)) {
if (L_ECHOPRT(tty)) {
if (!ldata->erasing) {
@@ -1071,7 +1067,11 @@ static inline void isig(int sig, struct tty_struct *tty)
* An RS232 break event has been hit in the incoming bitstream. This
* can cause a variety of events depending upon the termios settings.
*
- * Called from the receive_buf path so single threaded.
+ * n_tty_receive_buf()/producer path:
+ * caller holds non-exclusive termios_rwsem
+ * publishes read_head via put_tty_queue()
+ *
+ * Note: may get exclusive termios_rwsem if flushing input buffer
*/
static inline void n_tty_receive_break(struct tty_struct *tty)
@@ -1083,8 +1083,11 @@ static inline void n_tty_receive_break(struct tty_struct *tty)
if (I_BRKINT(tty)) {
isig(SIGINT, tty);
if (!L_NOFLSH(tty)) {
+ /* flushing needs exclusive termios_rwsem */
+ up_read(&tty->termios_rwsem);
n_tty_flush_buffer(tty);
tty_driver_flush_buffer(tty);
+ down_read(&tty->termios_rwsem);
}
return;
}
@@ -1131,7 +1134,11 @@ static inline void n_tty_receive_overrun(struct tty_struct *tty)
* @c: character
*
* Process a parity error and queue the right data to indicate
- * the error case if necessary. Locking as per n_tty_receive_buf.
+ * the error case if necessary.
+ *
+ * n_tty_receive_buf()/producer path:
+ * caller holds non-exclusive termios_rwsem
+ * publishes read_head via put_tty_queue()
*/
static inline void n_tty_receive_parity_error(struct tty_struct *tty,
unsigned char c)
@@ -1159,12 +1166,16 @@ static inline void n_tty_receive_parity_error(struct tty_struct *tty,
* Process an individual character of input received from the driver.
* This is serialized with respect to itself by the rules for the
* driver above.
+ *
+ * n_tty_receive_buf()/producer path:
+ * caller holds non-exclusive termios_rwsem
+ * publishes canon_head if canonical mode is active
+ * otherwise, publishes read_head via put_tty_queue()
*/
static inline void n_tty_receive_char(struct tty_struct *tty, unsigned char c)
{
struct n_tty_data *ldata = tty->disc_data;
- unsigned long flags;
int parmrk;
if (ldata->raw) {
@@ -1253,8 +1264,11 @@ static inline void n_tty_receive_char(struct tty_struct *tty, unsigned char c)
if (c == SUSP_CHAR(tty)) {
send_signal:
if (!L_NOFLSH(tty)) {
+ /* flushing needs exclusive termios_rwsem */
+ up_read(&tty->termios_rwsem);
n_tty_flush_buffer(tty);
tty_driver_flush_buffer(tty);
+ down_read(&tty->termios_rwsem);
}
if (I_IXON(tty))
start_tty(tty);
@@ -1355,11 +1369,9 @@ send_signal:
put_tty_queue(c, ldata);
handle_newline:
- raw_spin_lock_irqsave(&ldata->read_lock, flags);
set_bit(ldata->read_head & (N_TTY_BUF_SIZE - 1), ldata->read_flags);
- put_tty_queue_nolock(c, ldata);
+ put_tty_queue(c, ldata);
ldata->canon_head = ldata->read_head;
- raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
kill_fasync(&tty->fasync, SIGIO, POLL_IN);
if (waitqueue_active(&tty->read_wait))
wake_up_interruptible(&tty->read_wait);
@@ -1420,6 +1432,10 @@ static void n_tty_write_wakeup(struct tty_struct *tty)
* been received. This function must be called from soft contexts
* not from interrupt context. The driver is responsible for making
* calls one at a time and in order (or using flush_to_ldisc)
+ *
+ * n_tty_receive_buf()/producer path:
+ * claims non-exclusive termios_rwsem
+ * publishes read_head and canon_head
*/
static void __receive_buf(struct tty_struct *tty, const unsigned char *cp,
@@ -1430,10 +1446,8 @@ static void __receive_buf(struct tty_struct *tty, const unsigned char *cp,
char *f, flags = TTY_NORMAL;
int i;
char buf[64];
- unsigned long cpuflags;
if (ldata->real_raw) {
- raw_spin_lock_irqsave(&ldata->read_lock, cpuflags);
i = min(N_TTY_BUF_SIZE - read_cnt(ldata),
N_TTY_BUF_SIZE - (ldata->read_head & (N_TTY_BUF_SIZE - 1)));
i = min(count, i);
@@ -1447,7 +1461,6 @@ static void __receive_buf(struct tty_struct *tty, const unsigned char *cp,
i = min(count, i);
memcpy(read_buf_addr(ldata, ldata->read_head), cp, i);
ldata->read_head += i;
- raw_spin_unlock_irqrestore(&ldata->read_lock, cpuflags);
} else {
for (i = count, p = cp, f = fp; i; i--, p++) {
if (f)
@@ -1677,7 +1690,6 @@ static int n_tty_open(struct tty_struct *tty)
mutex_init(&ldata->atomic_read_lock);
mutex_init(&ldata->output_lock);
mutex_init(&ldata->echo_lock);
- raw_spin_lock_init(&ldata->read_lock);
/* These are ugly. Currently a malloc failure here can panic */
ldata->read_buf = kzalloc(N_TTY_BUF_SIZE, GFP_KERNEL);
@@ -1733,6 +1745,9 @@ static inline int input_available_p(struct tty_struct *tty, int amt)
*
* Called under the ldata->atomic_read_lock sem
*
+ * n_tty_read()/consumer path:
+ * caller holds non-exclusive termios_rwsem
+ * read_tail published
*/
static int copy_from_read_buf(struct tty_struct *tty,
@@ -1743,27 +1758,22 @@ static int copy_from_read_buf(struct tty_struct *tty,
struct n_tty_data *ldata = tty->disc_data;
int retval;
size_t n;
- unsigned long flags;
bool is_eof;
size_t tail = ldata->read_tail & (N_TTY_BUF_SIZE - 1);
retval = 0;
- raw_spin_lock_irqsave(&ldata->read_lock, flags);
n = min(read_cnt(ldata), N_TTY_BUF_SIZE - tail);
n = min(*nr, n);
- raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
if (n) {
retval = copy_to_user(*b, read_buf_addr(ldata, tail), n);
n -= retval;
is_eof = n == 1 && read_buf(ldata, tail) == EOF_CHAR(tty);
tty_audit_add_data(tty, read_buf_addr(ldata, tail), n,
ldata->icanon);
- raw_spin_lock_irqsave(&ldata->read_lock, flags);
ldata->read_tail += n;
/* Turn single EOF into zero-length read */
if (L_EXTPROC(tty) && ldata->icanon && is_eof && !read_cnt(ldata))
n = 0;
- raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
*b += n;
*nr -= n;
}
@@ -1781,6 +1791,10 @@ static int copy_from_read_buf(struct tty_struct *tty,
* character into the user-space buffer.
*
* Called under the atomic_read_lock mutex
+ *
+ * n_tty_read()/consumer path:
+ * caller holds non-exclusive termios_rwsem
+ * read_tail published
*/
static int canon_copy_from_read_buf(struct tty_struct *tty,
@@ -1788,21 +1802,15 @@ static int canon_copy_from_read_buf(struct tty_struct *tty,
size_t *nr)
{
struct n_tty_data *ldata = tty->disc_data;
- unsigned long flags;
size_t n, size, more, c;
size_t eol;
size_t tail;
int ret, found = 0;
/* N.B. avoid overrun if nr == 0 */
-
- raw_spin_lock_irqsave(&ldata->read_lock, flags);
-
n = min(*nr, read_cnt(ldata));
- if (!n) {
- raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
+ if (!n)
return 0;
- }
tail = ldata->read_tail & (N_TTY_BUF_SIZE - 1);
size = min_t(size_t, tail + n, N_TTY_BUF_SIZE);
@@ -1830,8 +1838,6 @@ static int canon_copy_from_read_buf(struct tty_struct *tty,
n_tty_trace("%s: eol:%zu found:%d n:%zu c:%zu size:%zu more:%zu\n",
__func__, eol, found, n, c, size, more);
- raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
-
if (n > size) {
ret = copy_to_user(*b, read_buf_addr(ldata, tail), size);
if (ret)
@@ -1845,11 +1851,10 @@ static int canon_copy_from_read_buf(struct tty_struct *tty,
*b += n;
*nr -= n;
- raw_spin_lock_irqsave(&ldata->read_lock, flags);
- ldata->read_tail += c;
if (found)
- __clear_bit(eol, ldata->read_flags);
- raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
+ clear_bit(eol, ldata->read_flags);
+ smp_mb__after_clear_bit();
+ ldata->read_tail += c;
if (found)
tty_audit_push(tty);
@@ -1913,6 +1918,10 @@ static int job_control(struct tty_struct *tty, struct file *file)
* a hangup. Always called in user context, may sleep.
*
* This code must be sure never to sleep through a hangup.
+ *
+ * n_tty_read()/consumer path:
+ * claims non-exclusive termios_rwsem
+ * publishes read_tail
*/
static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
@@ -2279,10 +2288,12 @@ static int n_tty_ioctl(struct tty_struct *tty, struct file *file,
case TIOCOUTQ:
return put_user(tty_chars_in_buffer(tty), (int __user *) arg);
case TIOCINQ:
- /* FIXME: Locking */
- retval = read_cnt(ldata);
+ down_write(&tty->termios_rwsem);
if (L_ICANON(tty))
retval = inq_canon(ldata);
+ else
+ retval = read_cnt(ldata);
+ up_write(&tty->termios_rwsem);
return put_user(retval, (unsigned int __user *) arg);
default:
return n_tty_ioctl_helper(tty, file, cmd, arg);
--
1.8.1.2
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