tty: Use raw spin lock to protect the TTY read section

The "normal" spin lock that guards the N_TTY line discipline read section
is replaced by a raw spin lock.

On a PREEMP_RT system this prevents unwanted scheduling overhead when data is
read at the same time as data is being received: while RX IRQ threaded handling
is busy a TTY read call is performed from a RT priority > threaded IRQ priority.
The read call tries to take the read section spin lock (held by the threaded
IRQ) which blocks and causes a context switch to/from the threaded IRQ handler
until the spin lock is unlocked.
Signed-off-by: Ivo Sieben <meltedpianoman@gmail.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_tty.c

@@ -100,7 +100,7 @@ struct n_tty_data {
 	struct mutex atomic_read_lock;
 	struct mutex output_lock;
 	struct mutex echo_lock;
-	spinlock_t read_lock;
+	raw_spinlock_t read_lock;
 };
 
 static inline int tty_put_user(struct tty_struct *tty, unsigned char x,
@@ -182,9 +182,9 @@ static void put_tty_queue(unsigned char c, struct n_tty_data *ldata)
 	 *	The problem of stomping on the buffers ends here.
 	 *	Why didn't anyone see this one coming? --AJK
 	*/
-	spin_lock_irqsave(&ldata->read_lock, flags);
+	raw_spin_lock_irqsave(&ldata->read_lock, flags);
 	put_tty_queue_nolock(c, ldata);
-	spin_unlock_irqrestore(&ldata->read_lock, flags);
+	raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
 }
 
 /**
@@ -218,9 +218,9 @@ static void reset_buffer_flags(struct tty_struct *tty)
 	struct n_tty_data *ldata = tty->disc_data;
 	unsigned long flags;
 
-	spin_lock_irqsave(&ldata->read_lock, flags);
+	raw_spin_lock_irqsave(&ldata->read_lock, flags);
 	ldata->read_head = ldata->read_tail = ldata->read_cnt = 0;
-	spin_unlock_irqrestore(&ldata->read_lock, flags);
+	raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
 
 	mutex_lock(&ldata->echo_lock);
 	ldata->echo_pos = ldata->echo_cnt = ldata->echo_overrun = 0;
@@ -276,7 +276,7 @@ static ssize_t n_tty_chars_in_buffer(struct tty_struct *tty)
 	unsigned long flags;
 	ssize_t n = 0;
 
-	spin_lock_irqsave(&ldata->read_lock, flags);
+	raw_spin_lock_irqsave(&ldata->read_lock, flags);
 	if (!ldata->icanon) {
 		n = ldata->read_cnt;
 	} else if (ldata->canon_data) {
@@ -284,7 +284,7 @@ static ssize_t n_tty_chars_in_buffer(struct tty_struct *tty)
 			ldata->canon_head - ldata->read_tail :
 			ldata->canon_head + (N_TTY_BUF_SIZE - ldata->read_tail);
 	}
-	spin_unlock_irqrestore(&ldata->read_lock, flags);
+	raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
 	return n;
 }
 
@@ -915,19 +915,19 @@ static void eraser(unsigned char c, struct tty_struct *tty)
 		kill_type = WERASE;
 	else {
 		if (!L_ECHO(tty)) {
-			spin_lock_irqsave(&ldata->read_lock, flags);
+			raw_spin_lock_irqsave(&ldata->read_lock, flags);
 			ldata->read_cnt -= ((ldata->read_head - ldata->canon_head) &
 					  (N_TTY_BUF_SIZE - 1));
 			ldata->read_head = ldata->canon_head;
-			spin_unlock_irqrestore(&ldata->read_lock, flags);
+			raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
 			return;
 		}
 		if (!L_ECHOK(tty) || !L_ECHOKE(tty) || !L_ECHOE(tty)) {
-			spin_lock_irqsave(&ldata->read_lock, flags);
+			raw_spin_lock_irqsave(&ldata->read_lock, flags);
 			ldata->read_cnt -= ((ldata->read_head - ldata->canon_head) &
 					  (N_TTY_BUF_SIZE - 1));
 			ldata->read_head = ldata->canon_head;
-			spin_unlock_irqrestore(&ldata->read_lock, flags);
+			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. */
@@ -961,10 +961,10 @@ static void eraser(unsigned char c, struct tty_struct *tty)
 				break;
 		}
 		cnt = (ldata->read_head - head) & (N_TTY_BUF_SIZE-1);
-		spin_lock_irqsave(&ldata->read_lock, flags);
+		raw_spin_lock_irqsave(&ldata->read_lock, flags);
 		ldata->read_head = head;
 		ldata->read_cnt -= cnt;
-		spin_unlock_irqrestore(&ldata->read_lock, flags);
+		raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
 		if (L_ECHO(tty)) {
 			if (L_ECHOPRT(tty)) {
 				if (!ldata->erasing) {
@@ -1344,12 +1344,12 @@ static inline void n_tty_receive_char(struct tty_struct *tty, unsigned char c)
 				put_tty_queue(c, ldata);
 
 handle_newline:
-			spin_lock_irqsave(&ldata->read_lock, flags);
+			raw_spin_lock_irqsave(&ldata->read_lock, flags);
 			set_bit(ldata->read_head, ldata->read_flags);
 			put_tty_queue_nolock(c, ldata);
 			ldata->canon_head = ldata->read_head;
 			ldata->canon_data++;
-			spin_unlock_irqrestore(&ldata->read_lock, flags);
+			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);
@@ -1423,7 +1423,7 @@ static void n_tty_receive_buf(struct tty_struct *tty, const unsigned char *cp,
 	unsigned long cpuflags;
 
 	if (ldata->real_raw) {
-		spin_lock_irqsave(&ldata->read_lock, cpuflags);
+		raw_spin_lock_irqsave(&ldata->read_lock, cpuflags);
 		i = min(N_TTY_BUF_SIZE - ldata->read_cnt,
 			N_TTY_BUF_SIZE - ldata->read_head);
 		i = min(count, i);
@@ -1439,7 +1439,7 @@ static void n_tty_receive_buf(struct tty_struct *tty, const unsigned char *cp,
 		memcpy(ldata->read_buf + ldata->read_head, cp, i);
 		ldata->read_head = (ldata->read_head + i) & (N_TTY_BUF_SIZE-1);
 		ldata->read_cnt += i;
-		spin_unlock_irqrestore(&ldata->read_lock, cpuflags);
+		raw_spin_unlock_irqrestore(&ldata->read_lock, cpuflags);
 	} else {
 		for (i = count, p = cp, f = fp; i; i--, p++) {
 			if (f)
@@ -1635,7 +1635,7 @@ 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);
-	spin_lock_init(&ldata->read_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);
@@ -1703,10 +1703,10 @@ static int copy_from_read_buf(struct tty_struct *tty,
 	bool is_eof;
 
 	retval = 0;
-	spin_lock_irqsave(&ldata->read_lock, flags);
+	raw_spin_lock_irqsave(&ldata->read_lock, flags);
 	n = min(ldata->read_cnt, N_TTY_BUF_SIZE - ldata->read_tail);
 	n = min(*nr, n);
-	spin_unlock_irqrestore(&ldata->read_lock, flags);
+	raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
 	if (n) {
 		retval = copy_to_user(*b, &ldata->read_buf[ldata->read_tail], n);
 		n -= retval;
@@ -1714,13 +1714,13 @@ static int copy_from_read_buf(struct tty_struct *tty,
 			ldata->read_buf[ldata->read_tail] == EOF_CHAR(tty);
 		tty_audit_add_data(tty, &ldata->read_buf[ldata->read_tail], n,
 				ldata->icanon);
-		spin_lock_irqsave(&ldata->read_lock, flags);
+		raw_spin_lock_irqsave(&ldata->read_lock, flags);
 		ldata->read_tail = (ldata->read_tail + n) & (N_TTY_BUF_SIZE-1);
 		ldata->read_cnt -= n;
 		/* Turn single EOF into zero-length read */
 		if (L_EXTPROC(tty) && ldata->icanon && is_eof && !ldata->read_cnt)
 			n = 0;
-		spin_unlock_irqrestore(&ldata->read_lock, flags);
+		raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
 		*b += n;
 		*nr -= n;
 	}
@@ -1900,7 +1900,7 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
 
 		if (ldata->icanon && !L_EXTPROC(tty)) {
 			/* N.B. avoid overrun if nr == 0 */
-			spin_lock_irqsave(&ldata->read_lock, flags);
+			raw_spin_lock_irqsave(&ldata->read_lock, flags);
 			while (nr && ldata->read_cnt) {
 				int eol;
 
@@ -1918,25 +1918,25 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
 					if (--ldata->canon_data < 0)
 						ldata->canon_data = 0;
 				}
-				spin_unlock_irqrestore(&ldata->read_lock, flags);
+				raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
 
 				if (!eol || (c != __DISABLED_CHAR)) {
 					if (tty_put_user(tty, c, b++)) {
 						retval = -EFAULT;
 						b--;
-						spin_lock_irqsave(&ldata->read_lock, flags);
+						raw_spin_lock_irqsave(&ldata->read_lock, flags);
 						break;
 					}
 					nr--;
 				}
 				if (eol) {
 					tty_audit_push(tty);
-					spin_lock_irqsave(&ldata->read_lock, flags);
+					raw_spin_lock_irqsave(&ldata->read_lock, flags);
 					break;
 				}
-				spin_lock_irqsave(&ldata->read_lock, flags);
+				raw_spin_lock_irqsave(&ldata->read_lock, flags);
 			}
-			spin_unlock_irqrestore(&ldata->read_lock, flags);
+			raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
 			if (retval)
 				break;
 		} else {