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>

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 19083efa2314660b25e0fb5bc793af6fb7e9af57..095072899b5edea12eee71da689367cce048e234 100644 (file)
--- a/drivers/tty/n_tty.c
+++ b/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 @@ send_signal:
                                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 @@ do_it_again:
 
                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 @@ do_it_again:
                                        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 {