tty: Use raw spin lock to protect TTY ldisc administration

The global "normal" spin lock that guards the line discipline
administration is replaced by a raw spin lock. On a PREEMPT_RT system this
prevents unwanted scheduling overhead around the line discipline administration.

On a 200 MHz AT91SAM9261 processor setup this fixes about 100us of scheduling
overhead on a TTY read or write call.

Signed-off-by: Ivo Sieben <meltedpianoman@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index f4e6754..c578229 100644 (file)
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -26,7 +26,7 @@
  *     callers who will do ldisc lookups and cannot sleep.
  */
 
-static DEFINE_SPINLOCK(tty_ldisc_lock);
+static DEFINE_RAW_SPINLOCK(tty_ldisc_lock);
 static DECLARE_WAIT_QUEUE_HEAD(tty_ldisc_wait);
 /* Line disc dispatch table */
 static struct tty_ldisc_ops *tty_ldiscs[NR_LDISCS];
@@ -49,21 +49,21 @@ static void put_ldisc(struct tty_ldisc *ld)
         * If this is the last user, free the ldisc, and
         * release the ldisc ops.
         *
-        * We really want an "atomic_dec_and_lock_irqsave()",
+        * We really want an "atomic_dec_and_raw_lock_irqsave()",
         * but we don't have it, so this does it by hand.
         */
-       local_irq_save(flags);
-       if (atomic_dec_and_lock(&ld->users, &tty_ldisc_lock)) {
+       raw_spin_lock_irqsave(&tty_ldisc_lock, flags);
+       if (atomic_dec_and_test(&ld->users)) {
                struct tty_ldisc_ops *ldo = ld->ops;
 
                ldo->refcount--;
                module_put(ldo->owner);
-               spin_unlock_irqrestore(&tty_ldisc_lock, flags);
+               raw_spin_unlock_irqrestore(&tty_ldisc_lock, flags);
 
                kfree(ld);
                return;
        }
-       local_irq_restore(flags);
+       raw_spin_unlock_irqrestore(&tty_ldisc_lock, flags);
        wake_up(&ld->wq_idle);
 }
 
@@ -88,11 +88,11 @@ int tty_register_ldisc(int disc, struct tty_ldisc_ops *new_ldisc)
        if (disc < N_TTY || disc >= NR_LDISCS)
                return -EINVAL;
 
-       spin_lock_irqsave(&tty_ldisc_lock, flags);
+       raw_spin_lock_irqsave(&tty_ldisc_lock, flags);
        tty_ldiscs[disc] = new_ldisc;
        new_ldisc->num = disc;
        new_ldisc->refcount = 0;
-       spin_unlock_irqrestore(&tty_ldisc_lock, flags);
+       raw_spin_unlock_irqrestore(&tty_ldisc_lock, flags);
 
        return ret;
 }
@@ -118,12 +118,12 @@ int tty_unregister_ldisc(int disc)
        if (disc < N_TTY || disc >= NR_LDISCS)
                return -EINVAL;
 
-       spin_lock_irqsave(&tty_ldisc_lock, flags);
+       raw_spin_lock_irqsave(&tty_ldisc_lock, flags);
        if (tty_ldiscs[disc]->refcount)
                ret = -EBUSY;
        else
                tty_ldiscs[disc] = NULL;
-       spin_unlock_irqrestore(&tty_ldisc_lock, flags);
+       raw_spin_unlock_irqrestore(&tty_ldisc_lock, flags);
 
        return ret;
 }
@@ -134,7 +134,7 @@ static struct tty_ldisc_ops *get_ldops(int disc)
        unsigned long flags;
        struct tty_ldisc_ops *ldops, *ret;
 
-       spin_lock_irqsave(&tty_ldisc_lock, flags);
+       raw_spin_lock_irqsave(&tty_ldisc_lock, flags);
        ret = ERR_PTR(-EINVAL);
        ldops = tty_ldiscs[disc];
        if (ldops) {
@@ -144,7 +144,7 @@ static struct tty_ldisc_ops *get_ldops(int disc)
                        ret = ldops;
                }
        }
-       spin_unlock_irqrestore(&tty_ldisc_lock, flags);
+       raw_spin_unlock_irqrestore(&tty_ldisc_lock, flags);
        return ret;
 }
 
@@ -152,10 +152,10 @@ static void put_ldops(struct tty_ldisc_ops *ldops)
 {
        unsigned long flags;
 
-       spin_lock_irqsave(&tty_ldisc_lock, flags);
+       raw_spin_lock_irqsave(&tty_ldisc_lock, flags);
        ldops->refcount--;
        module_put(ldops->owner);
-       spin_unlock_irqrestore(&tty_ldisc_lock, flags);
+       raw_spin_unlock_irqrestore(&tty_ldisc_lock, flags);
 }
 
 /**
@@ -287,11 +287,11 @@ static struct tty_ldisc *tty_ldisc_try(struct tty_struct *tty)
        unsigned long flags;
        struct tty_ldisc *ld;
 
-       spin_lock_irqsave(&tty_ldisc_lock, flags);
+       raw_spin_lock_irqsave(&tty_ldisc_lock, flags);
        ld = NULL;
        if (test_bit(TTY_LDISC, &tty->flags))
                ld = get_ldisc(tty->ldisc);
-       spin_unlock_irqrestore(&tty_ldisc_lock, flags);
+       raw_spin_unlock_irqrestore(&tty_ldisc_lock, flags);
        return ld;
 }