Commit c9739941 authored by Ivo Sieben's avatar Ivo Sieben Committed by Greg Kroah-Hartman
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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: default avatarIvo Sieben <meltedpianoman@gmail.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent ad3d1e5f

drivers/tty/tty_ldisc.c

+16 −16
Original line number Diff line number Diff line
@@ -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;

}

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