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Message-Id: <1219209726.21386.23.camel@pasglop>
Date: Wed, 20 Aug 2008 15:22:06 +1000
From: Benjamin Herrenschmidt <benh@...nel.crashing.org>
To: Sebastien Dugue <sebastien.dugue@...l.net>
Cc: linuxppc-dev@...abs.org, linux-kernel@...r.kernel.org,
linux-rt-users@...r.kernel.org, paulus@...ba.org,
michael@...erman.id.au, jean-pierre.dion@...l.net,
gilles.carry@....bull.net, tinytim@...ibm.com, tglx@...utronix.de,
rostedt@...dmis.org, dwalker@...sta.com
Subject: Re: [PATCH 2/2] powerpc - Make the irq reverse mapping radix tree
lockless
On Wed, 2008-08-06 at 15:30 +0200, Sebastien Dugue wrote:
> The radix trees used by interrupt controllers for their irq reverse mapping
> (currently only the XICS found on pSeries) have a complex locking scheme
> dating back to before the advent of the lockless radix tree.
>
> Take advantage of this and of the fact that the items of the tree are
> pointers to a static array (irq_map) elements which can never go under us
> to simplify the locking.
>
> Concurrency between readers and writers is handled by the intrinsic
> properties of the lockless radix tree. Concurrency between writers is handled
> with a spinlock added to the irq_host structure.
No need for a spinlock in the irq_host. Make it one global lock, it's
not like scalability of irq_create_mapping() was a big deal and there's
usually only one of those type of hosts anyway.
>
> Signed-off-by: Sebastien Dugue <sebastien.dugue@...l.net>
> Cc: Paul Mackerras <paulus@...ba.org>
> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Cc: Michael Ellerman <michael@...erman.id.au>
> ---
> arch/powerpc/include/asm/irq.h | 1 +
> arch/powerpc/kernel/irq.c | 74 ++++++----------------------------------
> 2 files changed, 12 insertions(+), 63 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/irq.h b/arch/powerpc/include/asm/irq.h
> index 0a51376..72fd036 100644
> --- a/arch/powerpc/include/asm/irq.h
> +++ b/arch/powerpc/include/asm/irq.h
> @@ -119,6 +119,7 @@ struct irq_host {
> } linear;
> struct radix_tree_root tree;
> } revmap_data;
> + spinlock_t tree_lock;
> struct irq_host_ops *ops;
> void *host_data;
> irq_hw_number_t inval_irq;
> diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
> index dc8663a..7a19103 100644
> --- a/arch/powerpc/kernel/irq.c
> +++ b/arch/powerpc/kernel/irq.c
> @@ -439,8 +439,6 @@ void do_softirq(void)
>
> static LIST_HEAD(irq_hosts);
> static DEFINE_SPINLOCK(irq_big_lock);
> -static DEFINE_PER_CPU(unsigned int, irq_radix_reader);
> -static unsigned int irq_radix_writer;
> static atomic_t revmap_trees_allocated = ATOMIC_INIT(0);
> struct irq_map_entry irq_map[NR_IRQS];
> static unsigned int irq_virq_count = NR_IRQS;
> @@ -584,57 +582,6 @@ void irq_set_virq_count(unsigned int count)
> irq_virq_count = count;
> }
>
> -/* radix tree not lockless safe ! we use a brlock-type mecanism
> - * for now, until we can use a lockless radix tree
> - */
> -static void irq_radix_wrlock(unsigned long *flags)
> -{
> - unsigned int cpu, ok;
> -
> - spin_lock_irqsave(&irq_big_lock, *flags);
> - irq_radix_writer = 1;
> - smp_mb();
> - do {
> - barrier();
> - ok = 1;
> - for_each_possible_cpu(cpu) {
> - if (per_cpu(irq_radix_reader, cpu)) {
> - ok = 0;
> - break;
> - }
> - }
> - if (!ok)
> - cpu_relax();
> - } while(!ok);
> -}
> -
> -static void irq_radix_wrunlock(unsigned long flags)
> -{
> - smp_wmb();
> - irq_radix_writer = 0;
> - spin_unlock_irqrestore(&irq_big_lock, flags);
> -}
> -
> -static void irq_radix_rdlock(unsigned long *flags)
> -{
> - local_irq_save(*flags);
> - __get_cpu_var(irq_radix_reader) = 1;
> - smp_mb();
> - if (likely(irq_radix_writer == 0))
> - return;
> - __get_cpu_var(irq_radix_reader) = 0;
> - smp_wmb();
> - spin_lock(&irq_big_lock);
> - __get_cpu_var(irq_radix_reader) = 1;
> - spin_unlock(&irq_big_lock);
> -}
> -
> -static void irq_radix_rdunlock(unsigned long flags)
> -{
> - __get_cpu_var(irq_radix_reader) = 0;
> - local_irq_restore(flags);
> -}
> -
> static int irq_setup_virq(struct irq_host *host, unsigned int virq,
> irq_hw_number_t hwirq)
> {
> @@ -789,7 +736,6 @@ void irq_dispose_mapping(unsigned int virq)
> {
> struct irq_host *host;
> irq_hw_number_t hwirq;
> - unsigned long flags;
>
> if (virq == NO_IRQ)
> return;
> @@ -825,9 +771,9 @@ void irq_dispose_mapping(unsigned int virq)
> /* Check if radix tree allocated yet */
> if (atomic_read(&revmap_trees_allocated) == 0)
> break;
> - irq_radix_wrlock(&flags);
> + spin_lock(&host->tree_lock);
> radix_tree_delete(&host->revmap_data.tree, hwirq);
> - irq_radix_wrunlock(flags);
> + spin_unlock(&host->tree_lock);
> break;
> }
>
> @@ -881,7 +827,6 @@ unsigned int irq_radix_revmap_lookup(struct irq_host *host,
> {
> struct irq_map_entry *ptr;
> unsigned int virq = NO_IRQ;
> - unsigned long flags;
>
> WARN_ON(host->revmap_type != IRQ_HOST_MAP_TREE);
>
> @@ -893,9 +838,11 @@ unsigned int irq_radix_revmap_lookup(struct irq_host *host,
> return irq_find_mapping(host, hwirq);
>
> /* Now try to resolve */
> - irq_radix_rdlock(&flags);
> + /*
> + * No rcu_read_lock(ing) needed, the ptr returned can't go under us
> + * as it's referencing an entry in the static irq_map table.
> + */
> ptr = radix_tree_lookup(&host->revmap_data.tree, hwirq);
> - irq_radix_rdunlock(flags);
>
> /* Found it, return */
> if (ptr)
> @@ -907,7 +854,6 @@ unsigned int irq_radix_revmap_lookup(struct irq_host *host,
> void irq_radix_revmap_insert(struct irq_host *host, unsigned int virq,
> irq_hw_number_t hwirq)
> {
> - unsigned long flags;
>
> WARN_ON(host->revmap_type != IRQ_HOST_MAP_TREE);
>
> @@ -920,10 +866,10 @@ void irq_radix_revmap_insert(struct irq_host *host, unsigned int virq,
> return;
>
> if (virq != NO_IRQ) {
> - irq_radix_wrlock(&flags);
> + spin_lock(&host->tree_lock);
> radix_tree_insert(&host->revmap_data.tree, hwirq,
> &irq_map[virq]);
> - irq_radix_wrunlock(flags);
> + spin_unlock(&host->tree_lock);
> }
> }
>
> @@ -1041,8 +987,10 @@ static int irq_late_init(void)
> * revmap_trees_allocated.
> */
> list_for_each_entry(h, &irq_hosts, link) {
> - if (h->revmap_type == IRQ_HOST_MAP_TREE)
> + if (h->revmap_type == IRQ_HOST_MAP_TREE) {
> INIT_RADIX_TREE(&h->revmap_data.tree, GFP_ATOMIC);
> + spin_lock_init(&h->tree_lock);
> + }
> }
>
> /*
--
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