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Message-ID: <20061117023316.GA3707@us.ibm.com>
Date: Thu, 16 Nov 2006 18:33:16 -0800
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Linus Torvalds <torvalds@...l.org>
Cc: Thomas Gleixner <tglx@...esys.com>, Ingo Molnar <mingo@...e.hu>,
Alan Stern <stern@...land.harvard.edu>,
LKML <linux-kernel@...r.kernel.org>,
john stultz <johnstul@...ibm.com>,
David Miller <davem@...emloft.net>,
Arjan van de Ven <arjan@...radead.org>,
Andrew Morton <akpm@...l.org>, Andi Kleen <ak@...e.de>
Subject: Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync
On Thu, Nov 16, 2006 at 01:47:48PM -0800, Linus Torvalds wrote:
>
>
> On Thu, 16 Nov 2006, Thomas Gleixner wrote:
> >
> > Here is the i386/sparc fixup
>
> Gag me with a volvo.
No can do -- my wife drives a Ford and my car is a bicycle.
> This is disgusting, but I would actually prefer the following version over
> the patches I've seen, because
>
> - it doesn't end up having any architecture-specific parts
>
> - it doesn't use the new "xxx_sync()" thing that I'm not even sure we
> should be using.
>
> - it makes it clear that this should be fixed, preferably by just having
> some way to initialize SRCU structs staticalyl. If we get that, the fix
> is to just replace the horrible "initialize by hand" with a static
> initializer once and for all.
>
> Hmm?
>
> Totally untested, but it compiles and it _looks_ sane. The overhead of the
> function call should be minimal, once things are initialized.
>
> Paul, it would be _really_ nice to have some way to just initialize that
> SRCU thing statically. This kind of crud is just crazy.
Static initialization is a bit of a tarpit for SRCU. Before this week,
I would have protested bitterly over the overhead of a dynamic runtime
check, but Jens is running into another issue that looks to require a
bit more read-side overhead as well (synchronize_srcu() is too expensive
for his situation). Now if I can get one of the local weak-memory model
torture-chamber boxes to deal with a recent kernel...
Hardware whines aside, shouldn't be too hard. Will put something
together...
Thanx, Paul
> Comments?
>
> Linus
>
> ----
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 86e69b7..02326b2 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -52,14 +52,39 @@ static void handle_update(void *data);
> * The mutex locks both lists.
> */
> static BLOCKING_NOTIFIER_HEAD(cpufreq_policy_notifier_list);
> -static struct srcu_notifier_head cpufreq_transition_notifier_list;
>
> -static int __init init_cpufreq_transition_notifier_list(void)
> +/*
> + * This is horribly horribly ugly.
> + *
> + * We really want to initialize the transition notifier list
> + * statically and just once, but there is no static way to
> + * initialize a srcu lock, so we instead make up all this nasty
> + * infrastructure to make sure it's initialized when we use it.
> + *
> + * Bleaargh.
> + */
> +static struct srcu_notifier_head *cpufreq_transition_notifier_list(void)
> {
> - srcu_init_notifier_head(&cpufreq_transition_notifier_list);
> - return 0;
> + static struct srcu_notifier_head *initialized;
> + struct srcu_notifier_head *ret;
> +
> + ret = initialized;
> + if (!ret) {
> + static DEFINE_MUTEX(init_lock);
> +
> + mutex_lock(&init_lock);
> + ret = initialized;
> + if (!ret) {
> + static struct srcu_notifier_head list_head;
> + ret = &list_head;
> + srcu_init_notifier_head(ret);
> + smp_wmb();
> + initialized = ret;
> + }
> + mutex_unlock(&init_lock);
> + }
> + return ret;
> }
> -core_initcall(init_cpufreq_transition_notifier_list);
>
> static LIST_HEAD(cpufreq_governor_list);
> static DEFINE_MUTEX (cpufreq_governor_mutex);
> @@ -268,14 +293,14 @@ void cpufreq_notify_transition(struct cp
> freqs->old = policy->cur;
> }
> }
> - srcu_notifier_call_chain(&cpufreq_transition_notifier_list,
> + srcu_notifier_call_chain(cpufreq_transition_notifier_list(),
> CPUFREQ_PRECHANGE, freqs);
> adjust_jiffies(CPUFREQ_PRECHANGE, freqs);
> break;
>
> case CPUFREQ_POSTCHANGE:
> adjust_jiffies(CPUFREQ_POSTCHANGE, freqs);
> - srcu_notifier_call_chain(&cpufreq_transition_notifier_list,
> + srcu_notifier_call_chain(cpufreq_transition_notifier_list(),
> CPUFREQ_POSTCHANGE, freqs);
> if (likely(policy) && likely(policy->cpu == freqs->cpu))
> policy->cur = freqs->new;
> @@ -1055,7 +1080,7 @@ static int cpufreq_suspend(struct sys_de
> freqs.old = cpu_policy->cur;
> freqs.new = cur_freq;
>
> - srcu_notifier_call_chain(&cpufreq_transition_notifier_list,
> + srcu_notifier_call_chain(cpufreq_transition_notifier_list(),
> CPUFREQ_SUSPENDCHANGE, &freqs);
> adjust_jiffies(CPUFREQ_SUSPENDCHANGE, &freqs);
>
> @@ -1137,7 +1162,7 @@ static int cpufreq_resume(struct sys_dev
> freqs.new = cur_freq;
>
> srcu_notifier_call_chain(
> - &cpufreq_transition_notifier_list,
> + cpufreq_transition_notifier_list(),
> CPUFREQ_RESUMECHANGE, &freqs);
> adjust_jiffies(CPUFREQ_RESUMECHANGE, &freqs);
>
> @@ -1183,7 +1208,7 @@ int cpufreq_register_notifier(struct not
> switch (list) {
> case CPUFREQ_TRANSITION_NOTIFIER:
> ret = srcu_notifier_chain_register(
> - &cpufreq_transition_notifier_list, nb);
> + cpufreq_transition_notifier_list(), nb);
> break;
> case CPUFREQ_POLICY_NOTIFIER:
> ret = blocking_notifier_chain_register(
> @@ -1215,7 +1240,7 @@ int cpufreq_unregister_notifier(struct n
> switch (list) {
> case CPUFREQ_TRANSITION_NOTIFIER:
> ret = srcu_notifier_chain_unregister(
> - &cpufreq_transition_notifier_list, nb);
> + cpufreq_transition_notifier_list(), nb);
> break;
> case CPUFREQ_POLICY_NOTIFIER:
> ret = blocking_notifier_chain_unregister(
-
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