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Message-ID: <20170802143748.dmthd3hvbq5pbelt@hirez.programming.kicks-ass.net>
Date: Wed, 2 Aug 2017 16:37:48 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Marc Zyngier <marc.zyngier@....com>
Cc: Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org, Leo Yan <leo.yan@...aro.org>,
Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v2 3/4] jump_label: Provide hotplug context variants
On Tue, Aug 01, 2017 at 09:02:56AM +0100, Marc Zyngier wrote:
> As using the normal static key API under the hotplug lock is
> pretty much impossible, let's provide a variant of some of them
> that require the hotplug lock to have already been taken.
>
> These function are only meant to be used in CPU hotplug callbacks.
>
> Signed-off-by: Marc Zyngier <marc.zyngier@....com>
> ---
> Documentation/static-keys.txt | 15 +++++++++++++++
> include/linux/jump_label.h | 11 +++++++++--
> kernel/jump_label.c | 20 ++++++++++++++++++++
> 3 files changed, 44 insertions(+), 2 deletions(-)
I stuffed them in my locking/core branch on top of Paolo's patches:
https://lkml.kernel.org/r/1501601046-35683-1-git-send-email-pbonzini@redhat.com
This patch now looks like so.
---
Subject: jump_label: Provide hotplug context variants
From: Marc Zyngier <marc.zyngier@....com>
Date: Tue, 1 Aug 2017 09:02:56 +0100
As using the normal static key API under the hotplug lock is
pretty much impossible, let's provide a variant of some of them
that require the hotplug lock to have already been taken.
These function are only meant to be used in CPU hotplug callbacks.
Cc: linux-arm-kernel@...ts.infradead.org
Cc: Leo Yan <leo.yan@...aro.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Marc Zyngier <marc.zyngier@....com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: http://lkml.kernel.org/r/20170801080257.5056-4-marc.zyngier@arm.com
---
Documentation/static-keys.txt | 15 +++++++++++++++
include/linux/jump_label.h | 11 +++++++++--
kernel/jump_label.c | 22 ++++++++++++++++++----
3 files changed, 42 insertions(+), 6 deletions(-)
--- a/Documentation/static-keys.txt
+++ b/Documentation/static-keys.txt
@@ -154,6 +154,21 @@ and 'static_key_count()'. In general, i
should be protected with the same mutex used around the enable/disable
or increment/decrement function.
+Note that switching branches results in some locks being taken,
+particularly the CPU hotplug lock (in order to avoid races against
+CPUs being brought in the kernel whilst the kernel is getting
+patched). Calling the static key API from within a hotplug notifier is
+thus a sure deadlock recipe. In order to still allow use of the
+functionnality, the following functions are provided:
+
+ static_key_enable_cpuslocked()
+ static_key_disable_cpuslocked()
+ static_branch_enable_cpuslocked()
+ static_branch_disable_cpuslocked()
+
+These functions are *not* general purpose, and must only be used when
+you really know that you're in the above context, and no other.
+
Where an array of keys is required, it can be defined as::
DEFINE_STATIC_KEY_ARRAY_TRUE(keys, count);
--- a/include/linux/jump_label.h
+++ b/include/linux/jump_label.h
@@ -163,6 +163,8 @@ extern void jump_label_apply_nops(struct
extern int static_key_count(struct static_key *key);
extern void static_key_enable(struct static_key *key);
extern void static_key_disable(struct static_key *key);
+extern void static_key_enable_cpuslocked(struct static_key *key);
+extern void static_key_disable_cpuslocked(struct static_key *key);
/*
* We should be using ATOMIC_INIT() for initializing .enabled, but
@@ -254,6 +256,9 @@ static inline void static_key_disable(st
atomic_set(&key->enabled, 0);
}
+#define static_key_enable_cpuslocked(k) static_key_enable((k))
+#define static_key_disable_cpuslocked(k) static_key_disable((k))
+
#define STATIC_KEY_INIT_TRUE { .enabled = ATOMIC_INIT(1) }
#define STATIC_KEY_INIT_FALSE { .enabled = ATOMIC_INIT(0) }
@@ -415,8 +420,10 @@ extern bool ____wrong_branch_error(void)
* Normal usage; boolean enable/disable.
*/
-#define static_branch_enable(x) static_key_enable(&(x)->key)
-#define static_branch_disable(x) static_key_disable(&(x)->key)
+#define static_branch_enable(x) static_key_enable(&(x)->key)
+#define static_branch_disable(x) static_key_disable(&(x)->key)
+#define static_branch_enable_cpuslocked(x) static_key_enable_cpuslocked(&(x)->key)
+#define static_branch_disable_cpuslocked(x) static_key_disable_cpuslocked(&(x)->key)
#endif /* __ASSEMBLY__ */
--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -126,15 +126,15 @@ void static_key_slow_inc(struct static_k
}
EXPORT_SYMBOL_GPL(static_key_slow_inc);
-void static_key_enable(struct static_key *key)
+void static_key_enable_cpuslocked(struct static_key *key)
{
STATIC_KEY_CHECK_USE();
+
if (atomic_read(&key->enabled) > 0) {
WARN_ON_ONCE(atomic_read(&key->enabled) != 1);
return;
}
- cpus_read_lock();
jump_label_lock();
if (atomic_read(&key->enabled) == 0) {
atomic_set(&key->enabled, -1);
@@ -145,23 +145,37 @@ void static_key_enable(struct static_key
atomic_set_release(&key->enabled, 1);
}
jump_label_unlock();
+}
+EXPORT_SYMBOL_GPL(static_key_enable_cpuslocked);
+
+void static_key_enable(struct static_key *key)
+{
+ cpus_read_lock();
+ static_key_enable_cpuslocked(key);
cpus_read_unlock();
}
EXPORT_SYMBOL_GPL(static_key_enable);
-void static_key_disable(struct static_key *key)
+void static_key_disable_cpuslocked(struct static_key *key)
{
STATIC_KEY_CHECK_USE();
+
if (atomic_read(&key->enabled) != 1) {
WARN_ON_ONCE(atomic_read(&key->enabled) != 0);
return;
}
- cpus_read_lock();
jump_label_lock();
if (atomic_cmpxchg(&key->enabled, 1, 0))
jump_label_update(key);
jump_label_unlock();
+}
+EXPORT_SYMBOL_GPL(static_key_disable_cpuslocked);
+
+void static_key_disable(struct static_key *key)
+{
+ cpus_read_lock();
+ static_key_disable_cpuslocked(key);
cpus_read_unlock();
}
EXPORT_SYMBOL_GPL(static_key_disable);
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