[PATCHv2] arm64/cpufeature: don't use mutex in bringup path

[Mark Rutland]

Currently, cpus_set_cap() calls static_branch_enable_cpuslocked(), which
must take the jump_label mutex.

We call cpus_set_cap() in the secondary bringup path, from the idle
thread where interrupts are disabled. Taking a mutex in this path "is a
NONO" regardless of whether it's contended, and something we must avoid.
Additionally, the secondary CPU doesn't hold the percpu rwsem (as this
is held by the primary CPU), so this triggers a lockdep splat.

This patch fixes both issues by moving the static_key poking from
cpus_set_cap() into enable_cpu_capabilities(). To account for the static
keys being set later, cpus_have_const_cap() is updated to use another
static key to check whether the const cap keys have been initialised.

This means that users of cpus_have_const_cap() gain should only gain a
single additional NOP in the fast path once the const caps are
initialised, but should always see the current cap value.

This rework means that we can remove the *_cpuslocked() helpers added in
commit d54bb72551b999dd ("arm64/cpufeature: Use
static_branch_enable_cpuslocked()").

Fixes: efd9e03facd075f5 ("arm64: Use static keys for CPU features")
Signed-off-by: Mark Rutland <mark.rutland at arm.com>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Marc Zyniger <marc.zyngier at arm.com>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Sebastian Sewior <bigeasy at linutronix.de>
Cc: Suzuki Poulose <suzuki.poulose at arm.com>
Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: Will Deacon <will.deacon at arm.com>
---
 arch/arm64/include/asm/cpufeature.h | 13 ++++++++++---
 arch/arm64/kernel/cpu_errata.c      |  9 +--------
 arch/arm64/kernel/cpufeature.c      | 25 ++++++++++++++++++++++---
 3 files changed, 33 insertions(+), 14 deletions(-)

Catalin, Will, assuming you're happy with the patch, it will need to go via the
tip tree.

Since v1 [1]:
* Kill redundant update_cpu_errata_workarounds() prototype
* Introduce arm64_const_caps_ready

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2017-May/505731.html

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index 8a7ff73..428ee1f 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -115,6 +115,7 @@ struct arm64_cpu_capabilities {
 
 extern DECLARE_BITMAP(cpu_hwcaps, ARM64_NCAPS);
 extern struct static_key_false cpu_hwcap_keys[ARM64_NCAPS];
+extern struct static_key_false arm64_const_caps_ready;
 
 bool this_cpu_has_cap(unsigned int cap);
 
@@ -124,7 +125,7 @@ static inline bool cpu_have_feature(unsigned int num)
 }
 
 /* System capability check for constant caps */
-static inline bool cpus_have_const_cap(int num)
+static inline bool __cpus_have_const_cap(int num)
 {
 	if (num >= ARM64_NCAPS)
 		return false;
@@ -138,6 +139,14 @@ static inline bool cpus_have_cap(unsigned int num)
 	return test_bit(num, cpu_hwcaps);
 }
 
+static inline bool cpus_have_const_cap(int num)
+{
+	if (static_branch_likely(&arm64_const_caps_ready))
+		return __cpus_have_const_cap(num);
+	else
+		return cpus_have_cap(num);
+}
+
 static inline void cpus_set_cap(unsigned int num)
 {
 	if (num >= ARM64_NCAPS) {
@@ -145,7 +154,6 @@ static inline void cpus_set_cap(unsigned int num)
 			num, ARM64_NCAPS);
 	} else {
 		__set_bit(num, cpu_hwcaps);
-		static_branch_enable_cpuslocked(&cpu_hwcap_keys[num]);
 	}
 }
 
@@ -223,7 +231,6 @@ void update_cpu_capabilities(const struct arm64_cpu_capabilities *caps,
 void check_local_cpu_capabilities(void);
 
 void update_cpu_errata_workarounds(void);
-void update_cpu_errata_workarounds_cpuslocked(void);
 void __init enable_errata_workarounds(void);
 void verify_local_cpu_errata_workarounds(void);
 
diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
index 57d60fa..2ed2a76 100644
--- a/arch/arm64/kernel/cpu_errata.c
+++ b/arch/arm64/kernel/cpu_errata.c
@@ -190,16 +190,9 @@ void verify_local_cpu_errata_workarounds(void)
 		}
 }
 
-void update_cpu_errata_workarounds_cpuslocked(void)
-{
-	update_cpu_capabilities(arm64_errata, "enabling workaround for");
-}
-
 void update_cpu_errata_workarounds(void)
 {
-	get_online_cpus();
-	update_cpu_errata_workarounds_cpuslocked();
-	put_online_cpus();
+	update_cpu_capabilities(arm64_errata, "enabling workaround for");
 }
 
 void __init enable_errata_workarounds(void)
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 803afae..4a89f59 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -986,8 +986,16 @@ void update_cpu_capabilities(const struct arm64_cpu_capabilities *caps,
  */
 void __init enable_cpu_capabilities(const struct arm64_cpu_capabilities *caps)
 {
-	for (; caps->matches; caps++)
-		if (caps->enable && cpus_have_cap(caps->capability))
+	for (; caps->matches; caps++) {
+		unsigned int num = caps->capability;
+
+		if (!cpus_have_cap(num))
+			continue;
+
+		/* Ensure cpus_have_const_cap(num) works */
+		static_branch_enable(&cpu_hwcap_keys[num]);
+
+		if (caps->enable) {
 			/*
 			 * Use stop_machine() as it schedules the work allowing
 			 * us to modify PSTATE, instead of on_each_cpu() which
@@ -995,6 +1003,8 @@ void __init enable_cpu_capabilities(const struct arm64_cpu_capabilities *caps)
 			 * we return.
 			 */
 			stop_machine(caps->enable, NULL, cpu_online_mask);
+		}
+	}
 }
 
 /*
@@ -1086,7 +1096,7 @@ void check_local_cpu_capabilities(void)
 	 * advertised capabilities.
 	 */
 	if (!sys_caps_initialised)
-		update_cpu_errata_workarounds_cpuslocked();
+		update_cpu_errata_workarounds();
 	else
 		verify_local_cpu_capabilities();
 }
@@ -1099,6 +1109,14 @@ static void __init setup_feature_capabilities(void)
 	enable_cpu_capabilities(arm64_features);
 }
 
+DEFINE_STATIC_KEY_FALSE(arm64_const_caps_ready);
+EXPORT_SYMBOL(arm64_const_caps_ready);
+
+static void __init mark_const_caps_ready(void)
+{
+	static_branch_enable(&arm64_const_caps_ready);
+}
+
 /*
  * Check if the current CPU has a given feature capability.
  * Should be called from non-preemptible context.
@@ -1134,6 +1152,7 @@ void __init setup_cpu_features(void)
 	/* Set the CPU feature capabilies */
 	setup_feature_capabilities();
 	enable_errata_workarounds();
+	mark_const_caps_ready();
 	setup_elf_hwcaps(arm64_elf_hwcaps);
 
 	if (system_supports_32bit_el0())
-- 
1.9.1