Re: [PATCH] arm64: cpufeature: use static_branch_enable_cpuslocked() (was: Re: [patch V2 00/24] cpu/hotplug: Convert get_online_cpus() to a percpu_rwsem)

[Suzuki K Poulose <Suzuki.Poulose@arm.com>]

On Thu, Apr 27, 2017 at 04:48:06PM +0100, Mark Rutland wrote:
> Hi Catalin/Will,
> 
> The below addresses a boot failure Catalin spotted in next-20170424,
> based on Sebastian's patch [1]. I've given it a spin on Juno R1, where I
> can reproduce the issue prior to applying this patch.
> 
> I believe this would need to go via tip, as the issue is a result of
> change in the tip smp/hotplug branch, and the fix depends on
> infrastructure introduced there.
> 
> Are you happy with the fix, and for it to go via the tip tree?
> 
> Thanks,
> Mark.
> 
> [1] https://lkml.kernel.org/r/20170425172838.mr3kyccsdteyjso5@linutronix.de
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/log/?h=smp/hotplug
> 
> ---->8----
> From 6cdb503b060f74743769c9f601c35f985d3c58eb Mon Sep 17 00:00:00 2001
> From: Mark Rutland <mark.rutland@arm.com>
> Date: Wed, 26 Apr 2017 09:46:47 +0100
> Subject: [PATCH] arm64: cpufeature: use static_branch_enable_cpuslocked()
> 
> Recently, the hotplug locking was conveted to use a percpu rwsem. Unlike
> the existing {get,put}_online_cpus() logic, this can't nest.
> Unfortunately, in arm64's secondary boot path we can end up nesting via
> static_branch_enable() in cpus_set_cap() when we detect an erratum.
> 
> This leads to a stream of messages as below, where the secondary
> attempts to schedule before it has been fully onlined. As the CPU
> orchestrating the onlining holds the rswem, this hangs the system.
> 
> [    0.250334] BUG: scheduling while atomic: swapper/1/0/0x00000002
> [    0.250337] Modules linked in:
> [    0.250346] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.11.0-rc7-next-20170424 #2
> [    0.250349] Hardware name: ARM Juno development board (r1) (DT)
> [    0.250353] Call trace:
> [    0.250365] [<ffff000008088510>] dump_backtrace+0x0/0x238
> [    0.250371] [<ffff00000808880c>] show_stack+0x14/0x20
> [    0.250377] [<ffff00000839d854>] dump_stack+0x9c/0xc0
> [    0.250384] [<ffff0000080e3540>] __schedule_bug+0x50/0x70
> [    0.250391] [<ffff000008932ecc>] __schedule+0x52c/0x5a8
> [    0.250395] [<ffff000008932f80>] schedule+0x38/0xa0
> [    0.250400] [<ffff000008935e8c>] rwsem_down_read_failed+0xc4/0x108
> [    0.250407] [<ffff0000080fe8e0>] __percpu_down_read+0x100/0x118
> [    0.250414] [<ffff0000080c0b60>] get_online_cpus+0x70/0x78
> [    0.250420] [<ffff0000081749e8>] static_key_enable+0x28/0x48
> [    0.250425] [<ffff00000808de90>] update_cpu_capabilities+0x78/0xf8
> [    0.250430] [<ffff00000808d14c>] update_cpu_errata_workarounds+0x1c/0x28
> [    0.250435] [<ffff00000808e004>] check_local_cpu_capabilities+0xf4/0x128
> [    0.250440] [<ffff00000808e894>] secondary_start_kernel+0x8c/0x118
> [    0.250444] [<000000008093d1b4>] 0x8093d1b4
> 
> We call cpus_set_cap() from update_cpu_capabilities(), which is called
> from the secondary boot path (where the CPU orchestrating the onlining
> holds the hotplug rwsem), and in the primary boot path, where this is
> not held.
> 
> This patch makes cpus_set_cap() use static_branch_enable_cpuslocked(),
> and updates the primary CPU boot path to hold the rwsem so as to keep
> the *_cpuslocked() code happy.
> 
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Reported-by: Catalin Marinas <catalin.marinas@arm.com>
> Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Suggested-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Suzuki Poulose <suzuki,poulose@arm.com>
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> ---
>  arch/arm64/include/asm/cpufeature.h | 2 +-
>  arch/arm64/kernel/smp.c             | 8 +++++---
>  2 files changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index f31c48d..349b5cd 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -145,7 +145,7 @@ static inline void cpus_set_cap(unsigned int num)
>  			num, ARM64_NCAPS);
>  	} else {
>  		__set_bit(num, cpu_hwcaps);
> -		static_branch_enable(&cpu_hwcap_keys[num]);
> +		static_branch_enable_cpuslocked(&cpu_hwcap_keys[num]);
>  	}
>  }
>  
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 9b10365..c2ce9aa 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -447,11 +447,13 @@ void __init smp_prepare_boot_cpu(void)
>  	cpuinfo_store_boot_cpu();
>  	save_boot_cpu_run_el();
>  	/*
> -	 * Run the errata work around checks on the boot CPU, once we have
> -	 * initialised the cpu feature infrastructure from
> -	 * cpuinfo_store_boot_cpu() above.
> +	 * Run the errata work around checks on the boot CPU, now that
> +	 * cpuinfo_store_boot_cpu() has set things up. We hold the percpu rwsem
> +	 * to keep the workaround setup code happy.
>  	 */
> +	get_online_cpus();
>  	update_cpu_errata_workarounds();
> +	put_online_cpus();

We need similar locking for updat_cpu_capabilities() called
from setup_cpu_feature_capabilities(). i.e., the following fixup is required.

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 94b8f7f..62bdab4 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -966,6 +966,7 @@ static void __init setup_elf_hwcaps(const struct arm64_cpu_capabilities *hwcaps)
 			cap_set_elf_hwcap(hwcaps);
 }
 
+/* Should be called with CPU hotplug lock held */
 void update_cpu_capabilities(const struct arm64_cpu_capabilities *caps,
 			    const char *info)
 {
@@ -1092,7 +1093,9 @@ void check_local_cpu_capabilities(void)
 
 static void __init setup_feature_capabilities(void)
 {
-	update_cpu_capabilities(arm64_features, "detected feature:");
+	get_online_cpus();
+	update_cpu_capabilities(arm6_features, "detected feature:");
+	put_online_cpus();
 	enable_cpu_capabilities(arm64_features);
 }
 
-- 
---

Also, I think having update_cpu_errata_workarounds() called with and without
the CPU hotplug lock makes it a bit confusing without proper explanation.
How about the following patch which makes things a bit more reader friendly ?

---8>---

>From f3b0809224e4915197d3ae4a38ebe7f210e74abf Mon Sep 17 00:00:00 2001
From: Mark Rutland <mark.rutland@arm.com>
Date: Thu, 27 Apr 2017 16:48:06 +0100
Subject: [PATCH] arm64: cpufeature: use static_branch_enable_cpuslocked()

Recently, the hotplug locking was conveted to use a percpu rwsem. Unlike
the existing {get,put}_online_cpus() logic, this can't nest.
Unfortunately, in arm64's secondary boot path we can end up nesting via
static_branch_enable() in cpus_set_cap() when we detect an erratum.

This leads to a stream of messages as below, where the secondary
attempts to schedule before it has been fully onlined. As the CPU
orchestrating the onlining holds the rswem, this hangs the system.

[    0.250334] BUG: scheduling while atomic: swapper/1/0/0x00000002
[    0.250337] Modules linked in:
[    0.250346] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.11.0-rc7-next-20170424 #2
[    0.250349] Hardware name: ARM Juno development board (r1) (DT)
[    0.250353] Call trace:
[    0.250365] [<ffff000008088510>] dump_backtrace+0x0/0x238
[    0.250371] [<ffff00000808880c>] show_stack+0x14/0x20
[    0.250377] [<ffff00000839d854>] dump_stack+0x9c/0xc0
[    0.250384] [<ffff0000080e3540>] __schedule_bug+0x50/0x70
[    0.250391] [<ffff000008932ecc>] __schedule+0x52c/0x5a8
[    0.250395] [<ffff000008932f80>] schedule+0x38/0xa0
[    0.250400] [<ffff000008935e8c>] rwsem_down_read_failed+0xc4/0x108
[    0.250407] [<ffff0000080fe8e0>] __percpu_down_read+0x100/0x118
[    0.250414] [<ffff0000080c0b60>] get_online_cpus+0x70/0x78
[    0.250420] [<ffff0000081749e8>] static_key_enable+0x28/0x48
[    0.250425] [<ffff00000808de90>] update_cpu_capabilities+0x78/0xf8
[    0.250430] [<ffff00000808d14c>] update_cpu_errata_workarounds+0x1c/0x28
[    0.250435] [<ffff00000808e004>] check_local_cpu_capabilities+0xf4/0x128
[    0.250440] [<ffff00000808e894>] secondary_start_kernel+0x8c/0x118
[    0.250444] [<000000008093d1b4>] 0x8093d1b4

We call cpus_set_cap() from update_cpu_capabilities(), which is called
from the secondary boot path (where the CPU orchestrating the onlining
holds the hotplug rwsem), and in the primary boot path, where this is
not held.

This patch makes cpus_set_cap() use static_branch_enable_cpuslocked(),
and updates all the callers of update_cpu_capabilities() consistent with
the change.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reported-by: Catalin Marinas <catalin.marinas@arm.com>
Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Suzuki Poulose <suzuki,poulose@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
---
 arch/arm64/include/asm/cpufeature.h |  5 +++--
 arch/arm64/kernel/cpu_errata.c      | 13 ++++++++++++-
 arch/arm64/kernel/cpufeature.c      |  5 ++++-
 arch/arm64/kernel/smp.c             |  7 +++----
 4 files changed, 22 insertions(+), 8 deletions(-)

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index e7f84a7..2a832c6 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -145,7 +145,7 @@ static inline void cpus_set_cap(unsigned int num)
 			num, ARM64_NCAPS);
 	} else {
 		__set_bit(num, cpu_hwcaps);
-		static_branch_enable(&cpu_hwcap_keys[num]);
+		static_branch_enable_cpuslocked(&cpu_hwcap_keys[num]);
 	}
 }
 
@@ -222,7 +222,8 @@ void update_cpu_capabilities(const struct arm64_cpu_capabilities *caps,
 void enable_cpu_capabilities(const struct arm64_cpu_capabilities *caps);
 void check_local_cpu_capabilities(void);
 
-void update_cpu_errata_workarounds(void);
+void update_secondary_cpu_errata_workarounds(void);
+void update_boot_cpu_errata_workarounds(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 2ed2a76..f2d889b 100644
--- a/arch/arm64/kernel/cpu_errata.c
+++ b/arch/arm64/kernel/cpu_errata.c
@@ -190,9 +190,20 @@ void verify_local_cpu_errata_workarounds(void)
 		}
 }
 
-void update_cpu_errata_workarounds(void)
+/*
+ * Secondary CPUs are booted with the waker holding the
+ * CPU hotplug lock, hence we don't need to lock it here again.
+ */
+void update_secondary_cpu_errata_workarounds(void)
+{
+	update_cpu_capabilities(arm64_errata, "enabling workaround for");
+}
+
+void update_boot_cpu_errata_workarounds(void)
 {
+	get_online_cpus();
 	update_cpu_capabilities(arm64_errata, "enabling workaround for");
+	put_online_cpus();
 }
 
 void __init enable_errata_workarounds(void)
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 94b8f7f..62bdab4 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -966,6 +966,7 @@ static void __init setup_elf_hwcaps(const struct arm64_cpu_capabilities *hwcaps)
 			cap_set_elf_hwcap(hwcaps);
 }
 
+/* Should be called with CPU hotplug lock held */
 void update_cpu_capabilities(const struct arm64_cpu_capabilities *caps,
 			    const char *info)
 {
@@ -1092,7 +1093,9 @@ void check_local_cpu_capabilities(void)
 
 static void __init setup_feature_capabilities(void)
 {
-	update_cpu_capabilities(arm64_features, "detected feature:");
+	get_online_cpus();
+	update_cpu_capabilities(arm6_features, "detected feature:");
+	put_online_cpus();
 	enable_cpu_capabilities(arm64_features);
 }
 
diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 6e0e16a..51ba91f 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -450,11 +450,10 @@ void __init smp_prepare_boot_cpu(void)
 	cpuinfo_store_boot_cpu();
 	save_boot_cpu_run_el();
 	/*
-	 * Run the errata work around checks on the boot CPU, once we have
-	 * initialised the cpu feature infrastructure from
-	 * cpuinfo_store_boot_cpu() above.
+	 * Run the errata work around checks on the boot CPU, now that
+	 * cpuinfo_store_boot_cpu() has set things up.
 	 */
-	update_cpu_errata_workarounds();
+	update_boot_cpu_errata_workarounds();
 }
 
 static u64 __init of_get_cpu_mpidr(struct device_node *dn)
-- 
2.7.4