linux-kernel - [PATCH] arm64/cpufeature: Convert hook_lock to raw_spin_lock_t in cpu_enable

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Date:   Thu, 30 May 2019 12:30:58 +0100
From:   Julien Grall <julien.grall@....com>
To:     linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Cc:     tglx@...utronix.de, rostedt@...dmis.org, bigeasy@...utronix.de,
        suzuki.poulose@....com, catalin.marinas@....com,
        will.deacon@....com, dave.martin@....com,
        Julien Grall <julien.grall@....com>
Subject: [PATCH] arm64/cpufeature: Convert hook_lock to raw_spin_lock_t in cpu_enable_ssbs()

cpu_enable_ssbs() is called via stop_machine() as part of the cpu_enable
callback. A spin lock is used to ensure the hook is registered before
the rest of the callback is executed.

On -RT spin_lock() may sleep. However, all the callees in stop_machine()
are expected to not sleep. Therefore a raw_spin_lock() is required here.

Given this is already done under stop_machine() and the work done under
the lock is quite small, the latency should not increase too much.

Signed-off-by: Julien Grall <julien.grall@....com>

---

It was noticed when looking at the current use of spin_lock in
arch/arm64. I don't have a platform calling that callback, so I have
hacked the code to reproduce the error and check it is now fixed.
---
 arch/arm64/kernel/cpufeature.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index ca27e08e3d8a..2a7159fda3ce 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -1194,14 +1194,14 @@ static struct undef_hook ssbs_emulation_hook = {
 static void cpu_enable_ssbs(const struct arm64_cpu_capabilities *__unused)
 {
 	static bool undef_hook_registered = false;
-	static DEFINE_SPINLOCK(hook_lock);
+	static DEFINE_RAW_SPINLOCK(hook_lock);
 
-	spin_lock(&hook_lock);
+	raw_spin_lock(&hook_lock);
 	if (!undef_hook_registered) {
 		register_undef_hook(&ssbs_emulation_hook);
 		undef_hook_registered = true;
 	}
-	spin_unlock(&hook_lock);
+	raw_spin_unlock(&hook_lock);
 
 	if (arm64_get_ssbd_state() == ARM64_SSBD_FORCE_DISABLE) {
 		sysreg_clear_set(sctlr_el1, 0, SCTLR_ELx_DSSBS);
-- 
2.11.0