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@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

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);