From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Wed, 25 Jul 2018 14:02:38 +0200
Subject: [PATCH] arm64: fpsimd: use preemp_disable in addition to
 local_bh_disable()
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/4.18/older/patches-4.18.5-rt3.tar.xz

In v4.16-RT I noticed a number of warnings from task_fpsimd_load(). The
code disables BH and expects that it is not preemptible. On -RT the
task remains preemptible but remains the same CPU. This may corrupt the
content of the SIMD registers if the task is preempted during
saving/restoring those registers.

Add preempt_disable()/enable() to enfore the required semantic on -RT.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 arch/arm64/kernel/fpsimd.c |   31 +++++++++++++++++++++++++++++--
 1 file changed, 29 insertions(+), 2 deletions(-)

--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -159,6 +159,16 @@ static void sve_free(struct task_struct
 	__sve_free(task);
 }
 
+static void *sve_free_atomic(struct task_struct *task)
+{
+	void *sve_state = task->thread.sve_state;
+
+	WARN_ON(test_tsk_thread_flag(task, TIF_SVE));
+
+	task->thread.sve_state = NULL;
+	return sve_state;
+}
+
 static void change_cpacr(u64 val, u64 mask)
 {
 	u64 cpacr = read_sysreg(CPACR_EL1);
@@ -566,6 +576,7 @@ int sve_set_vector_length(struct task_st
 	 * non-SVE thread.
 	 */
 	if (task == current) {
+		preempt_disable();
 		local_bh_disable();
 
 		fpsimd_save();
@@ -576,8 +587,10 @@ int sve_set_vector_length(struct task_st
 	if (test_and_clear_tsk_thread_flag(task, TIF_SVE))
 		sve_to_fpsimd(task);
 
-	if (task == current)
+	if (task == current) {
 		local_bh_enable();
+		preempt_enable();
+	}
 
 	/*
 	 * Force reallocation of task SVE state to the correct size
@@ -832,6 +845,7 @@ asmlinkage void do_sve_acc(unsigned int
 
 	sve_alloc(current);
 
+	preempt_disable();
 	local_bh_disable();
 
 	fpsimd_save();
@@ -845,6 +859,7 @@ asmlinkage void do_sve_acc(unsigned int
 		WARN_ON(1); /* SVE access shouldn't have trapped */
 
 	local_bh_enable();
+	preempt_enable();
 }
 
 /*
@@ -911,10 +926,12 @@ void fpsimd_thread_switch(struct task_st
 void fpsimd_flush_thread(void)
 {
 	int vl, supported_vl;
+	void *mem = NULL;
 
 	if (!system_supports_fpsimd())
 		return;
 
+	preempt_disable();
 	local_bh_disable();
 
 	memset(&current->thread.uw.fpsimd_state, 0,
@@ -923,7 +940,7 @@ void fpsimd_flush_thread(void)
 
 	if (system_supports_sve()) {
 		clear_thread_flag(TIF_SVE);
-		sve_free(current);
+		mem = sve_free_atomic(current);
 
 		/*
 		 * Reset the task vector length as required.
@@ -959,6 +976,8 @@ void fpsimd_flush_thread(void)
 	set_thread_flag(TIF_FOREIGN_FPSTATE);
 
 	local_bh_enable();
+	preempt_enable();
+	kfree(mem);
 }
 
 /*
@@ -970,9 +989,11 @@ void fpsimd_preserve_current_state(void)
 	if (!system_supports_fpsimd())
 		return;
 
+	preempt_disable();
 	local_bh_disable();
 	fpsimd_save();
 	local_bh_enable();
+	preempt_enable();
 }
 
 /*
@@ -1030,6 +1051,7 @@ void fpsimd_restore_current_state(void)
 	if (!system_supports_fpsimd())
 		return;
 
+	preempt_disable();
 	local_bh_disable();
 
 	if (test_and_clear_thread_flag(TIF_FOREIGN_FPSTATE)) {
@@ -1038,6 +1060,7 @@ void fpsimd_restore_current_state(void)
 	}
 
 	local_bh_enable();
+	preempt_enable();
 }
 
 /*
@@ -1050,6 +1073,7 @@ void fpsimd_update_current_state(struct
 	if (!system_supports_fpsimd())
 		return;
 
+	preempt_disable();
 	local_bh_disable();
 
 	current->thread.uw.fpsimd_state = *state;
@@ -1062,6 +1086,7 @@ void fpsimd_update_current_state(struct
 	clear_thread_flag(TIF_FOREIGN_FPSTATE);
 
 	local_bh_enable();
+	preempt_enable();
 }
 
 /*
@@ -1107,6 +1132,7 @@ void kernel_neon_begin(void)
 
 	BUG_ON(!may_use_simd());
 
+	preempt_disable();
 	local_bh_disable();
 
 	__this_cpu_write(kernel_neon_busy, true);
@@ -1120,6 +1146,7 @@ void kernel_neon_begin(void)
 	preempt_disable();
 
 	local_bh_enable();
+	preempt_enable();
 }
 EXPORT_SYMBOL(kernel_neon_begin);