[PATCH RT v3] arm64: fpsimd: use preemp_disable in addition to local_bh_disable()

[Sebastian Andrzej Siewior]

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@xxxxxxxxxxxxx>
---
This should work. Compiling currently gcc-6 on the box to see what
happens. Since the crypto disables preemption "frequently" and I don't
expect or see anything to worry about.

 arch/arm64/kernel/fpsimd.c |   30 ++++++++++++++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -157,6 +157,15 @@ 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;
+}
 
 /* Offset of FFR in the SVE register dump */
 static size_t sve_ffr_offset(int vl)
@@ -594,6 +603,7 @@ int sve_set_vector_length(struct task_st
 	 * non-SVE thread.
 	 */
 	if (task == current) {
+		preempt_disable();
 		local_bh_disable();
 
 		task_fpsimd_save();
@@ -604,8 +614,10 @@ 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
@@ -837,6 +849,7 @@ asmlinkage void do_sve_acc(unsigned int
 
 	sve_alloc(current);
 
+	preempt_disable();
 	local_bh_disable();
 
 	task_fpsimd_save();
@@ -850,6 +863,7 @@ asmlinkage void do_sve_acc(unsigned int
 		WARN_ON(1); /* SVE access shouldn't have trapped */
 
 	local_bh_enable();
+	preempt_enable();
 }
 
 /*
@@ -921,10 +935,12 @@ 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.fpsimd_state, 0, sizeof(struct fpsimd_state));
@@ -932,7 +948,7 @@ 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.
@@ -968,6 +984,8 @@ void fpsimd_flush_thread(void)
 	set_thread_flag(TIF_FOREIGN_FPSTATE);
 
 	local_bh_enable();
+	preempt_enable();
+	kfree(mem);
 }
 
 /*
@@ -979,9 +997,11 @@ void fpsimd_preserve_current_state(void)
 	if (!system_supports_fpsimd())
 		return;
 
+	preempt_disable();
 	local_bh_disable();
 	task_fpsimd_save();
 	local_bh_enable();
+	preempt_enable();
 }
 
 /*
@@ -1021,6 +1041,7 @@ 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)) {
@@ -1029,6 +1050,7 @@ void fpsimd_restore_current_state(void)
 	}
 
 	local_bh_enable();
+	preempt_enable();
 }
 
 /*
@@ -1041,6 +1063,7 @@ void fpsimd_update_current_state(struct
 	if (!system_supports_fpsimd())
 		return;
 
+	preempt_disable();
 	local_bh_disable();
 
 	current->thread.fpsimd_state.user_fpsimd = *state;
@@ -1053,6 +1076,7 @@ void fpsimd_update_current_state(struct
 		fpsimd_bind_to_cpu();
 
 	local_bh_enable();
+	preempt_enable();
 }
 
 /*
@@ -1115,6 +1139,7 @@ void kernel_neon_begin(void)
 
 	BUG_ON(!may_use_simd());
 
+	preempt_disable();
 	local_bh_disable();
 
 	__this_cpu_write(kernel_neon_busy, true);
@@ -1131,6 +1156,7 @@ void kernel_neon_begin(void)
 	preempt_disable();
 
 	local_bh_enable();
+	preempt_enable();
 }
 EXPORT_SYMBOL(kernel_neon_begin);