arm64/sve: Delay freeing memory in fpsimd_flush_thread()

fpsimd_flush_thread() invokes kfree() via sve_free()+sme_free() within a
preempt disabled section which is not working on -RT.

Delay freeing of memory until preemption is enabled again.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220505163207.85751-2-bigeasy@linutronix.de
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: Sebastian Andrzej Siewior

arch/arm64/kernel/fpsimd.c

@@ -1562,6 +1562,9 @@ static void fpsimd_flush_thread_vl(enum vec_type type)
 
 void fpsimd_flush_thread(void)
 {
+	void *sve_state = NULL;
+	void *za_state = NULL;
+
 	if (!system_supports_fpsimd())
 		return;
 
@@ -1573,18 +1576,28 @@ void fpsimd_flush_thread(void)
 
 	if (system_supports_sve()) {
 		clear_thread_flag(TIF_SVE);
-		sve_free(current);
+
+		/* Defer kfree() while in atomic context */
+		sve_state = current->thread.sve_state;
+		current->thread.sve_state = NULL;
+
 		fpsimd_flush_thread_vl(ARM64_VEC_SVE);
 	}
 
 	if (system_supports_sme()) {
 		clear_thread_flag(TIF_SME);
-		sme_free(current);
+
+		/* Defer kfree() while in atomic context */
+		za_state = current->thread.za_state;
+		current->thread.za_state = NULL;
+
 		fpsimd_flush_thread_vl(ARM64_VEC_SME);
 		current->thread.svcr = 0;
 	}
 
 	put_cpu_fpsimd_context();
+	kfree(sve_state);
+	kfree(za_state);
 }
 
 /*