mips: ftrace: fix static function graph tracing

BugLink: https://bugs.launchpad.net/bugs/1784382

commit 6fb86566 upstream.

ftrace_graph_caller was never run after calling ftrace_trace_function,
breaking the function graph tracer. Fix this, bringing it in line with the
x86 implementation.

While we're at it, also streamline the control flow of _mcount a bit to
reduce the number of branches.

This issue was reported before:
https://www.linux-mips.org/archives/linux-mips/2014-11/msg00295.htmlSigned-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Tested-by: Matt Redfearn <matt.redfearn@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/18929/Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: stable@vger.kernel.org # v3.17+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

arch/mips/kernel/mcount.S

@@ -116,10 +116,20 @@ ftrace_stub:
 NESTED(_mcount, PT_SIZE, ra)
 	PTR_LA	t1, ftrace_stub
 	PTR_L	t2, ftrace_trace_function /* Prepare t2 for (1) */
-	bne	t1, t2, static_trace
+	beq	t1, t2, fgraph_trace
 	 nop
 
+	MCOUNT_SAVE_REGS
+
+	move	a0, ra		/* arg1: self return address */
+	jalr	t2		/* (1) call *ftrace_trace_function */
+	 move	a1, AT		/* arg2: parent's return address */
+
+	MCOUNT_RESTORE_REGS
+
+fgraph_trace:
 #ifdef	CONFIG_FUNCTION_GRAPH_TRACER
+	PTR_LA	t1, ftrace_stub
 	PTR_L	t3, ftrace_graph_return
 	bne	t1, t3, ftrace_graph_caller
 	 nop
@@ -128,24 +138,11 @@ NESTED(_mcount, PT_SIZE, ra)
 	bne	t1, t3, ftrace_graph_caller
 	 nop
 #endif
-	b	ftrace_stub
-#ifdef CONFIG_32BIT
-	 addiu sp, sp, 8
-#else
-	 nop
-#endif
 
-static_trace:
-	MCOUNT_SAVE_REGS
-
-	move	a0, ra		/* arg1: self return address */
-	jalr	t2		/* (1) call *ftrace_trace_function */
-	 move	a1, AT		/* arg2: parent's return address */
-
-	MCOUNT_RESTORE_REGS
 #ifdef CONFIG_32BIT
 	addiu sp, sp, 8
 #endif
+
 	.globl ftrace_stub
 ftrace_stub:
 	RETURN_BACK