linux-kernel - [PATCH 3.16 305/366] tracing/x86/xen: Remove zero data size trace events trace_xen_mmu_flush_tlb{

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Message-ID: <lsq.1539530741.710172338@decadent.org.uk>
Date:   Sun, 14 Oct 2018 16:25:41 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        "Juergen Gross" <jgross@...e.com>
Subject: [PATCH 3.16 305/366] tracing/x86/xen: Remove zero data size trace
 events trace_xen_mmu_flush_tlb{_all}

3.16.60-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>

commit 45dd9b0666a162f8e4be76096716670cf1741f0e upstream.

Doing an audit of trace events, I discovered two trace events in the xen
subsystem that use a hack to create zero data size trace events. This is not
what trace events are for. Trace events add memory footprint overhead, and
if all you need to do is see if a function is hit or not, simply make that
function noinline and use function tracer filtering.

Worse yet, the hack used was:

 __array(char, x, 0)

Which creates a static string of zero in length. There's assumptions about
such constructs in ftrace that this is a dynamic string that is nul
terminated. This is not the case with these tracepoints and can cause
problems in various parts of ftrace.

Nuke the trace events!

Link: http://lkml.kernel.org/r/20180509144605.5a220327@gandalf.local.home

Fixes: 95a7d76897c1e ("xen/mmu: Use Xen specific TLB flush instead of the generic one.")
Reviewed-by: Juergen Gross <jgross@...e.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
[bwh: Backported to 3.16: adjust filename, context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -1283,8 +1283,6 @@ void xen_flush_tlb_all(void)
 	struct mmuext_op *op;
 	struct multicall_space mcs;
 
-	trace_xen_mmu_flush_tlb_all(0);
-
 	preempt_disable();
 
 	mcs = xen_mc_entry(sizeof(*op));
@@ -1297,13 +1295,11 @@ void xen_flush_tlb_all(void)
 
 	preempt_enable();
 }
-static void xen_flush_tlb(void)
+static noinline void xen_flush_tlb(void)
 {
 	struct mmuext_op *op;
 	struct multicall_space mcs;
 
-	trace_xen_mmu_flush_tlb(0);
-
 	preempt_disable();
 
 	mcs = xen_mc_entry(sizeof(*op));
--- a/include/trace/events/xen.h
+++ b/include/trace/events/xen.h
@@ -377,22 +377,6 @@ DECLARE_EVENT_CLASS(xen_mmu_pgd,
 DEFINE_XEN_MMU_PGD_EVENT(xen_mmu_pgd_pin);
 DEFINE_XEN_MMU_PGD_EVENT(xen_mmu_pgd_unpin);
 
-TRACE_EVENT(xen_mmu_flush_tlb_all,
-	    TP_PROTO(int x),
-	    TP_ARGS(x),
-	    TP_STRUCT__entry(__array(char, x, 0)),
-	    TP_fast_assign((void)x),
-	    TP_printk("%s", "")
-	);
-
-TRACE_EVENT(xen_mmu_flush_tlb,
-	    TP_PROTO(int x),
-	    TP_ARGS(x),
-	    TP_STRUCT__entry(__array(char, x, 0)),
-	    TP_fast_assign((void)x),
-	    TP_printk("%s", "")
-	);
-
 TRACE_EVENT(xen_mmu_flush_tlb_single,
 	    TP_PROTO(unsigned long addr),
 	    TP_ARGS(addr),