Le 24/03/2022 à 23:33, Steven Rostedt a écrit :

On Thu, 24 Mar 2022 11:01:45 +0100
Christophe Leroy [off-list ref] wrote:

quoted

@@ -1006,7 +1006,20 @@ unsigned long ftrace_graph_ret_addr(struct task_struct *task, int *idx,
  extern int register_ftrace_graph(struct fgraph_ops *ops);
  extern void unregister_ftrace_graph(struct fgraph_ops *ops);
  
-extern bool ftrace_graph_is_dead(void);
+/**
+ * ftrace_graph_is_dead - returns true if ftrace_graph_stop() was called
+ *
+ * ftrace_graph_stop() is called when a severe error is detected in
+ * the function graph tracing. This function is called by the critical
+ * paths of function graph to keep those paths from doing any more harm.
+ */
+extern bool kill_ftrace_graph;
+
+static inline bool ftrace_graph_is_dead(void)
+{
+	return kill_ftrace_graph;
+}
+
  extern void ftrace_graph_stop(void);

The reason I did not expose that variable, is because I didn't want it to
be touched outside of the kernel/trace directory. Or the ftrace.c file for
that matter (although, I could put it in fgraph.c :-/)

What would be better, is to make it a static branch.

extern struct static_key fgraph_dead;

static inline bool ftrace_graph_is_dead(void)
{
	if (static_key_false(&fgraph_dead))
		return true;
	return false;
}

That way we even get rid of the conditional branch.

Yeah, the fgraph_dead is still exposed for anyone to touch, but it still
requires a function to modify it, so I'm not as worried it will be touched
as easily.

If your concern is about kill_ftrace_graph being visible in ftrace.h, we 
could have hidden it with:

static inline bool ftrace_graph_is_dead(void)
{
	extern bool kill_ftrace_graph;

	return kill_ftrace_graph;
}


But using a static branch is even better, I'll do that.

By the way, I was wondering if that call is still necessary.

In 2014, in commit 545d47b8f359 ("ftrace-graph: Remove usage of 
ftrace_stop() in ftrace_graph_stop()") you wrote: "All archs now use 
ftrace_graph_is_dead() to stop function graph tracing".

But when I grep it nowadays, it seems not all architecture are using it:

$ git grep "prepare_ftrace_return(" arch/*ftrace.c

arch/arm/kernel/ftrace.c:void prepare_ftrace_return(unsigned long 
*parent, unsigned long self_addr,
arch/arm64/kernel/ftrace.c:void prepare_ftrace_return(unsigned long 
self_addr, unsigned long *parent,
arch/csky/kernel/ftrace.c:void prepare_ftrace_return(unsigned long 
*parent, unsigned long self_addr,
arch/microblaze/kernel/ftrace.c:void prepare_ftrace_return(unsigned long 
*parent, unsigned long self_addr)
arch/mips/kernel/ftrace.c:void prepare_ftrace_return(unsigned long 
*parent_ra_addr, unsigned long self_ra,
arch/nds32/kernel/ftrace.c:void prepare_ftrace_return(unsigned long 
*parent, unsigned long self_addr,
arch/nds32/kernel/ftrace.c:	prepare_ftrace_return(parent_ip, selfpc, 
frame_pointer);
arch/parisc/kernel/ftrace.c:static void __hot 
prepare_ftrace_return(unsigned long *parent,
arch/parisc/kernel/ftrace.c:		prepare_ftrace_return(parent_rp, self_addr);
arch/powerpc/kernel/trace/ftrace.c:unsigned long 
prepare_ftrace_return(unsigned long parent, unsigned long ip,
arch/powerpc/kernel/trace/ftrace.c:	fregs->regs.link = 
prepare_ftrace_return(parent_ip, ip, fregs->regs.gpr[1]);
arch/riscv/kernel/ftrace.c:void prepare_ftrace_return(unsigned long 
*parent, unsigned long self_addr,
arch/s390/kernel/ftrace.c:unsigned long prepare_ftrace_return(unsigned 
long ra, unsigned long sp,
arch/sh/kernel/ftrace.c:void prepare_ftrace_return(unsigned long 
*parent, unsigned long self_addr)
arch/sparc/kernel/ftrace.c:unsigned long prepare_ftrace_return(unsigned 
long parent,
arch/x86/kernel/ftrace.c:void prepare_ftrace_return(unsigned long ip, 
unsigned long *parent,
arch/x86/kernel/ftrace.c:void prepare_ftrace_return(unsigned long ip, 
unsigned long *parent,
arch/x86/kernel/ftrace.c:	prepare_ftrace_return(ip, (unsigned long 
*)stack, 0);

$ git grep "ftrace_graph_is_dead()" arch/*ftrace.c

arch/microblaze/kernel/ftrace.c:	if (unlikely(ftrace_graph_is_dead()))
arch/mips/kernel/ftrace.c:	if (unlikely(ftrace_graph_is_dead()))
arch/parisc/kernel/ftrace.c:	if (unlikely(ftrace_graph_is_dead()))
arch/powerpc/kernel/trace/ftrace.c:	if (unlikely(ftrace_graph_is_dead()))
arch/s390/kernel/ftrace.c:	if (unlikely(ftrace_graph_is_dead()))
arch/sh/kernel/ftrace.c:	if (unlikely(ftrace_graph_is_dead()))
arch/x86/kernel/ftrace.c:	if (unlikely(ftrace_graph_is_dead()))

But back in commit 545d47b8f359, there was already architectures 
implementing prepare_ftrace_return() without calling 
ftrace_graph_is_dead(), for instance s390. So I must be missing 
something I guess ?

Thanks
Christophe