On Tue, Jan 24, 2023 at 05:12:14PM +0000, Mark Rutland wrote:
> On Tue, Jan 24, 2023 at 03:44:35PM +0100, Peter Zijlstra wrote:
> > On Mon, Jan 23, 2023 at 05:07:53PM -0500, Steven Rostedt wrote:
> >
> > > Actually, perhaps we can just add this, and all you need to do is create
> > > and set CONFIG_NO_RCU_TRACING (or some other name).
> >
> > Elsewhere I've used CONFIG_ARCH_WANTS_NO_INSTR for this.
>
> Yes please; if we use CONFIG_ARCH_WANTS_NO_INSTR then arm64 will get this "for
> free" once we add the missing checks (which I assume we need) in our ftrace_prepare_return().
>
> > Anyway, I took it for a spin and it .... doesn't seems to do the job.
> >
> > With my patch the first splat is
> >
> > "RCU not on for: cpuidle_poll_time+0x0/0x70"
> >
> > While with yours I seems to get the endless:
> >
> > "WARNING: suspicious RCU usage"
> >
> > thing. Let me see if I can figure out where it goes side-ways.
>
> Hmmm... for WARN_ONCE() don't we need to wake RCU first also? I thought we
> needed that at least for the printk machinery?
OK, the below seems to work nice for me -- although I'm still on a
hacked up printk, but the recursive RCU not watching fail seems to be
tamed.
Ofc. Paul might have an opinion on this glorious bodge ;-)
---
diff --git a/include/linux/trace_recursion.h b/include/linux/trace_recursion.h
index c303f7a114e9..d48cd92d2364 100644
--- a/include/linux/trace_recursion.h
+++ b/include/linux/trace_recursion.h
@@ -135,6 +135,21 @@ extern void ftrace_record_recursion(unsigned long ip, unsigned long parent_ip);
# define do_ftrace_record_recursion(ip, pip) do { } while (0)
#endif
+#ifdef CONFIG_ARCH_WANTS_NO_INSTR
+# define trace_warn_on_no_rcu(ip) \
+ ({ \
+ bool __ret = !rcu_is_watching(); \
+ if (__ret && !trace_recursion_test(TRACE_RECORD_RECURSION_BIT)) { \
+ trace_recursion_set(TRACE_RECORD_RECURSION_BIT); \
+ WARN_ONCE(true, "RCU not on for: %pS\n", (void *)ip); \
+ trace_recursion_clear(TRACE_RECORD_RECURSION_BIT); \
+ } \
+ __ret; \
+ })
+#else
+# define trace_warn_on_no_rcu(ip) false
+#endif
+
/*
* Preemption is promised to be disabled when return bit >= 0.
*/
@@ -144,6 +159,9 @@ static __always_inline int trace_test_and_set_recursion(unsigned long ip, unsign
unsigned int val = READ_ONCE(current->trace_recursion);
int bit;
+ if (trace_warn_on_no_rcu(ip))
+ return -1;
+
bit = trace_get_context_bit() + start;
if (unlikely(val & (1 << bit))) {
/*
diff --git a/lib/bug.c b/lib/bug.c
index c223a2575b72..0a10643ea168 100644
--- a/lib/bug.c
+++ b/lib/bug.c
@@ -47,6 +47,7 @@
#include <linux/sched.h>
#include <linux/rculist.h>
#include <linux/ftrace.h>
+#include <linux/context_tracking.h>
extern struct bug_entry __start___bug_table[], __stop___bug_table[];
@@ -153,7 +154,7 @@ struct bug_entry *find_bug(unsigned long bugaddr)
return module_find_bug(bugaddr);
}
-enum bug_trap_type report_bug(unsigned long bugaddr, struct pt_regs *regs)
+static enum bug_trap_type __report_bug(unsigned long bugaddr, struct pt_regs *regs)
{
struct bug_entry *bug;
const char *file;
@@ -209,6 +210,30 @@ enum bug_trap_type report_bug(unsigned long bugaddr, struct pt_regs *regs)
return BUG_TRAP_TYPE_BUG;
}
+enum bug_trap_type report_bug(unsigned long bugaddr, struct pt_regs *regs)
+{
+ enum bug_trap_type ret;
+ bool rcu = false;
+
+#ifdef CONFIG_CONTEXT_TRACKING_IDLE
+ /*
+ * Horrible hack to shut up recursive RCU isn't watching fail since
+ * lots of the actual reporting also relies on RCU.
+ */
+ if (!rcu_is_watching()) {
+ rcu = true;
+ ct_state_inc(RCU_DYNTICKS_IDX);
+ }
+#endif
+
+ ret = __report_bug(bugaddr, regs);
+
+ if (rcu)
+ ct_state_inc(RCU_DYNTICKS_IDX);
+
+ return ret;
+}
+
static void clear_once_table(struct bug_entry *start, struct bug_entry *end)
{
struct bug_entry *bug;
