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SubjectRe: [PATCH v4 0/7] tracing: 'hist' triggers
Hi Tom,

On 04/10/2015 06:05 PM, Tom Zanussi wrote:
> This is v4 of the 'hist triggers' patchset, following feedback from
> v3.
> This version fixes the race in tracing_map_insert() noted in v3, where
> map.val.key could be checked even if map.val wasn't yet set. The
> simple fix for that in tracing_map_insert() introduces the possibility
> of duplicates in the map, which though rare, need to be accounted for
> in the output. To address that, duplicate-merging code was added to
> the map-printing code.

Note: I might be abusing your patch. So this could be a completely
bogus question/feedback.

Most important information here is that I placed additional tracepoints
during state transitions. I needed to disable the rcu_dereference
consistency checks to be able to get it working (not claiming that this
is the right thing to do)

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index c728513..f194324 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -125,7 +125,7 @@ extern void syscall_unregfunc(void);
return; \
prercu; \
rcu_read_lock_sched_notrace(); \
- it_func_ptr = rcu_dereference_sched((tp)->funcs); \
+ it_func_ptr = rcu_dereference_raw_notrace((tp)->funcs); \
if (it_func_ptr) { \
do { \
it_func = (it_func_ptr)->func; \
@@ -175,7 +175,7 @@ extern void syscall_unregfunc(void);
TP_CONDITION(cond),,); \
if (IS_ENABLED(CONFIG_LOCKDEP) && (cond)) { \
rcu_read_lock_sched_notrace(); \
- rcu_dereference_sched(__tracepoint_##name.funcs);\
+ rcu_dereference_raw_notrace(__tracepoint_##name.funcs);\
rcu_read_unlock_sched_notrace(); \
} \
} \

diff --git a/kernel/trace/trace_irqsoff.c b/kernel/trace/trace_irqsoff.c
index 8523ea3..03f42b9 100644
--- a/kernel/trace/trace_irqsoff.c
+++ b/kernel/trace/trace_irqsoff.c
@@ -14,6 +14,8 @@
#include <linux/module.h>
#include <linux/ftrace.h>

+#include <trace/events/sched.h>
#include "trace.h"

static struct trace_array *irqsoff_trace __read_mostly;
@@ -433,11 +435,13 @@ void start_critical_timings(void)
if (preempt_trace() || irq_trace())
start_critical_timing(CALLER_ADDR0, CALLER_ADDR1);
+ trace_sched_latency_critical_timing(1);

void stop_critical_timings(void)
+ trace_sched_latency_critical_timing(0);
if (preempt_trace() || irq_trace())
stop_critical_timing(CALLER_ADDR0, CALLER_ADDR1);
@@ -447,6 +451,7 @@ EXPORT_SYMBOL_GPL(stop_critical_timings);
void time_hardirqs_on(unsigned long a0, unsigned long a1)
+ trace_sched_latency_irqs(0);
if (!preempt_trace() && irq_trace())
stop_critical_timing(a0, a1);
@@ -455,6 +460,7 @@ void time_hardirqs_off(unsigned long a0, unsigned long a1)
if (!preempt_trace() && irq_trace())
start_critical_timing(a0, a1);
+ trace_sched_latency_irqs(1);

When activating now the hist trigger by

echo 'hist:key=common_pid:val=enabled' > \
the system crashes reliable after a very short time. Those tracepoints
do work normally, so I see them in /sys/kernel/debug/tracing/trace.

After some investigation I found out that event_hist_trigger() gets
called with rec=NULL. This is handed over to hist_field_s32() and
that function derefences the argument event (=NULL).

#19 event_hist_trigger (data=0xffff88007934c780, rec=0x0 <irq_stack_union>) at kernel/trace/trace_events_hist.c:885
#20 0xffffffff8113771b in event_triggers_call (file=<optimized out>, rec=0x0 <irq_stack_union>) at kernel/trace/trace_events_trigger.c:78
#21 0xffffffff81079def in ftrace_trigger_soft_disabled (file=<optimized out>) at include/linux/ftrace_event.h:453
#22 ftrace_raw_event_sched_latency_template (__data=0xffff88007c895aa8, enabled=0) at include/trace/events/sched.h:557
#23 0xffffffff8112cbec in trace_sched_latency_preempt (enabled=<optimized out>) at include/trace/events/sched.h:587
#24 trace_preempt_off (a0=18446744071579483412, a1=18446744071579485792) at kernel/trace/trace_irqsoff.c:532
#25 0xffffffff8107f449 in preempt_count_add (val=1) at kernel/sched/core.c:2554
#26 0xffffffff8109bd14 in get_lock_stats (class=0xffffffff82e235d0 <lock_classes+273296>) at kernel/locking/lockdep.c:249
#27 0xffffffff8109c660 in lock_release_holdtime (hlock=0xffff880079c8ee50) at kernel/locking/lockdep.c:267
#28 0xffffffff810a2cc9 in lock_release_holdtime (hlock=<optimized out>) at kernel/locking/lockdep.c:3464
#29 lock_release_nested (ip=<optimized out>, lock=<optimized out>, curr=<optimized out>) at kernel/locking/lockdep.c:3481
#30 __lock_release (ip=<optimized out>, nested=<optimized out>, lock=<optimized out>) at kernel/locking/lockdep.c:3507
#31 lock_release (lock=0xffffffff8226e5f0 <kernfs_mutex+112>, nested=<optimized out>, ip=18446744071590106190) at kernel/locking/lockdep.c:3628
#32 0xffffffff81abd306 in __mutex_unlock_common_slowpath (nested=<optimized out>, lock=<optimized out>) at kernel/locking/mutex.c:744
#33 __mutex_unlock_slowpath (lock_count=0xffff88007934c780) at kernel/locking/mutex.c:769
#34 0xffffffff81abd44e in mutex_unlock (lock=<optimized out>) at kernel/locking/mutex.c:446
#35 0xffffffff81238e84 in kernfs_dop_revalidate (dentry=0xffff88007acede00, flags=<optimized out>) at fs/kernfs/dir.c:470
#36 0xffffffff811c9813 in d_revalidate (flags=<optimized out>, dentry=<optimized out>) at fs/namei.c:607
#37 lookup_fast (nd=0xffff8800790e3e48, path=0xffff8800790e3ca8, inode=0xffff8800790e3cb8) at fs/namei.c:1465
#38 0xffffffff811cc05f in walk_component (follow=<optimized out>, path=<optimized out>, nd=<optimized out>) at fs/namei.c:1577
#39 link_path_walk (name=0xffff880079fa602d "virtual/block/loop6/subsystem", nd=0xffff8800790e3e48) at fs/namei.c:1836
#40 0xffffffff811cd327 in path_init (dfd=<optimized out>, name=0xffff880079fa6020 "/sys/devices/virtual/block/loop6/subsystem", flags=2051493056, nd=0xffff8800790e3e48)
at fs/namei.c:1952
#41 0xffffffff811cda90 in path_lookupat (dfd=<optimized out>, name=<optimized out>, flags=16448, nd=0xffff8800790e3e48) at fs/namei.c:1995
#42 0xffffffff811cec1a in filename_lookup (dfd=-100, name=0xffff880079fa6000, flags=16384, nd=0xffff8800790e3e48) at fs/namei.c:2030
#43 0xffffffff811d0d34 in user_path_at_empty (dfd=-100, name=<optimized out>, flags=16384, path=0xffff8800790e3f38, empty=<optimized out>) at fs/namei.c:2197
#44 0xffffffff811c285c in SYSC_readlinkat (bufsiz=<optimized out>, buf=<optimized out>, pathname=<optimized out>, dfd=<optimized out>) at fs/stat.c:327
#45 SyS_readlinkat (bufsiz=<optimized out>, buf=<optimized out>, pathname=<optimized out>, dfd=<optimized out>) at fs/stat.c:315
#46 SYSC_readlink (bufsiz=<optimized out>, buf=<optimized out>, path=<optimized out>) at fs/stat.c:352
#47 SyS_readlink (path=-131939361831040, buf=0, bufsiz=<optimized out>) at fs/stat.c:349
So I am wondering if the path from ftrace_trigger_soft_disabled()
to event_triggers_call() is supposed never to happen? The comment
on ftrace_trigger_soft_disabled() indicate this might happen on
normal operation:

* ftrace_trigger_soft_disabled - do triggers and test if soft disabled
* @file: The file pointer of the event to test
* If any triggers without filters are attached to this event, they
* will be called here. If the event is soft disabled and has no
* triggers that require testing the fields, it will return true,
* otherwise false.
static inline bool
ftrace_trigger_soft_disabled(struct ftrace_event_file *file)
unsigned long eflags = file->flags;
event_triggers_call(file, NULL);
return true;
return false;

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