On Fri, 12 Aug 2016, Jiri Kosina wrote:
> One thing is still beyond me though ... how the heck this doesn't happen
> without DEBUG_LOCK_ALLOC? The percpu area pointer should be corrupted
> nevertheless, shouldn't it?
The reason is that turning DEBUG_LOCK_ALLOC changing
trace_suspend_resume() from
ffffffff810c7280 <trace_suspend_resume>:
ffffffff810c7280: 55 push %rbp
ffffffff810c7281: 48 89 e5 mov %rsp,%rbp
ffffffff810c7284: 41 56 push %r14
ffffffff810c7286: 41 55 push %r13
ffffffff810c7288: 41 54 push %r12
ffffffff810c728a: 53 push %rbx
ffffffff810c728b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
ffffffff810c7290: 65 8b 05 59 2f f4 7e mov
%gs:0x7ef42f59(%rip),%eax # a1f0 <cpu_number>
ffffffff810c7297: 89 c0 mov %eax,%eax
ffffffff810c7299: 48 0f a3 05 9f 2f c4 bt %rax,0xc42f9f(%rip)
# ffffffff81d0a240 <__cpu_online_mask>
to
ffffffff810ad150 <trace_suspend_resume>:
ffffffff810ad150: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
ffffffff810ad155: c3 retq
ffffffff810ad156: 65 8b 05 93 d0 f5 7e mov
%gs:0x7ef5d093(%rip),%eax # a1f0 <cpu_number>
ffffffff810ad15d: 89 c0 mov %eax,%eax
ffffffff810ad15f: 48 0f a3 05 59 0b c4 bt %rax,0xc40b59(%rip)
# ffffffff81cedcc0 <__cpu_online_mask>
ffffffff810ad166: 00
IOW, with the config change, trace_suspend_resume() returns immediately
without trying to get the current SMP id. And it's because of
DEBUG_LOCK_ALLOC implies LOCKDEP, and that does this to __DECLARE_TRACE()
* When lockdep is enabled, we make sure to always do the RCU portions
of
* the tracepoint code, regardless of whether tracing is on. However,
* don't check if the condition is false, due to interaction with idle
* instrumentation. This lets us find RCU issues triggered with
tracepoints
* even when this tracepoint is off. This code has no purpose other than
* poking RCU a bit.
*/
#define __DECLARE_TRACE(name, proto, args, cond, data_proto, data_args)
\
extern struct tracepoint __tracepoint_##name;
\
static inline void trace_##name(proto)
\
{
\
if (static_key_false(&__tracepoint_##name.key))
\
__DO_TRACE(&__tracepoint_##name,
\
TP_PROTO(data_proto),
\
TP_ARGS(data_args),
\
TP_CONDITION(cond),,);
\
if (IS_ENABLED(CONFIG_LOCKDEP) && (cond)) {
\
rcu_read_lock_sched_notrace();
\
rcu_dereference_sched(__tracepoint_##name.funcs);\
rcu_read_unlock_sched_notrace();
\
}
\
}
That's pretty nasty, as turning LOCKDEP on has sideffects on the code
that'd normally not be expected to be run at all (tracepoint off).
Oh well.
Thanks again,
--
Jiri Kosina
SUSE Labs
