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SubjectRe: [PATCH] trace: Use rcu_dereference_raw for hooks from trace-event subsystem
On Thu,  2 Aug 2018 19:34:07 -0700
"Joel Fernandes (Google)" <joel@joelfernandes.org> wrote:

> Since we switched to using SRCU for tracepoints used in the idle path,
> we can no longer use rcu_dereference_sched for dereferencing points in
> trace-event hooks.
>
> Since tracepoints can now use either SRCU or sched-RCU, just use
> rcu_dereference_raw for traceevents just like we're doing when
> dereferencing the tracepoint table.

OK, I've tested that this fixes the bug.

/mnt/ftrace # ./ftracetest test.d/event/toplevel-enable.tc
=== Ftrace unit tests ===
[1] event tracing - enable/disable with top level files [PASS]

# of passed: 1
# of failed: 0
# of unresolved: 0
# of untested: 0
# of unsupported: 0
# of xfailed: 0
# of undefined(test bug): 0

Tested-by: Masami Hiramatsu <mhiramat@kernel.org>

Thank you!

>
> This prevents an RCU warning reported by Masami:
>
> [ 282.060593] WARNING: can't dereference registers at 00000000f3c7f62b
> [ 282.063200] =============================
> [ 282.064082] WARNING: suspicious RCU usage
> [ 282.064963] 4.18.0-rc6+ #15 Tainted: G W
> [ 282.066048] -----------------------------
> [ 282.066923] /home/mhiramat/ksrc/linux/kernel/trace/trace_events.c:242
> suspicious rcu_dereference_check() usage!
> [ 282.068974]
> [ 282.068974] other info that might help us debug this:
> [ 282.068974]
> [ 282.070770]
> [ 282.070770] RCU used illegally from idle CPU!
> [ 282.070770] rcu_scheduler_active = 2, debug_locks = 1
> [ 282.072938] RCU used illegally from extended quiescent state!
> [ 282.074183] no locks held by swapper/0/0.
> [ 282.075071]
> [ 282.075071] stack backtrace:
> [ 282.076121] CPU: 0 PID: 0 Comm: swapper/0 Tainted: G W
> [ 282.077782] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)
> [ 282.079604] Call Trace:
> [ 282.080212] <IRQ>
> [ 282.080755] dump_stack+0x85/0xcb
> [ 282.081523] trace_event_ignore_this_pid+0x66/0x70
> [ 282.082541] trace_event_raw_event_preemptirq_template+0xa2/0xb0
> [ 282.083774] ? interrupt_entry+0xc4/0xe0
> [ 282.084665] ? trace_hardirqs_off_thunk+0x1a/0x1c
> [ 282.085669] trace_hardirqs_off_caller+0x90/0xd0
> [ 282.086597] trace_hardirqs_off_thunk+0x1a/0x1c
> [ 282.087433] ? call_function_interrupt+0xa/0x20
> [ 282.088201] interrupt_entry+0xc4/0xe0
> [ 282.088848] ? call_function_interrupt+0xa/0x20
> [ 282.089579] </IRQ>
> [ 282.090029] ? native_safe_halt+0x2/0x10
> [ 282.090695] ? default_idle+0x1f/0x160
> [ 282.091330] ? default_idle_call+0x24/0x40
> [ 282.091997] ? do_idle+0x210/0x250
> [ 282.092658] ? cpu_startup_entry+0x6f/0x80
> [ 282.093338] ? start_kernel+0x49d/0x4bd
> [ 282.093987] ? secondary_startup_64+0xa5/0xb0
>
> Reported-by: Masami Hiramatsu <mhiramat@kernel.org>
> Fixes: e6753f23d961 ("tracepoint: Make rcuidle tracepoint callers use SRCU")
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
> kernel/trace/trace_events.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> index 14ff4ff3caab..7b508ce8ac44 100644
> --- a/kernel/trace/trace_events.c
> +++ b/kernel/trace/trace_events.c
> @@ -239,7 +239,7 @@ bool trace_event_ignore_this_pid(struct trace_event_file *trace_file)
> struct trace_array_cpu *data;
> struct trace_pid_list *pid_list;
>
> - pid_list = rcu_dereference_sched(tr->filtered_pids);
> + pid_list = rcu_dereference_raw(tr->filtered_pids);
> if (!pid_list)
> return false;
>
> @@ -512,7 +512,7 @@ event_filter_pid_sched_process_exit(void *data, struct task_struct *task)
> struct trace_pid_list *pid_list;
> struct trace_array *tr = data;
>
> - pid_list = rcu_dereference_sched(tr->filtered_pids);
> + pid_list = rcu_dereference_raw(tr->filtered_pids);
> trace_filter_add_remove_task(pid_list, NULL, task);
> }
>
> --
> 2.18.0.597.ga71716f1ad-goog
>


--
Masami Hiramatsu <mhiramat@kernel.org>

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