From: Peter Zijlstra <peterz@infradead.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@kernel.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Naresh Kamboju <naresh.kamboju@linaro.org>
Subject: Re: [PATCH v2] tracepoint: Fix out of sync data passing by static caller
Date: Fri, 2 Oct 2020 09:18:48 +0200 [thread overview]
Message-ID: <20201002071848.GS2628@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20201001212757.339a5520@oasis.local.home>
On Thu, Oct 01, 2020 at 09:27:57PM -0400, Steven Rostedt wrote:
> From: Steven Rostedt (VMware) <rostedt@goodmis.org>
>
> Naresh reported a bug discovered in linux-next that I can reliably
> trigger myself. It appears to be a side effect of the static calls. It
> happens when going from more than one tracepoint callback to a single
> one, and removing the first callback on the list. The list of
> tracepoint callbacks holds data and a function to call with the
> parameters of that tracepoint and a handler to the associated data.
>
> old_list:
> 0: func = foo; data = NULL;
> 1: func = bar; data = &bar_struct;
>
> new_list:
> 0: func = bar; data = &bar_struct;
>
>
> CPU 0 CPU 1
> ----- -----
> tp_funcs = old_list;
> tp_static_caller = tp_interator
>
> __DO_TRACE()
>
> data = tp_funcs[0].data = NULL;
>
> tp_funcs = new_list;
> tracepoint_update_call()
> tp_static_caller = tp_funcs[0] = bar;
> tp_static_caller(data)
> bar(data)
> x = data->item = NULL->item
>
> BOOM!
> To solve this, add a tracepoint_synchronize_unregister() between
> changing tp_funcs and updating the static tracepoint, that does both a
> synchronize_rcu() and synchronize_srcu(). This will ensure that when
> the static call is updated to the single callback that it will be
> receiving the data that it registered with.
> Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
> Fixes: d25e37d89dd2f ("tracepoint: Optimize using static_call()")
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> ---
Urgh :/
I'll go stick this in tip/core/static_call.
> diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
> index 1b4be44d1d2b..3f659f855074 100644
> --- a/kernel/tracepoint.c
> +++ b/kernel/tracepoint.c
> @@ -221,7 +221,7 @@ static void *func_remove(struct tracepoint_func **funcs,
> return old;
> }
>
> -static void tracepoint_update_call(struct tracepoint *tp, struct tracepoint_func *tp_funcs)
> +static void tracepoint_update_call(struct tracepoint *tp, struct tracepoint_func *tp_funcs, bool sync)
> {
> void *func = tp->iterator;
>
> @@ -229,8 +229,17 @@ static void tracepoint_update_call(struct tracepoint *tp, struct tracepoint_func
> if (!tp->static_call_key)
> return;
>
> - if (!tp_funcs[1].func)
> + if (!tp_funcs[1].func) {
> func = tp_funcs[0].func;
> + /*
> + * If going from the iterator back to a single caller,
> + * we need to synchronize with __DO_TRACE to make sure
> + * that the data passed to the callback is the one that
> + * belongs to that callback.
> + */
> + if (sync)
> + tracepoint_synchronize_unregister();
> + }
>
> __static_call_update(tp->static_call_key, tp->static_call_tramp, func);
> }
> @@ -265,7 +274,7 @@ static int tracepoint_add_func(struct tracepoint *tp,
> * include/linux/tracepoint.h using rcu_dereference_sched().
> */
> rcu_assign_pointer(tp->funcs, tp_funcs);
> - tracepoint_update_call(tp, tp_funcs);
> + tracepoint_update_call(tp, tp_funcs, false);
> static_key_enable(&tp->key);
>
> release_probes(old);
> @@ -297,11 +306,12 @@ static int tracepoint_remove_func(struct tracepoint *tp,
> tp->unregfunc();
>
> static_key_disable(&tp->key);
> + rcu_assign_pointer(tp->funcs, tp_funcs);
> } else {
> - tracepoint_update_call(tp, tp_funcs);
> + rcu_assign_pointer(tp->funcs, tp_funcs);
> + tracepoint_update_call(tp, tp_funcs,
> + tp_funcs[0].func != old[0].func);
> }
> -
> - rcu_assign_pointer(tp->funcs, tp_funcs);
> release_probes(old);
> return 0;
> }
prev parent reply other threads:[~2020-10-02 7:19 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-02 1:27 [PATCH v2] tracepoint: Fix out of sync data passing by static caller Steven Rostedt
2020-10-02 7:18 ` Peter Zijlstra [this message]
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