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Date: Sat, 31 Aug 2013 12:49:28 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Peter Zijlstra <peterz@...radead.org>,
Frederic Weisbecker <fweisbec@...il.com>,
Jiri Olsa <jolsa@...hat.com>
Subject: Re: [RFC][PATCH 10/18 v2] ftrace: Create a RCU unsafe checker
On Sat, Aug 31, 2013 at 01:11:27AM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>
>
> Knowing what functions are not safe to be traced by callbacks that use
> RCU read locks, is not easy to figure out. By adding a function tracer
> callback that is set as a non RCU safe callback that also uses
> rcu_read_lock() and enables PROVE_RCU, it can be used to find locations
> in the kernel that need to be tagged with FTRACE_UNSAFE_RCU().
>
> On boot up, this callback gets registered so that all functions are
> being traced, and there's no way to turn this off. In the future we
> can make this enabled or disabled at run time, but for now it's only
> used for debugging and should not be enabled by normal users.
>
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
> kernel/trace/Kconfig | 22 +++++++++++++++++++
> kernel/trace/ftrace.c | 12 ++++++++++
> kernel/trace/trace.h | 3 +++
> kernel/trace/trace_functions.c | 47 +++++++++++++++++++++++++++++++++++++++-
> 4 files changed, 83 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> index 015f85a..907b497 100644
> --- a/kernel/trace/Kconfig
> +++ b/kernel/trace/Kconfig
> @@ -489,6 +489,28 @@ config FTRACE_MCOUNT_RECORD
> config FTRACE_SELFTEST
> bool
>
> +config FTRACE_UNSAFE_RCU_CHECKER
> + bool "Trace for unsafe RCU callers"
> + depends on DYNAMIC_FTRACE
> + depends on PROVE_LOCKING
> + select PROVE_RCU
> + help
> + Some function tracing callbacks use RCU read lock (namely perf).
> + There are some RCU critical functions that are called out
> + of scope for RCU. For example, when NO_HZ_FULL is enabled,
> + and coming out of user space. The CPU is not being tracked by
> + RCU and all rcu_read_lock()s will be ignored. These functions
> + need to be tagged as unsafe for RCU so that only function callbacks
> + that specify that they do not use RCU will be called by them.
> +
> + This option will enable PROVE_RCU and will enable on boot up
> + a function callback that traces all functions and uses an
> + rcu_read_lock(). If any function that is not tagged as unsafe
> + for RCU is called, a debug splat will be shown. This is used
> + for finding unsafe RCU callers that need to be tagged.
> +
> + If you don't understand any of this, then say N.
> +
> config FTRACE_STARTUP_TEST
> bool "Perform a startup test on ftrace"
> depends on GENERIC_TRACER
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 33e4890..69b7f62 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -1359,6 +1359,13 @@ alloc_and_copy_ftrace_hash(int size_bits, struct ftrace_hash *hash)
> return NULL;
> }
>
> +#ifdef CONFIG_FTRACE_UNSAFE_RCU_CHECKER
> +bool ftrace_rcu_unsafe(unsigned long addr)
> +{
> + return ftrace_lookup_ip(ftrace_unsafe_rcu, addr) != NULL;
> +}
> +#endif
> +
> static void
> ftrace_hash_rec_disable(struct ftrace_ops *ops, int filter_hash);
> static void
> @@ -4391,6 +4398,11 @@ static void __init create_unsafe_rcu_hash(void)
> if (WARN_ON_ONCE(strcmp(str, finder->name) != 0))
> continue;
>
> +#ifdef CONFIG_FTRACE_UNSAFE_RCU_CHECKER
> + /* When checker is enabled, show what was marked unsafe */
> + pr_info("add ftrace rcu unsafe %p (%s)\n",
> + (void *)ip, finder->name);
> +#endif
> add_hash_entry(ftrace_unsafe_rcu, ip);
> }
> }
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 3578be6..e551316 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -760,6 +760,9 @@ static inline int ftrace_graph_addr(unsigned long addr)
>
> return 0;
> }
> +#ifdef CONFIG_FTRACE_UNSAFE_RCU_CHECKER
> +extern bool ftrace_rcu_unsafe(unsigned long addr);
> +#endif
> #else
> static inline int ftrace_graph_addr(unsigned long addr)
> {
> diff --git a/kernel/trace/trace_functions.c b/kernel/trace/trace_functions.c
> index 38fe148..9dd4627 100644
> --- a/kernel/trace/trace_functions.c
> +++ b/kernel/trace/trace_functions.c
> @@ -559,9 +559,54 @@ static inline int init_func_cmd_traceon(void)
> }
> #endif /* CONFIG_DYNAMIC_FTRACE */
>
> +#ifdef CONFIG_FTRACE_UNSAFE_RCU_CHECKER
> +static void
> +ftrace_unsafe_callback(unsigned long ip, unsigned long parent_ip,
> + struct ftrace_ops *op, struct pt_regs *pt_regs)
> +{
> + int bit;
> +
> + preempt_disable_notrace();
> +
> + bit = trace_test_and_set_recursion(TRACE_FTRACE_START, TRACE_FTRACE_MAX);
> + if (bit < 0)
> + goto out;
> +
> + if (WARN_ONCE(ftrace_rcu_unsafe(ip),
> + "UNSAFE RCU function called %pS",
> + (void *)ip))
> + goto out;
> +
> + /* Should trigger a RCU splat if called from unsafe RCU function */
> + rcu_read_lock();
> + rcu_read_unlock();
> +
> + trace_clear_recursion(bit);
> + out:
> + preempt_enable_notrace();
> +}
> +
> +static struct ftrace_ops ftrace_unsafe_ops = {
> + .func = ftrace_unsafe_callback,
> + .flags = FTRACE_OPS_FL_RECURSION_SAFE
> +};
> +
> +static __init void ftrace_start_unsafe_rcu(void)
> +{
> + register_ftrace_function(&ftrace_unsafe_ops);
> +}
> +#else
> +static inline void ftrace_start_unsafe_rcu(void) { }
> +#endif
> +
> static __init int init_function_trace(void)
> {
> + int ret;
> +
> init_func_cmd_traceon();
> - return register_tracer(&function_trace);
> + ret = register_tracer(&function_trace);
> + if (!ret)
> + ftrace_start_unsafe_rcu();
> + return ret;
> }
> core_initcall(init_function_trace);
> --
> 1.7.10.4
>
>
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