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Message-ID: <51ECE4EA.1020108@hitachi.com>
Date: Mon, 22 Jul 2013 16:53:14 +0900
From: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To: Tom Zanussi <tom.zanussi@...ux.intel.com>
Cc: rostedt@...dmis.org, jovi.zhangwei@...wei.com,
linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v3 1/9] tracing: Add support for SOFT_DISABLE to syscall
events
(2013/07/20 0:09), Tom Zanussi wrote:
> The original SOFT_DISABLE patches didn't add support for soft disable
> of syscall events; this adds it and paves the way for future patches
> allowing triggers to be added to syscall events, since triggers are
> built on top of SOFT_DISABLE.
>
> Add an array of ftrace_event_file pointers indexed by syscall number
> to the trace array alongside the existing enabled bitmaps. The
> ftrace_event_file structs in turn contain the soft disable flags we
> need for per-syscall soft disable accounting; later patches add
> additional 'trigger' flags and per-syscall triggers and filters.
>
> Signed-off-by: Tom Zanussi <tom.zanussi@...ux.intel.com>
> ---
> kernel/trace/trace.h | 2 ++
> kernel/trace/trace_syscalls.c | 14 ++++++++++++++
> 2 files changed, 16 insertions(+)
>
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 4a4f6e1..af6eb2c 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -202,6 +202,8 @@ struct trace_array {
> int sys_refcount_exit;
> DECLARE_BITMAP(enabled_enter_syscalls, NR_syscalls);
> DECLARE_BITMAP(enabled_exit_syscalls, NR_syscalls);
> + struct ftrace_event_file *enter_syscall_files[NR_syscalls];
> + struct ftrace_event_file *exit_syscall_files[NR_syscalls];
> #endif
> int stop_count;
> int clock_id;
> diff --git a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.c
> index 322e164..4915b69 100644
> --- a/kernel/trace/trace_syscalls.c
> +++ b/kernel/trace/trace_syscalls.c
> @@ -302,6 +302,7 @@ static int __init syscall_exit_define_fields(struct ftrace_event_call *call)
> static void ftrace_syscall_enter(void *data, struct pt_regs *regs, long id)
> {
> struct trace_array *tr = data;
> + struct ftrace_event_file *ftrace_file;
> struct syscall_trace_enter *entry;
> struct syscall_metadata *sys_data;
> struct ring_buffer_event *event;
> @@ -317,6 +318,10 @@ static void ftrace_syscall_enter(void *data, struct pt_regs *regs, long id)
> if (!test_bit(syscall_nr, tr->enabled_enter_syscalls))
> return;
>
> + ftrace_file = rcu_dereference_raw(tr->enter_syscall_files[syscall_nr]);
> + if (test_bit(FTRACE_EVENT_FL_SOFT_DISABLED_BIT, &ftrace_file->flags))
> + return;
> +
It seems here is no lock to protect ftrace_file. This could be racy with
unreg_event_syscall_enter(). For example,
<thread0> <thread1>
ftrace_syscall_enter() unreg_event_syscall_enter()
test_bit(enabled_enter_syscalls) lock(syscall_trace_lock);
clear_bit(enabled_enter_syscalls)
tr->enter_syscall_files[num] = NULL
ftrace_file = tr->enter_syscall_files[syscall_nr]
In this case, ftrace_file can be NULL.
And even if it is passed(checked there), unreg_event_syscall_enter() still
doesn't ensure that the other threads are using the ftrace_file
(Not completely disabled) as previous kprobe-tracer doesn't.
Please check the below patch I posted.
http://marc.info/?l=linux-kernel&m=137336270104492
AFAIK, tracepoint (on which syscall tracer depends) locks rcu_read_lock
too, so I think it is possible to use same approach in syscall tracer to
guarantee all running event handlers have done.
Thank you,
> sys_data = syscall_nr_to_meta(syscall_nr);
> if (!sys_data)
> return;
> @@ -345,6 +350,7 @@ static void ftrace_syscall_enter(void *data, struct pt_regs *regs, long id)
> static void ftrace_syscall_exit(void *data, struct pt_regs *regs, long ret)
> {
> struct trace_array *tr = data;
> + struct ftrace_event_file *ftrace_file;
> struct syscall_trace_exit *entry;
> struct syscall_metadata *sys_data;
> struct ring_buffer_event *event;
> @@ -359,6 +365,10 @@ static void ftrace_syscall_exit(void *data, struct pt_regs *regs, long ret)
> if (!test_bit(syscall_nr, tr->enabled_exit_syscalls))
> return;
>
> + ftrace_file = rcu_dereference_raw(tr->exit_syscall_files[syscall_nr]);
> + if (test_bit(FTRACE_EVENT_FL_SOFT_DISABLED_BIT, &ftrace_file->flags))
> + return;
> +
> sys_data = syscall_nr_to_meta(syscall_nr);
> if (!sys_data)
> return;
> @@ -397,6 +407,7 @@ static int reg_event_syscall_enter(struct ftrace_event_file *file,
> if (!tr->sys_refcount_enter)
> ret = register_trace_sys_enter(ftrace_syscall_enter, tr);
> if (!ret) {
> + rcu_assign_pointer(tr->enter_syscall_files[num], file);
> set_bit(num, tr->enabled_enter_syscalls);
> tr->sys_refcount_enter++;
> }
> @@ -416,6 +427,7 @@ static void unreg_event_syscall_enter(struct ftrace_event_file *file,
> mutex_lock(&syscall_trace_lock);
> tr->sys_refcount_enter--;
> clear_bit(num, tr->enabled_enter_syscalls);
> + rcu_assign_pointer(tr->enter_syscall_files[num], NULL);
> if (!tr->sys_refcount_enter)
> unregister_trace_sys_enter(ftrace_syscall_enter, tr);
> mutex_unlock(&syscall_trace_lock);
> @@ -435,6 +447,7 @@ static int reg_event_syscall_exit(struct ftrace_event_file *file,
> if (!tr->sys_refcount_exit)
> ret = register_trace_sys_exit(ftrace_syscall_exit, tr);
> if (!ret) {
> + rcu_assign_pointer(tr->exit_syscall_files[num], file);
> set_bit(num, tr->enabled_exit_syscalls);
> tr->sys_refcount_exit++;
> }
> @@ -454,6 +467,7 @@ static void unreg_event_syscall_exit(struct ftrace_event_file *file,
> mutex_lock(&syscall_trace_lock);
> tr->sys_refcount_exit--;
> clear_bit(num, tr->enabled_exit_syscalls);
> + rcu_assign_pointer(tr->exit_syscall_files[num], NULL);
> if (!tr->sys_refcount_exit)
> unregister_trace_sys_exit(ftrace_syscall_exit, tr);
> mutex_unlock(&syscall_trace_lock);
>
--
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com
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