linux-kernel - Re: [PATCH v3] tracing: Fix lock inversion in trace_event_enable_tgid

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Date:   Mon, 16 Dec 2019 11:04:04 +0530
From:   Prateek Sood <prsood@...eaurora.org>
To:     rostedt@...dmis.org, mingo@...hat.com
Cc:     linux-kernel@...r.kernel.org, kaushalk@...eaurora.org
Subject: Re: [PATCH v3] tracing: Fix lock inversion in
 trace_event_enable_tgid_record()

Hi Steve,

Please help in reviewing below patch


Thanks

Prateek

On 12/10/2019 2:45 PM, Prateek Sood wrote:
> Hi Steve,
>
> Are you suggesting something like below?
>
>> 8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8
>
>         Task T2                             Task T3
> trace_options_core_write()            subsystem_open()
>
>   mutex_lock(trace_types_lock)           mutex_lock(event_mutex)
>
>   set_tracer_flag()
>
>     trace_event_enable_tgid_record()       mutex_lock(trace_types_lock)
>
>      mutex_lock(event_mutex)
>
> This gives a circular dependency deadlock between trace_types_lock and
> event_mutex. To fix this invert the usage of trace_types_lock and
> event_mutex in trace_options_core_write(). This keeps the sequence of
> lock usage consistent.
>
> Signed-off-by: Prateek Sood <prsood@...eaurora.org>
> ---
>   kernel/trace/trace.c        | 8 ++++++++
>   kernel/trace/trace_events.c | 8 ++++----
>   2 files changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 6a0ee91..4dc93e3 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -4590,6 +4590,10 @@ int trace_keep_overwrite(struct tracer *tracer, u32 mask, int set)
>   
>   int set_tracer_flag(struct trace_array *tr, unsigned int mask, int enabled)
>   {
> +	if ((mask == TRACE_ITER_RECORD_TGID) ||
> +	    (mask == TRACE_ITER_RECORD_CMD))
> +		lockdep_assert_held(&event_mutex);
> +
>   	/* do nothing if flag is already set */
>   	if (!!(tr->trace_flags & mask) == !!enabled)
>   		return 0;
> @@ -4657,6 +4661,7 @@ static int trace_set_options(struct trace_array *tr, char *option)
>   
>   	cmp += len;
>   
> +	mutex_lock(&event_mutex);
>   	mutex_lock(&trace_types_lock);
>   
>   	ret = match_string(trace_options, -1, cmp);
> @@ -4667,6 +4672,7 @@ static int trace_set_options(struct trace_array *tr, char *option)
>   		ret = set_tracer_flag(tr, 1 << ret, !neg);
>   
>   	mutex_unlock(&trace_types_lock);
> +	mutex_unlock(&event_mutex);
>   
>   	/*
>   	 * If the first trailing whitespace is replaced with '\0' by strstrip,
> @@ -7972,9 +7978,11 @@ static void get_tr_index(void *data, struct trace_array **ptr,
>   	if (val != 0 && val != 1)
>   		return -EINVAL;
>   
> +	mutex_lock(&event_mutex);
>   	mutex_lock(&trace_types_lock);
>   	ret = set_tracer_flag(tr, 1 << index, val);
>   	mutex_unlock(&trace_types_lock);
> +	mutex_unlock(&event_mutex);
>   
>   	if (ret < 0)
>   		return ret;
> diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> index fba87d1..995061b 100644
> --- a/kernel/trace/trace_events.c
> +++ b/kernel/trace/trace_events.c
> @@ -320,7 +320,8 @@ void trace_event_enable_cmd_record(bool enable)
>   	struct trace_event_file *file;
>   	struct trace_array *tr;
>   
> -	mutex_lock(&event_mutex);
> +	lockdep_assert_held(&event_mutex);
> +
>   	do_for_each_event_file(tr, file) {
>   
>   		if (!(file->flags & EVENT_FILE_FL_ENABLED))
> @@ -334,7 +335,6 @@ void trace_event_enable_cmd_record(bool enable)
>   			clear_bit(EVENT_FILE_FL_RECORDED_CMD_BIT, &file->flags);
>   		}
>   	} while_for_each_event_file();
> -	mutex_unlock(&event_mutex);
>   }
>   
>   void trace_event_enable_tgid_record(bool enable)
> @@ -342,7 +342,8 @@ void trace_event_enable_tgid_record(bool enable)
>   	struct trace_event_file *file;
>   	struct trace_array *tr;
>   
> -	mutex_lock(&event_mutex);
> +	lockdep_assert_held(&event_mutex);
> +
>   	do_for_each_event_file(tr, file) {
>   		if (!(file->flags & EVENT_FILE_FL_ENABLED))
>   			continue;
> @@ -356,7 +357,6 @@ void trace_event_enable_tgid_record(bool enable)
>   				  &file->flags);
>   		}
>   	} while_for_each_event_file();
> -	mutex_unlock(&event_mutex);
>   }
>   
>   static int __ftrace_event_enable_disable(struct trace_event_file *file,

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