Re: [PATCH v2 1/3] tracing: Deference pointers without RCU checks

[Masami Hiramatsu]

On Wed, 24 Aug 2016 16:47:28 +0530
Binoy Jayan <binoy.jayan@xxxxxxxxxx> wrote:

> From: Daniel Wagner <daniel.wagner@xxxxxxxxxxxx>
> 
> The tracepoint can't be used in code section where we are in the
> middle of a state transition.
> 
> For example if we place a tracepoint inside start/stop_critical_section(),
> lockdep complains with
> 
> [    0.035589] WARNING: CPU: 0 PID: 3 at kernel/locking/lockdep.c:3560 \
> check_flags.part.36+0x1bc/0x210() [    0.036000] \
> DEBUG_LOCKS_WARN_ON(current->softirqs_enabled) [    0.036000] Kernel panic - not \
> syncing: panic_on_warn set ... [    0.036000]
> [    0.036000] CPU: 0 PID: 3 Comm: ksoftirqd/0 Not tainted 4.0.0-rc7+ #460
> [    0.036000] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS \
> 1.7.5-20140709_153950- 04/01/2014 [    0.036000]  ffffffff81f2463a ffff88007c93bb98 \
> ffffffff81afb317 0000000000000001 [    0.036000]  ffffffff81f212b3 ffff88007c93bc18 \
> ffffffff81af7bc2 ffff88007c93bbb8 [    0.036000]  ffffffff00000008 ffff88007c93bc28 \
> ffff88007c93bbc8 000000000093bbd8 [    0.036000] Call Trace:
> [    0.036000]  [<ffffffff81afb317>] dump_stack+0x4f/0x7b
> [    0.036000]  [<ffffffff81af7bc2>] panic+0xc0/0x1e9
> [    0.036000]  [<ffffffff81b07188>] ? _raw_spin_unlock_irqrestore+0x38/0x80
> [    0.036000]  [<ffffffff81052a60>] warn_slowpath_common+0xc0/0xc0
> [    0.036000]  [<ffffffff81057116>] ? __local_bh_enable+0x36/0x70
> [    0.036000]  [<ffffffff81052aa6>] warn_slowpath_fmt+0x46/0x50
> [    0.036000]  [<ffffffff810a468c>] check_flags.part.36+0x1bc/0x210
> [    0.036000]  [<ffffffff810a5168>] lock_is_held+0x78/0x90
> [    0.036000]  [<ffffffff81057116>] ? __local_bh_enable+0x36/0x70
> [    0.036000]  [<ffffffff810588ab>] ? __do_softirq+0x3db/0x500
> [    0.036000]  [<ffffffff81134495>] trace_preempt_on+0x255/0x260
> [    0.036000]  [<ffffffff81081c0b>] preempt_count_sub+0xab/0xf0
> [    0.036000]  [<ffffffff81057116>] __local_bh_enable+0x36/0x70
> [    0.036000]  [<ffffffff810588ab>] __do_softirq+0x3db/0x500
> [    0.036000]  [<ffffffff810589ef>] run_ksoftirqd+0x1f/0x60
> [    0.036000]  [<ffffffff81079653>] smpboot_thread_fn+0x193/0x2a0
> [    0.036000]  [<ffffffff810794c0>] ? SyS_setgroups+0x150/0x150
> [    0.036000]  [<ffffffff81075312>] kthread+0xf2/0x110
> [    0.036000]  [<ffffffff81b026b3>] ? wait_for_completion+0xc3/0x120
> [    0.036000]  [<ffffffff81081c0b>] ? preempt_count_sub+0xab/0xf0
> [    0.036000]  [<ffffffff81075220>] ? kthread_create_on_node+0x240/0x240
> [    0.036000]  [<ffffffff81b07cc8>] ret_from_fork+0x58/0x90
> [    0.036000]  [<ffffffff81075220>] ? kthread_create_on_node+0x240/0x240
> [    0.036000] ---[ end Kernel panic - not syncing: panic_on_warn set ...
> 
> PeterZ was so kind to explain it to me what is happening:
> 
> "__local_bh_enable() tests if this is the last SOFTIRQ_OFFSET, if so it
> tells lockdep softirqs are enabled with trace_softirqs_on() after that
> we go an actually modify the preempt_count with preempt_count_sub().
> Then in preempt_count_sub() you call into trace_preempt_on() if this
> was the last preempt_count increment but you do that _before_ you
> actually change the preempt_count with __preempt_count_sub() at this
> point lockdep and preempt_count think the world differs and *boom*"
> 
> So the simplest way to avoid this is by disabling the consistency
> checks.
> 
> We also need to take care of the iterating in trace_events_trigger.c
> to avoid a splatter in conjunction with the hist trigger.

Special care for lockdep inside tracepoint handler is reasonable.

Reviewed-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>

Steven, since this seems a bugfix, could you pick this from the series?

Thank you,

> 
> Signed-off-by: Daniel Wagner <daniel.wagner@xxxxxxxxxxxx>
> ---
>  include/linux/rculist.h             | 36 ++++++++++++++++++++++++++++++++++++
>  include/linux/tracepoint.h          |  4 ++--
>  kernel/trace/trace_events_trigger.c |  6 +++---
>  3 files changed, 41 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/rculist.h b/include/linux/rculist.h
> index 8beb98d..bee836b 100644
> --- a/include/linux/rculist.h
> +++ b/include/linux/rculist.h
> @@ -279,6 +279,24 @@ static inline void list_splice_tail_init_rcu(struct list_head *list,
>  	container_of(lockless_dereference(ptr), type, member)
>  
>  /**
> + * list_entry_rcu_notrace - get the struct for this entry (for tracing)
> + * @ptr:        the &struct list_head pointer.
> + * @type:       the type of the struct this is embedded in.
> + * @member:     the name of the list_head within the struct.
> + *
> + * This primitive may safely run concurrently with the _rcu list-mutation
> + * primitives such as list_add_rcu() as long as it's guarded by rcu_read_lock().
> + *
> + * This is the same as list_entry_rcu() except that it does
> + * not do any RCU debugging or tracing.
> + */
> +#define list_entry_rcu_notrace(ptr, type, member) \
> +({ \
> +	typeof(*ptr) __rcu *__ptr = (typeof(*ptr) __rcu __force *)ptr; \
> +	container_of((typeof(ptr))rcu_dereference_raw_notrace(__ptr), type, member); \
> +})
> +
> +/**
>   * Where are list_empty_rcu() and list_first_entry_rcu()?
>   *
>   * Implementing those functions following their counterparts list_empty() and
> @@ -391,6 +409,24 @@ static inline void list_splice_tail_init_rcu(struct list_head *list,
>  	     pos = list_entry_lockless(pos->member.next, typeof(*pos), member))
>  
>  /**
> + * list_for_each_entry_rcu_notrace	-	iterate over rcu list of given type (for tracing)
> + * @pos:	the type * to use as a loop cursor.
> + * @head:	the head for your list.
> + * @member:	the name of the list_head within the struct.
> + *
> + * This list-traversal primitive may safely run concurrently with
> + * the _rcu list-mutation primitives such as list_add_rcu()
> + * as long as the traversal is guarded by rcu_read_lock().
> + *
> + * This is the same as list_for_each_entry_rcu() except that it does
> + * not do any RCU debugging or tracing.
> + */
> +#define list_for_each_entry_rcu_notrace(pos, head, member) \
> +	for (pos = list_entry_rcu_notrace((head)->next, typeof(*pos), member); \
> +		&pos->member != (head); \
> +		pos = list_entry_rcu_notrace(pos->member.next, typeof(*pos), member))
> +
> +/**
>   * list_for_each_entry_continue_rcu - continue iteration over list of given type
>   * @pos:	the type * to use as a loop cursor.
>   * @head:	the head for your list.
> diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> index be586c6..d0e2a82 100644
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -138,7 +138,7 @@ extern void syscall_unregfunc(void);
>  			return;						\
>  		prercu;							\
>  		rcu_read_lock_sched_notrace();				\
> -		it_func_ptr = rcu_dereference_sched((tp)->funcs);	\
> +		it_func_ptr = rcu_dereference_raw_notrace((tp)->funcs);	\
>  		if (it_func_ptr) {					\
>  			do {						\
>  				it_func = (it_func_ptr)->func;		\
> @@ -189,7 +189,7 @@ extern void syscall_unregfunc(void);
>  				TP_CONDITION(cond),,);			\
>  		if (IS_ENABLED(CONFIG_LOCKDEP) && (cond)) {		\
>  			rcu_read_lock_sched_notrace();			\
> -			rcu_dereference_sched(__tracepoint_##name.funcs);\
> +			rcu_dereference_raw_notrace(__tracepoint_##name.funcs);\
>  			rcu_read_unlock_sched_notrace();		\
>  		}							\
>  	}								\
> diff --git a/kernel/trace/trace_events_trigger.c b/kernel/trace/trace_events_trigger.c
> index a975571..4489fb4 100644
> --- a/kernel/trace/trace_events_trigger.c
> +++ b/kernel/trace/trace_events_trigger.c
> @@ -71,14 +71,14 @@ event_triggers_call(struct trace_event_file *file, void *rec)
>  	if (list_empty(&file->triggers))
>  		return tt;
>  
> -	list_for_each_entry_rcu(data, &file->triggers, list) {
> +	list_for_each_entry_rcu_notrace(data, &file->triggers, list) {
>  		if (data->paused)
>  			continue;
>  		if (!rec) {
>  			data->ops->func(data, rec);
>  			continue;
>  		}
> -		filter = rcu_dereference_sched(data->filter);
> +		filter = rcu_dereference_raw_notrace(data->filter);
>  		if (filter && !filter_match_preds(filter, rec))
>  			continue;
>  		if (event_command_post_trigger(data->cmd_ops)) {
> @@ -111,7 +111,7 @@ event_triggers_post_call(struct trace_event_file *file,
>  {
>  	struct event_trigger_data *data;
>  
> -	list_for_each_entry_rcu(data, &file->triggers, list) {
> +	list_for_each_entry_rcu_notrace(data, &file->triggers, list) {
>  		if (data->paused)
>  			continue;
>  		if (data->cmd_ops->trigger_type & tt)
> -- 
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> a Linux Foundation Collaborative Project
> 


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>