Re: [PATCH] tracepoints: Code clean up

[Steven Rostedt]

On Thu, 4 Feb 2021 14:13:59 -0500 (EST)
Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:

> ----- On Feb 4, 2021, at 1:27 PM, rostedt rostedt@xxxxxxxxxxx wrote:
> 
> > From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
> > 
> > Restructure the code a bit to make it simpler, fix some formatting problems
> > and add READ_ONCE/WRITE_ONCE to make sure there's no compiler tear downs on  
> 
> compiler tear downs -> compiler load/store tearing.

Will change.

> 
> > changes to variables that can be accessed across CPUs.
> > 
> > Started with Mathieu Desnoyers's patch:
> > 
> >  Link:
> >  https://lore.kernel.org/lkml/20210203175741.20665-1-mathieu.desnoyers@xxxxxxxxxxxx/
> > 
> > And will keep his signature, but I will take the responsibility of this
> > being correct, and keep the authorship.
> > 
> > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> > ---
> > include/linux/tracepoint.h |  2 +-
> > kernel/tracepoint.c        | 92 +++++++++++++++-----------------------
> > 2 files changed, 37 insertions(+), 57 deletions(-)
> > 
> > diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> > index 966ed8980327..dc1d4c612cc3 100644
> > --- a/include/linux/tracepoint.h
> > +++ b/include/linux/tracepoint.h
> > @@ -309,7 +309,7 @@ static inline struct tracepoint
> > *tracepoint_ptr_deref(tracepoint_ptr_t *p)
> > 			rcu_dereference_raw((&__tracepoint_##_name)->funcs); \
> > 		if (it_func_ptr) {					\
> > 			do {						\
> > -				it_func = (it_func_ptr)->func;		\
> > +				it_func = READ_ONCE((it_func_ptr)->func); \
> > 				__data = (it_func_ptr)->data;		\
> > 				((void(*)(void *, proto))(it_func))(__data, args); \
> > 			} while ((++it_func_ptr)->func);		\
> > diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
> > index e8f20ae29c18..4b9de79bb927 100644
> > --- a/kernel/tracepoint.c
> > +++ b/kernel/tracepoint.c
> > @@ -136,9 +136,9 @@ func_add(struct tracepoint_func **funcs, struct
> > tracepoint_func *tp_func,
> > 	 int prio)
> > {
> > 	struct tracepoint_func *old, *new;
> > -	int nr_probes = 0;
> > -	int stub_funcs = 0;
> > -	int pos = -1;
> > +	int iter_probes;	/* Iterate over old probe array. */
> > +	int nr_probes = 0;	/* Counter for probes */
> > +	int pos = -1;		/* New position */  
> 
> New position -> Insertion position into new array

OK.

> 
> > 
> > 	if (WARN_ON(!tp_func->func))
> > 		return ERR_PTR(-EINVAL);
> > @@ -147,54 +147,39 @@ func_add(struct tracepoint_func **funcs, struct
> > tracepoint_func *tp_func,
> > 	old = *funcs;
> > 	if (old) {
> > 		/* (N -> N+1), (N != 0, 1) probes */
> > -		for (nr_probes = 0; old[nr_probes].func; nr_probes++) {
> > -			/* Insert before probes of lower priority */
> > -			if (pos < 0 && old[nr_probes].prio < prio)
> > -				pos = nr_probes;
> > -			if (old[nr_probes].func == tp_func->func &&
> > -			    old[nr_probes].data == tp_func->data)
> > +		for (iter_probes = 0; old[iter_probes].func; iter_probes++) {
> > +			if (old[iter_probes].func == tp_stub_func)
> > +				continue;	/* Skip stub functions. */
> > +			if (old[iter_probes].func == tp_func->func &&
> > +			    old[iter_probes].data == tp_func->data)
> > 				return ERR_PTR(-EEXIST);
> > -			if (old[nr_probes].func == tp_stub_func)
> > -				stub_funcs++;
> > +			nr_probes++;
> > 		}
> > 	}
> > -	/* + 2 : one for new probe, one for NULL func - stub functions */
> > -	new = allocate_probes(nr_probes + 2 - stub_funcs);
> > +	/* + 2 : one for new probe, one for NULL func */
> > +	new = allocate_probes(nr_probes + 2);
> > 	if (new == NULL)
> > 		return ERR_PTR(-ENOMEM);
> > 	if (old) {
> > -		if (stub_funcs) {
> > -			/* Need to copy one at a time to remove stubs */
> > -			int probes = 0;
> > -
> > -			pos = -1;
> > -			for (nr_probes = 0; old[nr_probes].func; nr_probes++) {
> > -				if (old[nr_probes].func == tp_stub_func)
> > -					continue;
> > -				if (pos < 0 && old[nr_probes].prio < prio)
> > -					pos = probes++;
> > -				new[probes++] = old[nr_probes];
> > -			}
> > -			nr_probes = probes;
> > -			if (pos < 0)
> > -				pos = probes;
> > -			else
> > -				nr_probes--; /* Account for insertion */
> > -
> > -		} else if (pos < 0) {
> > -			pos = nr_probes;
> > -			memcpy(new, old, nr_probes * sizeof(struct tracepoint_func));
> > -		} else {
> > -			/* Copy higher priority probes ahead of the new probe */
> > -			memcpy(new, old, pos * sizeof(struct tracepoint_func));
> > -			/* Copy the rest after it. */
> > -			memcpy(new + pos + 1, old + pos,
> > -			       (nr_probes - pos) * sizeof(struct tracepoint_func));
> > +		pos = -1;  
> 
> pos is already initialized to -1 at function beginning.

Ah. I noticed near the end of developing this, that we were calculating
"pos" twice. Once in the search for stub functions, and again later, where
the above assignment was necessary. I then realized that finding pos the
first time wasn't necessary and removed it, but didn't remove this second
initialization of pos.

Will remove it in v2.

> 
> > +		nr_probes = 0;
> > +		for (iter_probes = 0; old[iter_probes].func; iter_probes++) {
> > +			if (old[iter_probes].func == tp_stub_func)
> > +				continue;
> > +			/* Insert before probes of lower priority */
> > +			if (pos < 0 && old[iter_probes].prio < prio)
> > +				pos = nr_probes++;
> > +			new[nr_probes++] = old[iter_probes];
> > 		}
> > -	} else
> > +		if (pos < 0)
> > +			pos = nr_probes++;
> > +		/* nr_probes now points to the end of the new array */
> > +	} else {
> > 		pos = 0;
> > +		nr_probes = 1; /* must point at end of array */  
> 
> Yep, much nicer.
> 
> > +	}
> > 	new[pos] = *tp_func;
> > -	new[nr_probes + 1].func = NULL;
> > +	new[nr_probes].func = NULL;
> > 	*funcs = new;
> > 	debug_print_probes(*funcs);
> > 	return old;
> > @@ -237,11 +222,12 @@ static void *func_remove(struct tracepoint_func **funcs,
> > 		/* + 1 for NULL */
> > 		new = allocate_probes(nr_probes - nr_del + 1);
> > 		if (new) {
> > -			for (i = 0; old[i].func; i++)
> > -				if ((old[i].func != tp_func->func
> > -				     || old[i].data != tp_func->data)
> > -				    && old[i].func != tp_stub_func)
> > +			for (i = 0; old[i].func; i++) {
> > +				if ((old[i].func != tp_func->func ||
> > +				     old[i].data != tp_func->data) &&
> > +				    old[i].func != tp_stub_func)
> > 					new[j++] = old[i];
> > +			}
> > 			new[nr_probes - nr_del].func = NULL;
> > 			*funcs = new;
> > 		} else {
> > @@ -249,17 +235,11 @@ static void *func_remove(struct tracepoint_func **funcs,
> > 			 * Failed to allocate, replace the old function
> > 			 * with calls to tp_stub_func.
> > 			 */
> > -			for (i = 0; old[i].func; i++)
> > +			for (i = 0; old[i].func; i++) {
> > 				if (old[i].func == tp_func->func &&
> > -				    old[i].data == tp_func->data) {
> > -					old[i].func = tp_stub_func;
> > -					/* Set the prio to the next event. */
> > -					if (old[i + 1].func)
> > -						old[i].prio =
> > -							old[i + 1].prio;
> > -					else
> > -						old[i].prio = -1;
> > -				}
> > +				    old[i].data == tp_func->data)
> > +					WRITE_ONCE(old[i].func, tp_stub_func);
> > +			}  
> 
> The rest looks good, thanks!
> 
> You can keep my signed-off-by, and if needed may add my reviewed-by tag
> as well. ;-)
> 

I'll send a v2. Thanks for looking at it.

-- Steve