Re: [PATCH 1/1] tracepoint: cleanup: do not fail unregistration

Steven Rostedt
2021-02-04T17:49:14Z

On Wed, 3 Feb 2021 12:57:41 -0500
Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:

> This patch is only compile-tested.
>

Yes it is. (crashed on boot)

> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> ---
> include/linux/tracepoint.h | 2 +-
> kernel/tracepoint.c | 80 +++++++++++++-------------------------
> 2 files changed, 28 insertions(+), 54 deletions(-)
>
> diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> index 0f21617f1a66..fae8d6d9588c 100644
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -308,7 +308,7 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
> it_func_ptr = \
> rcu_dereference_raw((&__tracepoint_##_name)->funcs); \
> 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 3e261482296c..b1bec710f68a 100644
> --- a/kernel/tracepoint.c
> +++ b/kernel/tracepoint.c
> @@ -136,9 +136,10 @@ 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 = 0; /* Iterate over old probe array. */
> + int nr_old_probes = 0; /* Count non-stub functions in old. */
> + int nr_new_probes = 0; /* Count functions in new. */
> + int pos = -1; /* Insert position in new. */
>
> if (WARN_ON(!tp_func->func))
> return ERR_PTR(-EINVAL);
> @@ -147,54 +148,34 @@ 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++;
> + /* Insert before probes of lower priority */
> + if (pos < 0 && old[iter_probes].prio < prio)
> + pos = nr_old_probes;
> + nr_old_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_old_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 */

The above "trick" is to handle the case that we inserted a probe with the:

if (pos < 0 && old[nr_probes].prio < prio)
pos = probes++;

Which in the below code, you missed.

> -
> - } 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));
> + for (iter_probes = 0; old[iter_probes].func; iter_probes++) {
> + if (old[iter_probes].func == tp_stub_func)
> + continue; /* Skip stub functions. */
> + if (nr_new_probes != pos)
> + new[nr_new_probes] = old[iter_probes];

The problem is above, because we just skipped adding old[iter_probes].

> + nr_new_probes++;

I'm not sure this is any less confusing than my code. Because, I now have
to try and wrap my head around how to increment three different variables
instead of just two.

I agree that we can just make it one code, but I think I'll keep the
original logic, with slight modifications influenced by your code.

Instead of doing that "trick" that decrements nr_probes, I should increment
it if we are adding to the end (pos < 0):

if (pos < 0)
pos = nr_probes++;
/* nr_probes points to the end of the array */
} else { /* !old */
pos = 0;
nr_probes = 1; /* points to end of array */
}

new[pos] = *tp_func;
new[nr_probes].func = NULL;

> }
> } else
> pos = 0;
> new[pos] = *tp_func;
> - new[nr_probes + 1].func = NULL;
> + new[nr_new_probes + 1].func = NULL;
> *funcs = new;
> debug_print_probes(*funcs);
> return old;
> @@ -238,9 +219,9 @@ static void *func_remove(struct tracepoint_func **funcs,
> 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)
> + 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;
> @@ -251,15 +232,8 @@ static void *func_remove(struct tracepoint_func **funcs,
> */
> 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);

I'll add this change too.

> *funcs = old;
> }
> }

Here's an updated patch:

(tested against tools/testing/selftests/ftrace/ftracetest)

Index: linux-trace.git/include/linux/tracepoint.h
===================================================================
--- linux-trace.git.orig/include/linux/tracepoint.h
+++ linux-trace.git/include/linux/tracepoint.h
@@ -309,7 +309,7 @@ static inline struct tracepoint *tracepo
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); \
Index: linux-trace.git/kernel/tracepoint.c
===================================================================
--- linux-trace.git.orig/kernel/tracepoint.c
+++ linux-trace.git/kernel/tracepoint.c
@@ -136,9 +136,9 @@ func_add(struct tracepoint_func **funcs,
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 */

if (WARN_ON(!tp_func->func))
return ERR_PTR(-EINVAL);
@@ -147,54 +147,39 @@ func_add(struct tracepoint_func **funcs,
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;
+ 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 */
+ }
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 tracepoi
/* + 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 tracepoi
* 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);
+ }
*funcs = old;
}
}