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Message-ID: <aabd9668-bd22-44de-a4ef-e3bff9c48c28@lucifer.local>
Date: Wed, 13 Aug 2025 08:09:54 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: tglx@...utronix.de, linux-kernel@...r.kernel.org,
torvalds@...uxfoundation.org, mingo@...nel.org, namhyung@...nel.org,
acme@...hat.com, kees@...nel.org
Subject: Re: [PATCH v3 15/15] perf: Convert mmap() refcounts to refcount_t
On Tue, Aug 12, 2025 at 12:39:13PM +0200, Peter Zijlstra wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
>
> The recently fixed reference count leaks could have been detected by using
> refcount_t and refcount_t would have mitigated the potential overflow at
> least.
>
> Now that the code is properly structured, convert the mmap() related
> mmap_count variants over to refcount_t.
>
> No functional change intended.
>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
All LGTM, checked the mmap test passes and no splat, the atomic_inc() ->
atomic_set() on &rb->mmap_count (which here becomes refcount_set()) sorted
that out.
This is a nice fix up as is the rest of the series, kudos to tglx and
additionally you Peter for adding further nice changes + fixing up the bit
we missed.
This is a _massive_ improvement :)
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Link: https://lkml.kernel.org/r/20250811070620.716309215@linutronix.de
> ---
> include/linux/perf_event.h | 2 +-
> kernel/events/core.c | 40 ++++++++++++++++++++--------------------
> kernel/events/internal.h | 4 ++--
> kernel/events/ring_buffer.c | 2 +-
> 4 files changed, 24 insertions(+), 24 deletions(-)
>
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -859,7 +859,7 @@ struct perf_event {
>
> /* mmap bits */
> struct mutex mmap_mutex;
> - atomic_t mmap_count;
> + refcount_t mmap_count;
>
> struct perf_buffer *rb;
> struct list_head rb_entry;
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3968,7 +3968,7 @@ static noinline int visit_groups_merge(s
> */
> static inline bool event_update_userpage(struct perf_event *event)
> {
> - if (likely(!atomic_read(&event->mmap_count)))
> + if (likely(!refcount_read(&event->mmap_count)))
> return false;
>
> perf_event_update_time(event);
> @@ -6704,11 +6704,11 @@ static void perf_mmap_open(struct vm_are
> struct perf_event *event = vma->vm_file->private_data;
> mapped_f mapped = get_mapped(event, event_mapped);
>
> - atomic_inc(&event->mmap_count);
> - atomic_inc(&event->rb->mmap_count);
> + refcount_inc(&event->mmap_count);
> + refcount_inc(&event->rb->mmap_count);
>
> if (vma->vm_pgoff)
> - atomic_inc(&event->rb->aux_mmap_count);
> + refcount_inc(&event->rb->aux_mmap_count);
>
> if (mapped)
> mapped(event, vma->vm_mm);
> @@ -6743,7 +6743,7 @@ static void perf_mmap_close(struct vm_ar
> * to avoid complications.
> */
> if (rb_has_aux(rb) && vma->vm_pgoff == rb->aux_pgoff &&
> - atomic_dec_and_mutex_lock(&rb->aux_mmap_count, &rb->aux_mutex)) {
> + refcount_dec_and_mutex_lock(&rb->aux_mmap_count, &rb->aux_mutex)) {
> /*
> * Stop all AUX events that are writing to this buffer,
> * so that we can free its AUX pages and corresponding PMU
> @@ -6763,10 +6763,10 @@ static void perf_mmap_close(struct vm_ar
> mutex_unlock(&rb->aux_mutex);
> }
>
> - if (atomic_dec_and_test(&rb->mmap_count))
> + if (refcount_dec_and_test(&rb->mmap_count))
> detach_rest = true;
>
> - if (!atomic_dec_and_mutex_lock(&event->mmap_count, &event->mmap_mutex))
> + if (!refcount_dec_and_mutex_lock(&event->mmap_count, &event->mmap_mutex))
> goto out_put;
>
> ring_buffer_attach(event, NULL);
> @@ -6992,19 +6992,19 @@ static int perf_mmap_rb(struct vm_area_s
> if (data_page_nr(event->rb) != nr_pages)
> return -EINVAL;
>
> - if (atomic_inc_not_zero(&event->rb->mmap_count)) {
> + if (refcount_inc_not_zero(&event->rb->mmap_count)) {
> /*
> * Success -- managed to mmap() the same buffer
> * multiple times.
> */
> perf_mmap_account(vma, user_extra, extra);
> - atomic_inc(&event->mmap_count);
> + refcount_inc(&event->mmap_count);
> return 0;
> }
>
> /*
> * Raced against perf_mmap_close()'s
> - * atomic_dec_and_mutex_lock() remove the
> + * refcount_dec_and_mutex_lock() remove the
> * event and continue as if !event->rb
> */
> ring_buffer_attach(event, NULL);
> @@ -7023,7 +7023,7 @@ static int perf_mmap_rb(struct vm_area_s
> if (!rb)
> return -ENOMEM;
>
> - atomic_set(&rb->mmap_count, 1);
> + refcount_set(&rb->mmap_count, 1);
> rb->mmap_user = get_current_user();
> rb->mmap_locked = extra;
>
> @@ -7034,7 +7034,7 @@ static int perf_mmap_rb(struct vm_area_s
> perf_event_update_userpage(event);
>
> perf_mmap_account(vma, user_extra, extra);
> - atomic_set(&event->mmap_count, 1);
> + refcount_set(&event->mmap_count, 1);
>
> return 0;
> }
> @@ -7081,15 +7081,15 @@ static int perf_mmap_aux(struct vm_area_
> if (!is_power_of_2(nr_pages))
> return -EINVAL;
>
> - if (!atomic_inc_not_zero(&rb->mmap_count))
> + if (!refcount_inc_not_zero(&rb->mmap_count))
> return -EINVAL;
>
> if (rb_has_aux(rb)) {
> - atomic_inc(&rb->aux_mmap_count);
> + refcount_inc(&rb->aux_mmap_count);
>
> } else {
> if (!perf_mmap_calc_limits(vma, &user_extra, &extra)) {
> - atomic_dec(&rb->mmap_count);
> + refcount_dec(&rb->mmap_count);
> return -EPERM;
> }
>
> @@ -7101,16 +7101,16 @@ static int perf_mmap_aux(struct vm_area_
> ret = rb_alloc_aux(rb, event, vma->vm_pgoff, nr_pages,
> event->attr.aux_watermark, rb_flags);
> if (ret) {
> - atomic_dec(&rb->mmap_count);
> + refcount_dec(&rb->mmap_count);
> return ret;
> }
>
> - atomic_set(&rb->aux_mmap_count, 1);
> + refcount_set(&rb->aux_mmap_count, 1);
> rb->aux_mmap_locked = extra;
> }
>
> perf_mmap_account(vma, user_extra, extra);
> - atomic_inc(&event->mmap_count);
> + refcount_inc(&event->mmap_count);
>
> return 0;
> }
> @@ -13257,7 +13257,7 @@ perf_event_set_output(struct perf_event
> mutex_lock_double(&event->mmap_mutex, &output_event->mmap_mutex);
> set:
> /* Can't redirect output if we've got an active mmap() */
> - if (atomic_read(&event->mmap_count))
> + if (refcount_read(&event->mmap_count))
> goto unlock;
>
> if (output_event) {
> @@ -13270,7 +13270,7 @@ perf_event_set_output(struct perf_event
> goto unlock;
>
> /* did we race against perf_mmap_close() */
> - if (!atomic_read(&rb->mmap_count)) {
> + if (!refcount_read(&rb->mmap_count)) {
> ring_buffer_put(rb);
> goto unlock;
> }
> --- a/kernel/events/internal.h
> +++ b/kernel/events/internal.h
> @@ -35,7 +35,7 @@ struct perf_buffer {
> spinlock_t event_lock;
> struct list_head event_list;
>
> - atomic_t mmap_count;
> + refcount_t mmap_count;
> unsigned long mmap_locked;
> struct user_struct *mmap_user;
>
> @@ -47,7 +47,7 @@ struct perf_buffer {
> unsigned long aux_pgoff;
> int aux_nr_pages;
> int aux_overwrite;
> - atomic_t aux_mmap_count;
> + refcount_t aux_mmap_count;
> unsigned long aux_mmap_locked;
> void (*free_aux)(void *);
> refcount_t aux_refcount;
> --- a/kernel/events/ring_buffer.c
> +++ b/kernel/events/ring_buffer.c
> @@ -400,7 +400,7 @@ void *perf_aux_output_begin(struct perf_
> * the same order, see perf_mmap_close. Otherwise we end up freeing
> * aux pages in this path, which is a bug, because in_atomic().
> */
> - if (!atomic_read(&rb->aux_mmap_count))
> + if (!refcount_read(&rb->aux_mmap_count))
> goto err;
>
> if (!refcount_inc_not_zero(&rb->aux_refcount))
>
>
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