Re: [PATCH v3] staging: ion: Avoid using rt_mutexes directly

[Colin Cross]

On Tue, Dec 17, 2013 at 5:04 PM, John Stultz <john.stultz@xxxxxxxxxx> wrote:
> RT_MUTEXES can be configured out of the kernel, causing compile
> problems with ION.
>
> To quote Colin:
> "rt_mutexes were added with the deferred freeing feature.  Heaps need
> to return zeroed memory to userspace, but zeroing the memory on every
> allocation was causing performance issues.  We added a SCHED_IDLE
> thread to zero memory in the background after freeing, but locking the
> heap from the SCHED_IDLE thread might block a high priority allocation
> thread for a long time.
>
> The lock is only used to protect the heap's free_list and
> free_list_size members, and is not held for any long or sleeping
> operations.  Converting to a spinlock should prevent priority
> inversion without using the rt_mutex.  I'd also rename it to free_lock
> to so it doesn't get used as a general heap lock."
>
> Thus this patch converts the rt_mutex usage to a spinlock and
> renames the lock free_lock to be more clear as to its use.
>
> I also had to change a bit of logic in ion_heap_freelist_drain()
> to safely avoid list corruption.
>
> Cc: Colin Cross <ccross@xxxxxxxxxxx>
> Cc: Android Kernel Team <kernel-team@xxxxxxxxxxx>
> Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
> Reported-by: Jim Davis <jim.epost@xxxxxxxxx>
> Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
> ---
>
> Changes from v2:
> - Modified while loop to be more clear, per Colin's suggestion
>
>
>  drivers/staging/android/ion/ion_heap.c | 28 ++++++++++++++++------------
>  drivers/staging/android/ion/ion_priv.h |  2 +-
>  2 files changed, 17 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/staging/android/ion/ion_heap.c b/drivers/staging/android/ion/ion_heap.c
> index 9cf5622..296c74f 100644
> --- a/drivers/staging/android/ion/ion_heap.c
> +++ b/drivers/staging/android/ion/ion_heap.c
> @@ -160,10 +160,10 @@ int ion_heap_pages_zero(struct page *page, size_t size, pgprot_t pgprot)
>
>  void ion_heap_freelist_add(struct ion_heap *heap, struct ion_buffer *buffer)
>  {
> -       rt_mutex_lock(&heap->lock);
> +       spin_lock(&heap->free_lock);
>         list_add(&buffer->list, &heap->free_list);
>         heap->free_list_size += buffer->size;
> -       rt_mutex_unlock(&heap->lock);
> +       spin_unlock(&heap->free_lock);
>         wake_up(&heap->waitqueue);
>  }
>
> @@ -171,34 +171,38 @@ size_t ion_heap_freelist_size(struct ion_heap *heap)
>  {
>         size_t size;
>
> -       rt_mutex_lock(&heap->lock);
> +       spin_lock(&heap->free_lock);
>         size = heap->free_list_size;
> -       rt_mutex_unlock(&heap->lock);
> +       spin_unlock(&heap->free_lock);
>
>         return size;
>  }
>
>  size_t ion_heap_freelist_drain(struct ion_heap *heap, size_t size)
>  {
> -       struct ion_buffer *buffer, *tmp;
> +       struct ion_buffer *buffer;
>         size_t total_drained = 0;
>
>         if (ion_heap_freelist_size(heap) == 0)
>                 return 0;
>
> -       rt_mutex_lock(&heap->lock);
> +       spin_lock(&heap->free_lock);
>         if (size == 0)
>                 size = heap->free_list_size;
>
> -       list_for_each_entry_safe(buffer, tmp, &heap->free_list, list) {
> +       while (!list_empty(&heap->free_list)) {
>                 if (total_drained >= size)
>                         break;
> +               buffer = list_first_entry(&heap->free_list, struct ion_buffer,
> +                                         list);
>                 list_del(&buffer->list);
>                 heap->free_list_size -= buffer->size;
>                 total_drained += buffer->size;
> +               spin_unlock(&heap->free_lock);
>                 ion_buffer_destroy(buffer);
> +               spin_lock(&heap->free_lock);
>         }
> -       rt_mutex_unlock(&heap->lock);
> +       spin_unlock(&heap->free_lock);
>
>         return total_drained;
>  }
> @@ -213,16 +217,16 @@ static int ion_heap_deferred_free(void *data)
>                 wait_event_freezable(heap->waitqueue,
>                                      ion_heap_freelist_size(heap) > 0);
>
> -               rt_mutex_lock(&heap->lock);
> +               spin_lock(&heap->free_lock);
>                 if (list_empty(&heap->free_list)) {
> -                       rt_mutex_unlock(&heap->lock);
> +                       spin_unlock(&heap->free_lock);
>                         continue;
>                 }
>                 buffer = list_first_entry(&heap->free_list, struct ion_buffer,
>                                           list);
>                 list_del(&buffer->list);
>                 heap->free_list_size -= buffer->size;
> -               rt_mutex_unlock(&heap->lock);
> +               spin_unlock(&heap->free_lock);
>                 ion_buffer_destroy(buffer);
>         }
>
> @@ -235,7 +239,7 @@ int ion_heap_init_deferred_free(struct ion_heap *heap)
>
>         INIT_LIST_HEAD(&heap->free_list);
>         heap->free_list_size = 0;
> -       rt_mutex_init(&heap->lock);
> +       spin_lock_init(&heap->free_lock);
>         init_waitqueue_head(&heap->waitqueue);
>         heap->task = kthread_run(ion_heap_deferred_free, heap,
>                                  "%s", heap->name);
> diff --git a/drivers/staging/android/ion/ion_priv.h b/drivers/staging/android/ion/ion_priv.h
> index fc8a4c3..d986739 100644
> --- a/drivers/staging/android/ion/ion_priv.h
> +++ b/drivers/staging/android/ion/ion_priv.h
> @@ -159,7 +159,7 @@ struct ion_heap {
>         struct shrinker shrinker;
>         struct list_head free_list;
>         size_t free_list_size;
> -       struct rt_mutex lock;
> +       spinlock_t free_lock;
>         wait_queue_head_t waitqueue;
>         struct task_struct *task;
>         int (*debug_show)(struct ion_heap *heap, struct seq_file *, void *);
> --
> 1.8.3.2
>

Acked-by: Colin Cross <ccross@xxxxxxxxxxx>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/