On Tue, Dec 17, 2013 at 5:04 PM, John Stultz <john.stu...@linaro.org> 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 <ccr...@android.com>
> Cc: Android Kernel Team <kernel-t...@android.com>
> Cc: Greg KH <gre...@linuxfoundation.org>
> Reported-by: Jim Davis <jim.ep...@gmail.com>
> Signed-off-by: John Stultz <john.stu...@linaro.org>
> ---
>
> 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 <ccr...@android.com>
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