binder: reduce mmap_sem write-side lock

binder has used write-side mmap_sem semaphore to release memory
mapped at address space of the process. However, right lock to
release pages is down_read, not down_write because page table lock
already protects the race for parallel freeing.

Please do not use mmap_sem write-side lock which is well known
contented lock.

Cc: Todd Kjos <tkjos@google.com>
Cc: Martijn Coenen <maco@android.com>
Cc: Arve Hjønnevåg <arve@android.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
index 000dd4d..6389467 100644 (file)
--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -932,7 +932,7 @@ enum lru_status binder_alloc_free_page(struct list_head *item,
                if (!mmget_not_zero(alloc->vma_vm_mm))
                        goto err_mmget;
                mm = alloc->vma_vm_mm;
-               if (!down_write_trylock(&mm->mmap_sem))
+               if (!down_read_trylock(&mm->mmap_sem))
                        goto err_down_write_mmap_sem_failed;
        }
 
@@ -946,7 +946,7 @@ enum lru_status binder_alloc_free_page(struct list_head *item,
 
                trace_binder_unmap_user_end(alloc, index);
 
-               up_write(&mm->mmap_sem);
+               up_read(&mm->mmap_sem);
                mmput(mm);
        }