[PATCH] binder: reduce mmap_sem write-side lock

[Minchan Kim]

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 <tk...@google.com>
Cc: Martijn Coenen <m...@android.com>
Cc: Arve Hjønnevåg <a...@android.com>
Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org>
Signed-off-by: Minchan Kim <minc...@kernel.org>
---
 drivers/android/binder_alloc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
index 022cd80e80cc3..2a62553b20fcc 100644
--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -964,7 +964,7 @@ 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;
        }
 
@@ -980,7 +980,7 @@ 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);
        }
 
-- 
2.21.0.rc0.258.g878e2cd30e-goog