[PATCH] binder: reduce mmap_sem write-side lock

[Minchan Kim <minchan@kernel.org>]

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>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Minchan Kim <minchan@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