From: Minchan Kim <minchan@kernel.org>
Date: Mon, 18 Feb 2019 08:11:45 +0000 (+0900)
Subject: binder: reduce mmap_sem write-side lock
X-Git-Tag: v5.1-rc1~147^2~48
X-Git-Url: http://git.osdn.net/view?a=commitdiff_plain;h=3013bf62b67aef921bc2e9ba10e639a022002d02;p=uclinux-h8%2Flinux.git

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 000dd4d145ba..6389467670a0 100644
--- 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);
 	}