[PATCH] binder: Drop needless locking in binder_mmap()

[Dmitry Safonov]

binder_mmap() protected proc->buffer from concurrent changes.
It was introduced with commit bd1eff9741af ("Staging: android: binder:
Fix crashes when sharing a binder file between processes").

The commit fixed such races alike:

task1
=====
fd = open("/dev/binder",..)
fork()
---------------------------------------->task2
  |                                      =====
mmap(fd)                                mmap(fd)

Afterwards the situation was changed with the commit a79f41ed9786
("binder: don't allow mmap() by process other than proc->tsk").
That commit prohibited mmap'ing of fd, that was opened by some
other task.
That restriction has made locking in binder_mmap() unnecessary.

Cc: Al Viro <v...@zeniv.linux.org.uk>
Cc: "Arve Hjønnevåg" <a...@android.com>
Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org>
Cc: Riley Andrews <riandr...@android.com>
Cc: de...@driverdev.osuosl.org
Cc: Android Kernel Team <kernel-t...@android.com>
Signed-off-by: Dmitry Safonov <dsafo...@virtuozzo.com>
---
 drivers/android/binder.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index aae4d8d4be36..b5b32be85ab8 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -49,7 +49,6 @@
 
 static DEFINE_MUTEX(binder_main_lock);
 static DEFINE_MUTEX(binder_deferred_lock);
-static DEFINE_MUTEX(binder_mmap_lock);
 
 static HLIST_HEAD(binder_devices);
 static HLIST_HEAD(binder_procs);
@@ -3381,7 +3380,6 @@ static int binder_mmap(struct file *filp, struct 
vm_area_struct *vma)
        }
        vma->vm_flags = (vma->vm_flags | VM_DONTCOPY) & ~VM_MAYWRITE;
 
-       mutex_lock(&binder_mmap_lock);
        if (proc->buffer) {
                ret = -EBUSY;
                failure_string = "already mapped";
@@ -3396,7 +3394,6 @@ static int binder_mmap(struct file *filp, struct 
vm_area_struct *vma)
        }
        proc->buffer = area->addr;
        proc->user_buffer_offset = vma->vm_start - (uintptr_t)proc->buffer;
-       mutex_unlock(&binder_mmap_lock);
 
 #ifdef CONFIG_CPU_CACHE_VIPT
        if (cache_is_vipt_aliasing()) {
@@ -3441,12 +3438,10 @@ static int binder_mmap(struct file *filp, struct 
vm_area_struct *vma)
        kfree(proc->pages);
        proc->pages = NULL;
 err_alloc_pages_failed:
-       mutex_lock(&binder_mmap_lock);
        vfree(proc->buffer);
        proc->buffer = NULL;
 err_get_vm_area_failed:
 err_already_mapped:
-       mutex_unlock(&binder_mmap_lock);
 err_bad_arg:
        pr_err("binder_mmap: %d %lx-%lx %s failed %d\n",
               proc->pid, vma->vm_start, vma->vm_end, failure_string, ret);
-- 
2.12.2