linux-kernel - [PATCH] binder: Drop needless locking in binder

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Message-ID: <20170524145151.21310-1-dsafonov@virtuozzo.com>
Date:   Wed, 24 May 2017 17:51:51 +0300
From:   Dmitry Safonov <dsafonov@...tuozzo.com>
To:     <linux-kernel@...r.kernel.org>
CC:     <0x7f454c46@...il.com>, Dmitry Safonov <dsafonov@...tuozzo.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Arve Hjønnevåg <arve@...roid.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Riley Andrews <riandrews@...roid.com>,
        <devel@...verdev.osuosl.org>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: [PATCH] binder: Drop needless locking in binder_mmap()

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 <viro@...iv.linux.org.uk>
Cc: "Arve Hjønnevåg" <arve@...roid.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Riley Andrews <riandrews@...roid.com>
Cc: devel@...verdev.osuosl.org
Cc: Android Kernel Team <kernel-team@...roid.com>
Signed-off-by: Dmitry Safonov <dsafonov@...tuozzo.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