Re: [PATCH 06/19] bcache: explicitly destory mutex while exiting

Liang Chen
2017-07-05T11:59:00Z

Hi Coly,
Thanks for reviewing the patch! You raised a good point about the race. I also
think it should be addressed. Even though the time window is small, it will
still happen sooner or later.

I would like to keep this "destory mutex" patch unchanged, and send another
patch to fix the issue based on your approach. Please take a look. Thanks!

Thanks,
Liang

On Sun, Jul 2, 2017 at 2:43 AM, Coly Li <i@coly.li> wrote:
> On 2017/7/1 上午4:42, bcache@lists.ewheeler.net wrote:
>> From: Liang Chen <liangchen.linux@gmail.com>
>>
>> mutex_destroy does nothing most of time, but it's better to call
>> it to make the code future proof and it also has some meaning
>> for like mutex debug.
>>
>> Signed-off-by: Liang Chen <liangchen.linux@gmail.com>
>> Reviewed-by: Eric Wheeler <bcache@linux.ewheeler.net>
>> Cc: stable@vger.kernel.org
>> ---
>> drivers/md/bcache/super.c | 2 ++
>> 1 file changed, 2 insertions(+)
>>
>> diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
>> index 48b8c20..1f84791 100644
>> --- a/drivers/md/bcache/super.c
>> +++ b/drivers/md/bcache/super.c
>> @@ -2089,6 +2089,7 @@ static void bcache_exit(void)
>> if (bcache_major)
>> unregister_blkdev(bcache_major, "bcache");
>> unregister_reboot_notifier(&reboot);
>> + mutex_destroy(&bch_register_lock> }
>>
>> static int __init bcache_init(void)
>> @@ -2106,6 +2107,7 @@ static int __init bcache_init(void)
>>
>> bcache_major = register_blkdev(0, "bcache");
>> if (bcache_major < 0) {
>> + mutex_destroy(&bch_register_lock);
>> unregister_reboot_notifier(&reboot);
>> return bcache_major;
>> }
>>
>
> Hi Liang,
>
> Current code might have a potential race in a very corner case, see,
> 2084 static int __init bcache_init(void)
> 2085 {
> 2086 static const struct attribute *files[] = {
> 2087 &ksysfs_register.attr,
> 2088 &ksysfs_register_quiet.attr,
> 2089 NULL
> 2090 };
> 2091
> 2092 mutex_init(&bch_register_lock);
> 2093 init_waitqueue_head(&unregister_wait);
> 2094 register_reboot_notifier(&reboot);
> 2095 closure_debug_init();
> 2096
> 2097 bcache_major = register_blkdev(0, "bcache");
> 2098 if (bcache_major < 0) {
> 2099 unregister_reboot_notifier(&reboot);
> 2100 return bcache_major;
> 2101 }
> 2102
> 2103 if (!(bcache_wq = alloc_workqueue("bcache", WQ_MEM_RECLAIM,
> 0)) ||
> 2104 !(bcache_kobj = kobject_create_and_add("bcache",
> fs_kobj)) ||
> 2105 sysfs_create_files(bcache_kobj, files) ||
> 2106 bch_request_init() ||
> 2107 bch_debug_init(bcache_kobj))
> 2108 goto err;
> 2109
> 2110 return 0;
> 2111 err:
> 2112 bcache_exit();
> 2113 return -ENOMEM;
> 2114 }
>
> At line 2107, most of bache stuffs are ready to work, only a debugfs
> entry not created yet. If in the time gap between line 2106 and line
> 2017, another user space tool just registers cache and backing devices.
> Then bch_debug_init() failed, and bcache_exit() gets called. In this
> case, I doubt bcache_exit() can handle all the references correctly.
>
> The race is very rare, and almost won't happen in real life. So, if you
> don't care about it, the patch can be simpler like this,
> diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
> index e57353e39168..fb5453a46a03 100644
> --- a/drivers/md/bcache/super.c
> +++ b/drivers/md/bcache/super.c
> @@ -2070,6 +2070,7 @@ static struct notifier_block reboot = {
>
> static void bcache_exit(void)
> {
> + mutex_destroy(&bch_register_lock);
> bch_debug_exit();
> bch_request_exit();
> if (bcache_kobj)
> @@ -2089,7 +2090,6 @@ static int __init bcache_init(void)
> NULL
> };
>
> - mutex_init(&bch_register_lock);
> init_waitqueue_head(&unregister_wait);
> register_reboot_notifier(&reboot);
> closure_debug_init();
> @@ -2107,6 +2107,7 @@ static int __init bcache_init(void)
> bch_debug_init(bcache_kobj))
> goto err;
>
> + mutex_init(&bch_register_lock);
> return 0;
> err:
> bcache_exit();
> ---
> And if you do care about the race, maybe you should do something like this,
> diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
> index e57353e39168..ca1d8b7a7815 100644
> --- a/drivers/md/bcache/super.c
> +++ b/drivers/md/bcache/super.c
> @@ -2079,6 +2079,7 @@ static void bcache_exit(void)
> if (bcache_major)
> unregister_blkdev(bcache_major, "bcache");
> unregister_reboot_notifier(&reboot);
> + mutex_unlock(&bch_register_lock);
> }
>
> static int __init bcache_init(void)
> @@ -2090,6 +2091,7 @@ static int __init bcache_init(void)
> };
>
> mutex_init(&bch_register_lock);
> + mutex_lock(&bch_register_lock);
> init_waitqueue_head(&unregister_wait);
> register_reboot_notifier(&reboot);
> closure_debug_init();
> @@ -2097,6 +2099,8 @@ static int __init bcache_init(void)
> bcache_major = register_blkdev(0, "bcache");
> if (bcache_major < 0) {
> unregister_reboot_notifier(&reboot);
> + mutex_unlock(&bch_register_lock);
> + mutex_destroy(&bch_register_lock);
> return bcache_major;
> }
>
> @@ -2104,9 +2108,12 @@ static int __init bcache_init(void)
> !(bcache_kobj = kobject_create_and_add("bcache", fs_kobj)) ||
> sysfs_create_files(bcache_kobj, files) ||
> bch_request_init() ||
> - bch_debug_init(bcache_kobj))
> + bch_debug_init(bcache_kobj)) {
> + mutex_unlock(&bch_register_lock);
> goto err;
> + }
>
> + mutex_unlock(&bch_register_lock);
> return 0;
> err:
> bcache_exit();
> ---
>
> Personally I think the first approach with only one new line code added,
> your original version will add two new lines of code.
>
> Just FYI. Thanks.
>
> --
> Coly Li