Re: [PATCH] md: properly lock and unlock in rdev_attr_store()

[Song Liu]

On Sun, Apr 28, 2019 at 3:41 AM Lukas Bulwahn <lukas.bulwahn@xxxxxxxxx> wrote:
>
> rdev_attr_store() should lock and unlock mddev->reconfig_mutex in a
> balanced way with mddev_lock() and mddev_unlock().
>
> But when rdev->mddev is NULL, rdev_attr_store() would try to unlock
> without locking before. Resolve this locking issue..
>
> This locking issue was detected with Clang Thread Safety Analyser:
>
> drivers/md/md.c:3393:3: warning: releasing mutex 'mddev->reconfig_mutex' that was not held [-Wthread-safety-analysis]
>                 mddev_unlock(mddev);
>                 ^
>
> This warning was reported after annotating mutex functions and
> mddev_lock() and mddev_unlock().
>
> Fixes: 27c529bb8e90 ("md: lock access to rdev attributes properly")
> Link: https://groups.google.com/d/topic/clang-built-linux/CvBiiQLB0H4/discussion
> Signed-off-by: Lukas Bulwahn <lukas.bulwahn@xxxxxxxxx>
> ---
> Arnd, Neil, here a proposal to fix lock and unlocking asymmetry.
>
> I quite sure that if mddev is NULL, it should just return.

I think current code will just return -EBUSY when mddev is NULL, right?

Song


>
> I am still puzzled if the return value from mddev_lock() should be really
> return by rdev_attr_store() when it is not 0. But that was the behaviour
> before, so I will keep it that way.
>
>  drivers/md/md.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 05ffffb8b769..a9735d8f1e70 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -3384,7 +3384,9 @@ rdev_attr_store(struct kobject *kobj, struct attribute *attr,
>                 return -EIO;
>         if (!capable(CAP_SYS_ADMIN))
>                 return -EACCES;
> -       rv = mddev ? mddev_lock(mddev): -EBUSY;
> +       if (!mddev)
> +               return -EBUSY;
> +       rv = mddev_lock(mddev);
>         if (!rv) {
>                 if (rdev->mddev == NULL)
>                         rv = -EBUSY;
> --
> 2.17.1
>