On Sun, Apr 28 2019, Lukas Bulwahn wrote:
> rdev_attr_store() should lock and unlock mddev->reconfig_mutex in a
> balanced way with mddev_lock() and mddev_unlock().
It does.
>
> But when rdev->mddev is NULL, rdev_attr_store() would try to unlock
> without locking before. Resolve this locking issue..
This is incorrect.
>
> This locking issue was detected with Clang Thread Safety Analyser:
Either the Clang Thread Safety Analyser is broken, or you used it
incorrectly.
>
> 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.
If mddev is NULL, the code does return (with -EBUSY). All you've done
is change things so it returns from a different part of the code. You
haven't changed the behaviour at all.
>
> 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.
Certainly it should. mddev_lock() either returns 0 to indicate success
or -EINTR if it received a signal.
If it was interrupted by a signal, then rdev_attr_store() should return
-EINTR as well.
As Arnd tried to explain, the only possible problem here is that the C
compiler is allowed to assume that rdev->mddev never changes value, so
in
rv = mddev ? mddev_lock(mddev) : =EBUSY
it could load rdev->mddev, test if it is NULL, then load it again and
pass that value to mddev_lock() - the new value might be NULL which
would cause problems.
This could be fixed by changing
struct mddev *mddev = rdev->mddev;
to
struct mddev *mddev = READ_ONCE(rdev->mddev);
That is the only change that might be useful here.
NeilBrown
>
> 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
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