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    SubjectRe: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by WRITE_ONCE
    On 05/19/2016 06:21 PM, Jason Low wrote:
    > On Wed, 2016-05-18 at 12:58 -0700, Jason Low wrote:
    >> On Wed, 2016-05-18 at 14:29 -0400, Waiman Long wrote:
    >>> On 05/18/2016 01:21 PM, Jason Low wrote:
    >>>> On Wed, 2016-05-18 at 07:04 -0700, Davidlohr Bueso wrote:
    >>>>> On Tue, 17 May 2016, Waiman Long wrote:
    >>>>>
    >>>>>> Without using WRITE_ONCE(), the compiler can potentially break a
    >>>>>> write into multiple smaller ones (store tearing). So a read from the
    >>>>>> same data by another task concurrently may return a partial result.
    >>>>>> This can result in a kernel crash if the data is a memory address
    >>>>>> that is being dereferenced.
    >>>>>>
    >>>>>> This patch changes all write to rwsem->owner to use WRITE_ONCE()
    >>>>>> to make sure that store tearing will not happen. READ_ONCE() may
    >>>>>> not be needed for rwsem->owner as long as the value is only used for
    >>>>>> comparison and not dereferencing.
    >>>> It might be okay to leave out READ_ONCE() for reading rwsem->owner, but
    >>>> couldn't we include it to at least document that we're performing a
    >>>> "special" lockless read?
    >>>>
    >>> Using READ_ONCE() does have a bit of cost as it limits compiler
    >>> optimization. If we changes all access to rwsem->owner to READ_ONCE()
    >>> and WRITE_ONCE(), we may as well change its type to volatile and be done
    >>> with.
    >> Right, although there are still places like the init function where
    >> WRITE_ONCE isn't necessary.
    >>
    >>> I am not against doing that, but it feels a bit over-reach for me.
    >>> On the other hand, we may define a do-nothing macro that designates the
    >>> owner as a special variable for documentation purpose, but don't need
    >>> protection at that particular call site.
    >> It should be fine to use the standard READ_ONCE here, even if it's just
    >> for documentation, as it's probably not going to cost anything in
    >> practice. It would be better to avoid adding any special macros for this
    >> which may just add more complexity.
    > By the way, this potential "partial write" issue may also apply to
    > mutexes as well, so we should also make a similar change to
    > mutex_set_owner() and mutex_clear_owner().
    >
    > Jason
    >
    Yes, I am aware of that. I just don't have the time to to do a mutex
    patch yet. As you have sent out a patch on that, this is now covered.

    Cheers,
    Longman

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