Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by WRITE_ONCE()

[Davidlohr Bueso]

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.
>
> Signed-off-by: Waiman Long <Waiman.Long@xxxxxxx>

Yes, ->owner can obviously be handled locklessly during optimistic
spinning.

Acked-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>