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

[Waiman Long]

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. 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.

Cheers,
Longman