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

[Jason Low]

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