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

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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

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[PATCH v4 2/5] locking/rwsem: Protect all writes to owner by WRITE_ONCE() Waiman Long <Waiman.Long@hpe.com> - 2016-05-18 03:30 +0200
  Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by  WRITE_ONCE() Davidlohr Bueso <dave@stgolabs.net> - 2016-05-18 16:10 +0200
    Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by  WRITE_ONCE() Jason Low <jason.low2@hpe.com> - 2016-05-18 19:30 +0200
    Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by  WRITE_ONCE Jason Low <jason.low2@hpe.com> - 2016-05-18 19:30 +0200
      Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by  WRITE_ONCE Jason Low <jason.low2@hpe.com> - 2016-05-18 22:00 +0200
        Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by  WRITE_ONCE Jason Low <jason.low2@hpe.com> - 2016-05-20 00:40 +0200
        Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by  WRITE_ONCE Peter Hurley <peter@hurleysoftware.com> - 2016-05-21 18:10 +0200
          Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by  WRITE_ONCE Peter Zijlstra <peterz@infradead.org> - 2016-05-22 12:50 +0200
          Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by  WRITE_ONCE Jason Low <jason.low2@hpe.com> - 2016-05-23 20:50 +0200
            Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by  WRITE_ONCE Davidlohr Bueso <dave@stgolabs.net> - 2016-05-23 21:50 +0200
              Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by  WRITE_ONCE "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-05-23 22:20 +0200
                Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by  WRITE_ONCE Davidlohr Bueso <dave@stgolabs.net> - 2016-05-23 23:10 +0200
            Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by  WRITE_ONCE Waiman Long <waiman.long@hpe.com> - 2016-05-25 03:30 +0200

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