On Fri, Jul 19, 2013 at 03:08:36PM -0400, Waiman Long wrote:
> On 07/19/2013 02:31 PM, Peter Zijlstra wrote:
> >mutex_can_spin_on_owner() is broken in that it would allow the compiler
> >to load lock->owner twice, seeing a pointer first time and a MULL
> >pointer the second time.
> >
> >Signed-off-by: Peter Zijlstra<pet...@infradead.org>
> >---
> > kernel/mutex.c | 6 ++++--
> > 1 file changed, 4 insertions(+), 2 deletions(-)
> >
> >diff --git a/kernel/mutex.c b/kernel/mutex.c
> >index ff05f4b..7ff48c5 100644
> >--- a/kernel/mutex.c
> >+++ b/kernel/mutex.c
> >@@ -209,11 +209,13 @@ int mutex_spin_on_owner(struct mutex *lock, struct
> >task_struct *owner)
> > */
> > static inline int mutex_can_spin_on_owner(struct mutex *lock)
> > {
> >+ struct task_struct *owner;
> > int retval = 1;
> >
> > rcu_read_lock();
> >- if (lock->owner)
> >- retval = lock->owner->on_cpu;
> >+ owner = ACCESS_ONCE(lock->owner);
> >+ if (owner)
> >+ retval = owner->on_cpu;
> > rcu_read_unlock();
> > /*
> > * if lock->owner is not set, the mutex owner may have just acquired
>
> I am fine with this change. However, the compiler is smart enough to not do
> two memory accesses to the same memory location. So this will not change the
> generated code. Below is the relevant x86 code for that section of code:
Yes I know, but the compiler would be allowed to do so; not so after the
change.
Also, GCC can be surprisingly stupid at times, depending on the options
given, never rely/trust on anything you don't have to.
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