Re: Broken ARM (and powerpc ?) futex wrt memory barriers

[Mathieu Desnoyers]

* Mathieu Desnoyers (mathieu.desnoyers@xxxxxxxxxx) wrote:
> * Russell King - ARM Linux (linux@xxxxxxxxxxxxxxxx) wrote:
> > Hmm, the mutex is undocumented in the atomic ops document.  Does it
> > require ordering both before and after, or do some of those ops just
> > need it before acquire and after release?
> > 
> 
> I guess the mutex fast path should probably be added to atomic_ops.txt.
> If I look at PowerPC mutex.h, mutex lock provides acquire semantic (like
> spinlock) and mutex unlock provides release semantic (like spin unlock).
> 
> acquire :
> 
> take lock
> smp_mb()
> (critical section memory accesses)
> 
> release :
> 
> (critical section memory accesses)
> smp_mb()
> release lock

* ARM

I think we also have to deal with futexes. See
arch/arm/include/asm/futex.h :

1 - 

#ifdef CONFIG_SMP

#include <asm-generic/futex.h>

#else /* !SMP, we can work around lack of atomic ops by disabling
preemption */

(arm-specific code here, seems to deal with futexes)

#endif

-> is it just me or this ifdef condition is the exact opposite of what
it should be ? I thought those generic futexes were for UP-only
systems...

Given futexes are used as key element of userspace mutex slow path
implementation, I think we should consider adding memory barriers there
too.


* PowerPC

Powerpc futex.h seems to have a LWSYNC_ON_SMP/ISYNC_ON_SMP before/after
the futex atomic operation, which act as memory barriers.

Interestingly enough, powerpc futex.h:futex_atomic_cmpxchg_inatomic()
has both LWSYNC_ON_SMP (before atomic op) and ISYNC_ON_SMP (after); this
is typical for all powerpc atomic ops. However, __futex_atomic_op() only
has the LWSYNC_ON_SMP. Is there a reason for not having a ISYNC_ON_SMP
there ?

Mathieu

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Mathieu Desnoyers
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