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Re: [PATCH 3/3] arm64/locking: qspinlocks and qrwlocks support

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  Re: [PATCH 3/3] arm64/locking: qspinlocks and qrwlocks support Peter Zijlstra <peterz@infradead.org> - 2017-04-13 20:20 +0200
    Re: [PATCH 3/3] arm64/locking: qspinlocks and qrwlocks support Mark Rutland <mark.rutland@arm.com> - 2017-04-20 21:10 +0200
    Re: [PATCH 3/3] arm64/locking: qspinlocks and qrwlocks support Peter Zijlstra <peterz@infradead.org> - 2017-04-20 21:20 +0200

#1623241 — Re: [PATCH 3/3] arm64/locking: qspinlocks and qrwlocks support

On Tue, Apr 11, 2017 at 01:35:04AM +0400, Yury Norov wrote:

> +++ b/arch/arm64/include/asm/qspinlock.h
> @@ -0,0 +1,20 @@
> +#ifndef _ASM_ARM64_QSPINLOCK_H
> +#define _ASM_ARM64_QSPINLOCK_H
> +
> +#include <asm-generic/qspinlock_types.h>
> +
> +#define	queued_spin_unlock queued_spin_unlock
> +/**
> + * queued_spin_unlock - release a queued spinlock
> + * @lock : Pointer to queued spinlock structure
> + *
> + * A smp_store_release() on the least-significant byte.
> + */
> +static inline void queued_spin_unlock(struct qspinlock *lock)
> +{
> +	smp_store_release((u8 *)lock, 0);
> +}

I'm afraid this isn't enough for arm64. I suspect you want your own
variant of queued_spin_unlock_wait() and queued_spin_is_locked() as
well.

Much memory ordering fun to be had there.

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#1627754

On Thu, Apr 20, 2017 at 09:23:18PM +0300, Yury Norov wrote:
> On Thu, Apr 13, 2017 at 08:12:12PM +0200, Peter Zijlstra wrote:
> > On Tue, Apr 11, 2017 at 01:35:04AM +0400, Yury Norov wrote:
> > 
> > > +++ b/arch/arm64/include/asm/qspinlock.h
> > > @@ -0,0 +1,20 @@
> > > +#ifndef _ASM_ARM64_QSPINLOCK_H
> > > +#define _ASM_ARM64_QSPINLOCK_H
> > > +
> > > +#include <asm-generic/qspinlock_types.h>
> > > +
> > > +#define	queued_spin_unlock queued_spin_unlock
> > > +/**
> > > + * queued_spin_unlock - release a queued spinlock
> > > + * @lock : Pointer to queued spinlock structure
> > > + *
> > > + * A smp_store_release() on the least-significant byte.
> > > + */
> > > +static inline void queued_spin_unlock(struct qspinlock *lock)
> > > +{
> > > +	smp_store_release((u8 *)lock, 0);
> > > +}
> > 
> > I'm afraid this isn't enough for arm64. I suspect you want your own
> > variant of queued_spin_unlock_wait() and queued_spin_is_locked() as
> > well.
> > 
> > Much memory ordering fun to be had there.
> 
> Hi Peter,
> 
> Is there some test to reproduce the locking failure for the case. I
> ask because I run loctorture for many hours on my qemu (emulating
> cortex-a57), and I see no failures in the test reports.

Even with multi-threaded TCG, a system emulated with QEMU will have far
stronger memory ordering than a real platform. So stress tests on such a
system are useless for testing memory ordering properties.

I would strongly advise that you use a real platform for anything beyond
basic tests when touching code in this area.

> And Jan did it on ThunderX, and Adam on QDF2400 without any problems.
> So even if I rework those functions, how could I check them for
> correctness?

Given the variation the architecture permits, and how difficult it is to
diagnose issues in this area, testing isn't enough here.

You need at least some informal proof as to the primitives doing what
they should, i.e. you should be able to explain why the code is correct.

Thanks,
Mark.

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#1627757

On Thu, Apr 20, 2017 at 09:23:18PM +0300, Yury Norov wrote:
> Is there some test to reproduce the locking failure for the case.

Possibly sysvsem stress before commit:

  27d7be1801a4 ("ipc/sem.c: avoid using spin_unlock_wait()")

Although a similar scheme is also used in nf_conntrack, see commit:

  b316ff783d17 ("locking/spinlock, netfilter: Fix nf_conntrack_lock() barriers")

> I
> ask because I run loctorture for many hours on my qemu (emulating
> cortex-a57), and I see no failures in the test reports. And Jan did it
> on ThunderX, and Adam on QDF2400 without any problems. So even if I
> rework those functions, how could I check them for correctness?

Running them doesn't prove them correct. Memory ordering bugs have been
in the kernel for many years without 'ever' triggering. This is stuff
you have to think about.

> Anyway, regarding the queued_spin_unlock_wait(), is my understanding
> correct that you assume adding smp_mb() before entering the for(;;)
> cycle, and using ldaxr/strxr instead of atomic_read()?

You'll have to ask Will, I always forget the arm64 details.

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