From: Will Deacon <will@kernel.org>
To: Nicholas Piggin <npiggin@gmail.com>
Cc: Anton Blanchard <anton@ozlabs.org>,
Boqun Feng <boqun.feng@gmail.com>,
kvm-ppc@vger.kernel.org, linux-arch@vger.kernel.org,
linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
Waiman Long <longman@redhat.com>, Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
virtualization@lists.linux-foundation.org
Subject: Re: [PATCH 5/8] powerpc/64s: implement queued spinlocks and rwlocks
Date: Thu, 2 Jul 2020 11:48:46 +0100 [thread overview]
Message-ID: <20200702104845.GB16418@willie-the-truck> (raw)
Message-ID: <20200702104846.sInnYoao-aVLY2cqyoRke7Lw3PgjJNMHyiimvT53r5g@z> (raw)
In-Reply-To: <1593686722.w9psaqk7yp.astroid@bobo.none>
On Thu, Jul 02, 2020 at 08:47:05PM +1000, Nicholas Piggin wrote:
> Excerpts from Will Deacon's message of July 2, 2020 8:35 pm:
> > On Thu, Jul 02, 2020 at 08:25:43PM +1000, Nicholas Piggin wrote:
> >> Excerpts from Will Deacon's message of July 2, 2020 6:02 pm:
> >> > On Thu, Jul 02, 2020 at 05:48:36PM +1000, Nicholas Piggin wrote:
> >> >> diff --git a/arch/powerpc/include/asm/qspinlock.h b/arch/powerpc/include/asm/qspinlock.h
> >> >> new file mode 100644
> >> >> index 000000000000..f84da77b6bb7
> >> >> --- /dev/null
> >> >> +++ b/arch/powerpc/include/asm/qspinlock.h
> >> >> @@ -0,0 +1,20 @@
> >> >> +/* SPDX-License-Identifier: GPL-2.0 */
> >> >> +#ifndef _ASM_POWERPC_QSPINLOCK_H
> >> >> +#define _ASM_POWERPC_QSPINLOCK_H
> >> >> +
> >> >> +#include <asm-generic/qspinlock_types.h>
> >> >> +
> >> >> +#define _Q_PENDING_LOOPS (1 << 9) /* not tuned */
> >> >> +
> >> >> +#define smp_mb__after_spinlock() smp_mb()
> >> >> +
> >> >> +static __always_inline int queued_spin_is_locked(struct qspinlock *lock)
> >> >> +{
> >> >> + smp_mb();
> >> >> + return atomic_read(&lock->val);
> >> >> +}
> >> >
> >> > Why do you need the smp_mb() here?
> >>
> >> A long and sad tale that ends here 51d7d5205d338
> >>
> >> Should probably at least refer to that commit from here, since this one
> >> is not going to git blame back there. I'll add something.
> >
> > Is this still an issue, though?
> >
> > See 38b850a73034 (where we added a similar barrier on arm64) and then
> > c6f5d02b6a0f (where we removed it).
> >
>
> Oh nice, I didn't know that went away. Thanks for the heads up.
>
> I'm going to say I'm too scared to remove it while changing the
> spinlock algorithm, but I'll open an issue and we should look at
> removing it.
Makes sense to me -- it certainly needs a deeper look! In the meantime,
please put some of this in a comment next to the barrier.
Cheers,
Will
next prev parent reply other threads:[~2020-07-02 10:48 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-02 7:48 [PATCH 0/8] powerpc: queued spinlocks and rwlocks Nicholas Piggin
2020-07-02 7:48 ` [PATCH 1/8] powerpc/powernv: must include hvcall.h to get PAPR defines Nicholas Piggin
2020-07-02 7:48 ` Nicholas Piggin
2020-07-02 7:48 ` [PATCH 2/8] powerpc/pseries: use smp_rmb() in H_CONFER spin yield Nicholas Piggin
2020-07-02 7:48 ` Nicholas Piggin
2020-07-02 8:28 ` Peter Zijlstra
2020-07-02 8:28 ` Peter Zijlstra
2020-07-02 10:36 ` Nicholas Piggin
2020-07-02 10:36 ` Nicholas Piggin
2020-07-02 7:48 ` [PATCH 3/8] powerpc/pseries: move some PAPR paravirt functions to their own file Nicholas Piggin
2020-07-02 7:48 ` [PATCH 4/8] powerpc: move spinlock implementation to simple_spinlock Nicholas Piggin
2020-07-02 7:48 ` Nicholas Piggin
2020-07-02 7:48 ` [PATCH 5/8] powerpc/64s: implement queued spinlocks and rwlocks Nicholas Piggin
2020-07-02 8:02 ` Will Deacon
2020-07-02 8:02 ` Will Deacon
2020-07-02 10:25 ` Nicholas Piggin
2020-07-02 10:25 ` Nicholas Piggin
2020-07-02 10:35 ` Will Deacon
2020-07-02 10:47 ` Nicholas Piggin
2020-07-02 10:47 ` Nicholas Piggin
2020-07-02 10:48 ` Will Deacon [this message]
2020-07-02 10:48 ` Will Deacon
2020-07-03 10:52 ` Michael Ellerman
2020-07-03 10:52 ` Michael Ellerman
2020-07-02 7:48 ` [PATCH 6/8] powerpc/pseries: implement paravirt qspinlocks for SPLPAR Nicholas Piggin
2020-07-02 21:01 ` Waiman Long
2020-07-02 7:48 ` [PATCH 7/8] powerpc/qspinlock: optimised atomic_try_cmpxchg_lock that adds the lock hint Nicholas Piggin
2020-07-02 7:48 ` Nicholas Piggin
2020-07-02 7:48 ` [PATCH 8/8] powerpc/64s: remove paravirt from simple spinlocks (RFC only) Nicholas Piggin
2020-07-02 7:48 ` Nicholas Piggin
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