From: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Peter Zijlstra <peterz@infradead.org>,
Russell King <linux@armlinux.org.uk>,
Ingo Molnar <mingo@redhat.com>, Waiman Long <longman@redhat.com>,
Will Deacon <will@kernel.org>,
Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [RFC PATCH 0/3] Queued spinlocks/RW-locks for ARM
Date: Wed, 9 Oct 2019 00:02:38 +0200 [thread overview]
Message-ID: <20191008220238.neuy5iwrgbkum6m3@flow> (raw)
In-Reply-To: <CAK8P3a182o64NfheNEYixDsi=mSZCNVSgg=_EDnwy+fZ1hrzLw@mail.gmail.com>
On 2019-10-08 23:47:31 [+0200], Arnd Bergmann wrote:
…
> diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h
> index fde943d180e0..3c456ad1661b 100644
> --- a/include/asm-generic/qspinlock.h
> +++ b/include/asm-generic/qspinlock.h
…
> @@ -75,6 +79,11 @@ static __always_inline void queued_spin_lock(struct
> qspinlock *lock)
> {
> u32 val = 0;
>
> + if (!smp_enabled()) {
> + atomic_set(&lock->val, _Q_LOCKED_VAL);
> + return;
> + }
> +
> if (likely(atomic_try_cmpxchg_acquire(&lock->val, &val, _Q_LOCKED_VAL)))
> return;
>
> The above is likely incorrect, non-idiomatic or inefficient, but this
> is a way to
> avoid both a runtime check and the cmpxchg() in each spinlock.
You would have to put this in arch_spin_trylock() but I get the idea.
The current implementation does cmpxchg() in the try-lock case so by
switching to q-locks are not getting worse in the UP case. Therefore I
this is more of an optimisation for those that run SMP kernels on UP
machines.
> Arnd
Sebastian
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next prev parent reply other threads:[~2019-10-08 22:02 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-07 21:44 [RFC PATCH 0/3] Queued spinlocks/RW-locks for ARM Sebastian Andrzej Siewior
2019-10-07 21:44 ` [PATCH 1/3] ARM: Use qrwlock implementation Sebastian Andrzej Siewior
2019-10-07 21:44 ` [PATCH 2/3] ARM: Use qspinlock implementation Sebastian Andrzej Siewior
2019-10-07 21:44 ` [PATCH 3/3] ARM: Inline locking functions for !PREEMPTION Sebastian Andrzej Siewior
2019-10-08 11:42 ` [RFC PATCH 0/3] Queued spinlocks/RW-locks for ARM Arnd Bergmann
2019-10-08 13:36 ` Waiman Long
2019-10-08 14:32 ` Arnd Bergmann
2019-10-08 19:47 ` Sebastian Andrzej Siewior
2019-10-08 21:47 ` Arnd Bergmann
2019-10-08 22:02 ` Sebastian Andrzej Siewior [this message]
2019-10-09 8:15 ` Arnd Bergmann
2019-10-09 8:46 ` Peter Zijlstra
2019-10-09 8:57 ` Arnd Bergmann
2019-10-09 9:31 ` Peter Zijlstra
2019-10-09 10:31 ` Arnd Bergmann
2019-10-09 10:56 ` Peter Zijlstra
2019-10-09 12:00 ` Arnd Bergmann
2019-10-09 12:06 ` Peter Zijlstra
2019-10-09 12:52 ` Will Deacon
2019-10-09 13:50 ` Arnd Bergmann
2019-10-09 21:42 ` Sebastian Andrzej Siewior
2019-10-08 19:32 ` Sebastian Andrzej Siewior
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