From: David Laight <David.Laight@ACULAB.COM>
To: "'guoren@kernel.org'" <guoren@kernel.org>,
"palmer@rivosinc.com" <palmer@rivosinc.com>,
"arnd@arndb.de" <arnd@arndb.de>,
"mingo@redhat.com" <mingo@redhat.com>,
"will@kernel.org" <will@kernel.org>,
"longman@redhat.com" <longman@redhat.com>,
"boqun.feng@gmail.com" <boqun.feng@gmail.com>
Cc: "linux-riscv@lists.infradead.org"
<linux-riscv@lists.infradead.org>,
"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Guo Ren <guoren@linux.alibaba.com>,
Peter Zijlstra <peterz@infradead.org>
Subject: RE: [PATCH V7 1/5] asm-generic: ticket-lock: Remove unnecessary atomic_read
Date: Wed, 29 Jun 2022 08:27:19 +0000 [thread overview]
Message-ID: <8eaf85d4e4d9401ea187366de12e7269@AcuMS.aculab.com> (raw)
In-Reply-To: <20220628081707.1997728-2-guoren@kernel.org>
From: guoren@kernel.org
> Sent: 28 June 2022 09:17
>
> From: Guo Ren <guoren@linux.alibaba.com>
>
> Remove unnecessary atomic_read in arch_spin_value_unlocked(lock),
> because the value has been in lock. This patch could prevent
> arch_spin_value_unlocked contend spin_lock data again.
I'm confused (as usual).
Isn't atomic_read() pretty much free?
..
> diff --git a/include/asm-generic/spinlock.h b/include/asm-generic/spinlock.h
> index fdfebcb050f4..f1e4fa100f5a 100644
> --- a/include/asm-generic/spinlock.h
> +++ b/include/asm-generic/spinlock.h
> @@ -84,7 +84,9 @@ static __always_inline int arch_spin_is_contended(arch_spinlock_t *lock)
>
> static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
> {
> - return !arch_spin_is_locked(&lock);
> + u32 val = lock.counter;
> +
> + return ((val >> 16) == (val & 0xffff));
That almost certainly needs a READ_ONCE().
The result is also inherently stale.
So the uses must be pretty limited.
David
> }
>
> #include <asm/qrwlock.h>
> --
> 2.36.1
-
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next prev parent reply other threads:[~2022-06-29 8:27 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-28 8:17 [PATCH V7 0/5] riscv: Add qspinlock support with combo style guoren
2022-06-28 8:17 ` [PATCH V7 1/5] asm-generic: ticket-lock: Remove unnecessary atomic_read guoren
2022-06-28 18:05 ` Waiman Long
2022-06-29 2:12 ` Guo Ren
2022-06-29 8:27 ` David Laight [this message]
2022-07-01 15:18 ` Guo Ren
2022-07-04 9:52 ` Peter Zijlstra
2022-07-04 11:10 ` Guo Ren
2022-06-28 8:17 ` [PATCH V7 2/5] asm-generic: ticket-lock: Use the same struct definitions with qspinlock guoren
2022-06-28 8:17 ` [PATCH V7 3/5] asm-generic: ticket-lock: Move into ticket_spinlock.h guoren
2022-06-28 8:17 ` [PATCH V7 4/5] asm-generic: spinlock: Add combo spinlock (ticket & queued) guoren
2022-06-28 18:13 ` Waiman Long
2022-06-29 1:17 ` Guo Ren
2022-06-29 1:34 ` Waiman Long
2022-06-29 2:29 ` Guo Ren
2022-06-29 7:08 ` Arnd Bergmann
2022-06-29 8:24 ` Guo Ren
2022-06-29 8:29 ` Arnd Bergmann
2022-07-01 12:18 ` Guo Ren
2022-06-29 12:53 ` Waiman Long
2022-07-04 9:57 ` Peter Zijlstra
2022-07-04 13:13 ` Guo Ren
2022-07-04 13:45 ` Peter Zijlstra
2022-06-28 8:17 ` [PATCH V7 5/5] riscv: Add qspinlock support guoren
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