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 - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
WARNING: multiple messages have this Message-ID (diff)
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 - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales) _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv
next prev parent reply other threads:[~2022-06-29 8:27 UTC|newest] Thread overview: 48+ 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 ` guoren 2022-06-28 8:17 ` [PATCH V7 1/5] asm-generic: ticket-lock: Remove unnecessary atomic_read guoren 2022-06-28 8:17 ` guoren 2022-06-28 18:05 ` Waiman Long 2022-06-28 18:05 ` Waiman Long 2022-06-29 2:12 ` Guo Ren 2022-06-29 2:12 ` Guo Ren 2022-06-29 8:27 ` David Laight [this message] 2022-06-29 8:27 ` David Laight 2022-07-01 15:18 ` Guo Ren 2022-07-01 15:18 ` Guo Ren 2022-07-04 9:52 ` Peter Zijlstra 2022-07-04 9:52 ` Peter Zijlstra 2022-07-04 11:10 ` Guo Ren 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 ` 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 ` guoren 2022-06-28 8:17 ` [PATCH V7 4/5] asm-generic: spinlock: Add combo spinlock (ticket & queued) guoren 2022-06-28 8:17 ` guoren 2022-06-28 18:13 ` Waiman Long 2022-06-28 18:13 ` Waiman Long 2022-06-29 1:17 ` Guo Ren 2022-06-29 1:17 ` Guo Ren 2022-06-29 1:34 ` Waiman Long 2022-06-29 1:34 ` Waiman Long 2022-06-29 2:29 ` Guo Ren 2022-06-29 2:29 ` Guo Ren 2022-06-29 7:08 ` Arnd Bergmann 2022-06-29 7:08 ` Arnd Bergmann 2022-06-29 8:24 ` Guo Ren 2022-06-29 8:24 ` Guo Ren 2022-06-29 8:29 ` Arnd Bergmann 2022-06-29 8:29 ` Arnd Bergmann 2022-07-01 12:18 ` Guo Ren 2022-07-01 12:18 ` Guo Ren 2022-06-29 12:53 ` Waiman Long 2022-06-29 12:53 ` Waiman Long 2022-07-04 9:57 ` Peter Zijlstra 2022-07-04 9:57 ` Peter Zijlstra 2022-07-04 13:13 ` Guo Ren 2022-07-04 13:13 ` Guo Ren 2022-07-04 13:45 ` Peter Zijlstra 2022-07-04 13:45 ` Peter Zijlstra 2022-06-28 8:17 ` [PATCH V7 5/5] riscv: Add qspinlock support guoren 2022-06-28 8:17 ` guoren
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