From: Peter Zijlstra <peterz@infradead.org>
To: Guo Ren <guoren@kernel.org>
Cc: "Christoph Müllner" <christophm30@gmail.com>,
"Palmer Dabbelt" <palmer@dabbelt.com>,
"Anup Patel" <anup@brainfault.org>,
linux-riscv <linux-riscv@lists.infradead.org>,
"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
"Guo Ren" <guoren@linux.alibaba.com>,
"Catalin Marinas" <catalin.marinas@arm.com>,
"Will Deacon" <will.deacon@arm.com>,
"Arnd Bergmann" <arnd@arndb.de>,
"Jonas Bonn" <jonas@southpole.se>,
"Stefan Kristiansson" <stefan.kristiansson@saunalahti.fi>,
"Stafford Horne" <shorne@gmail.com>
Subject: Re: [RFC][PATCH] locking: Generic ticket-lock
Date: Wed, 14 Apr 2021 14:55:57 +0200 [thread overview]
Message-ID: <YHbmXXvuG442ZDfN@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CAJF2gTRsQQ=RunxK6R9MfK70dULt=RJOXXGCOT9oDPEsBgvKtQ@mail.gmail.com>
On Wed, Apr 14, 2021 at 08:39:33PM +0800, Guo Ren wrote:
> I've tested it on csky SMP*4 hw (860) & riscv SMP*4 hw (c910) and it's okay.
W00t :-)
> Hope you can keep
> typedef struct {
> union {
> atomic_t lock;
> struct __raw_tickets {
> #ifdef __BIG_ENDIAN
> u16 next;
> u16 owner;
> #else
> u16 owner;
> u16 next;
> #endif
> } tickets;
> };
> } arch_spinlock_t;
>
> Using owner & next is much more readable.
That almost doubles the line-count of the thing ;-)
> > + * It further assumes atomic_*_release() + atomic_*_acquire() is RCpc and hence
> > + * uses atomic_fetch_add() which is SC to create an RCsc lock.
This ^^^ then vvv
> > +static __always_inline void ticket_lock(arch_spinlock_t *lock)
> > +{
> > + u32 val = atomic_fetch_add(1<<16, lock); /* SC, gives us RCsc */
> atomic_fetch_add_acquire ?
Then we must rely on the arch to implement RCsc atomics. And I for one
can never tell wth Risc-V actually does.
> > +static __always_inline int ticket_is_locked(arch_spinlock_t *lock)
> > +{
> > + u32 val = atomic_read(lock);
> > +
> > + return ((val >> 16) != (val & 0xffff));
> I perfer:
> return !arch_spin_value_unlocked(READ_ONCE(*lock));
> > +}
> > +}
> > +
> > +static __always_inline int ticket_value_unlocked(arch_spinlock_t lock)
> > +{
> > + return !ticket_is_locked(&lock);
> Are you sure to let ticket_is_locked->atomic_read(lock) again, the
> lock has contained all information?
>
> return lock.tickets.owner == lock.tickets.next;
Yeah, I wrote then the wrong way around. Couldn't be bothered to go back
when I figured it out.
> > +
> > +static __always_inline int ticket_is_contended(arch_spinlock_t *lock)
> > +{
> > + u32 val = atomic_read(lock);
> > +
> > + return (s16)((val >> 16) - (val & 0xffff)) > 1;
> How big-endian ?
How not? Endian-ness only matters when you go poke at sub-words, which
the above does not. Only ticket_unlock() does and cares about that.
next prev parent reply other threads:[~2021-04-14 12:57 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-24 10:14 [PATCH] riscv: locks: introduce ticket-based spinlock implementation guoren
2021-03-24 11:09 ` Peter Zijlstra
2021-03-24 12:10 ` Guo Ren
[not found] ` <CAM4kBBK7_s9U2vJbq68yC8WdDEfPQTaCOvn1xds3Si5B-Wpw+A@mail.gmail.com>
2021-03-24 12:23 ` Peter Zijlstra
2021-03-24 12:24 ` Guo Ren
2021-03-24 12:31 ` Peter Zijlstra
2021-03-24 12:28 ` Anup Patel
2021-03-24 12:37 ` Peter Zijlstra
2021-03-24 12:53 ` Anup Patel
2021-04-11 21:11 ` Palmer Dabbelt
2021-04-12 13:32 ` Christoph Müllner
2021-04-12 14:51 ` Peter Zijlstra
2021-04-12 21:21 ` Christoph Müllner
2021-04-12 17:33 ` Palmer Dabbelt
2021-04-12 21:54 ` Christoph Müllner
2021-04-13 8:03 ` Peter Zijlstra
2021-04-13 8:17 ` Peter Zijlstra
2021-04-14 2:26 ` Guo Ren
2021-04-14 7:08 ` Peter Zijlstra
2021-04-14 9:05 ` Peter Zijlstra
2021-04-14 10:16 ` [RFC][PATCH] locking: Generic ticket-lock Peter Zijlstra
2021-04-14 12:39 ` Guo Ren
2021-04-14 12:55 ` Peter Zijlstra [this message]
2021-04-14 13:08 ` Peter Zijlstra
2021-04-14 15:59 ` David Laight
2021-04-14 12:45 ` Peter Zijlstra
2021-04-14 21:02 ` Stafford Horne
2021-04-14 20:47 ` Stafford Horne
2021-04-15 8:09 ` Peter Zijlstra
2021-04-15 9:02 ` Catalin Marinas
2021-04-15 9:22 ` Will Deacon
2021-04-15 9:24 ` Peter Zijlstra
2021-04-19 17:35 ` Will Deacon
2021-04-23 6:44 ` Palmer Dabbelt
2021-04-13 9:22 ` [PATCH] riscv: locks: introduce ticket-based spinlock implementation Christoph Müllner
2021-04-13 9:30 ` Catalin Marinas
2021-04-13 9:55 ` Christoph Müllner
2021-04-14 0:23 ` Guo Ren
2021-04-14 9:17 ` Catalin Marinas
2021-04-13 9:35 ` Peter Zijlstra
2021-04-13 10:25 ` Christoph Müllner
2021-04-13 10:45 ` Catalin Marinas
2021-04-13 10:54 ` David Laight
2021-04-14 5:54 ` Guo Ren
2021-04-13 11:04 ` Christoph Müllner
2021-04-13 13:19 ` Guo Ren
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