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Date:   Thu, 17 Mar 2022 21:57:19 +0800
From:   Boqun Feng <boqun.feng@gmail.com>
To:     Palmer Dabbelt <palmer@rivosinc.com>
Cc:     linux-riscv@lists.infradead.org, peterz@infradead.org,
        jonas@southpole.se, stefan.kristiansson@saunalahti.fi,
        shorne@gmail.com, mingo@redhat.com, Will Deacon <will@kernel.org>,
        longman@redhat.com, Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>, aou@eecs.berkeley.edu,
        Arnd Bergmann <arnd@arndb.de>, jszhang@kernel.org,
        wangkefeng.wang@huawei.com, openrisc@lists.librecores.org,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org
Subject: Re: [PATCH 2/5] asm-generic: ticket-lock: New generic ticket-based
 spinlock
Message-ID: <YjM+P32I4fENIqGV@boqun-archlinux>
References: <20220316232600.20419-1-palmer@rivosinc.com>
 <20220316232600.20419-3-palmer@rivosinc.com>
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On Wed, Mar 16, 2022 at 04:25:57PM -0700, Palmer Dabbelt wrote:
> From: Peter Zijlstra <peterz@infradead.org>
> 
> This is a simple, fair spinlock.  Specifically it doesn't have all the
> subtle memory model dependencies that qspinlock has, which makes it more
> suitable for simple systems as it is more likely to be correct.
> 
> [Palmer: commit text]
> Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
> 
> --
> 
> I have specifically not included Peter's SOB on this, as he sent his
> original patch
> <https://lore.kernel.org/lkml/YHbBBuVFNnI4kjj3@hirez.programming.kicks-ass.net/>
> without one.
> ---
>  include/asm-generic/ticket-lock-types.h | 11 ++++
>  include/asm-generic/ticket-lock.h       | 86 +++++++++++++++++++++++++
>  2 files changed, 97 insertions(+)
>  create mode 100644 include/asm-generic/ticket-lock-types.h
>  create mode 100644 include/asm-generic/ticket-lock.h
> 
> diff --git a/include/asm-generic/ticket-lock-types.h b/include/asm-generic/ticket-lock-types.h
> new file mode 100644
> index 000000000000..829759aedda8
> --- /dev/null
> +++ b/include/asm-generic/ticket-lock-types.h
> @@ -0,0 +1,11 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef __ASM_GENERIC_TICKET_LOCK_TYPES_H
> +#define __ASM_GENERIC_TICKET_LOCK_TYPES_H
> +
> +#include <linux/types.h>
> +typedef atomic_t arch_spinlock_t;
> +
> +#define __ARCH_SPIN_LOCK_UNLOCKED	ATOMIC_INIT(0)
> +
> +#endif /* __ASM_GENERIC_TICKET_LOCK_TYPES_H */
> diff --git a/include/asm-generic/ticket-lock.h b/include/asm-generic/ticket-lock.h
> new file mode 100644
> index 000000000000..3f0d53e21a37
> --- /dev/null
> +++ b/include/asm-generic/ticket-lock.h
> @@ -0,0 +1,86 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +/*
> + * 'Generic' ticket-lock implementation.
> + *
> + * It relies on atomic_fetch_add() having well defined forward progress
> + * guarantees under contention. If your architecture cannot provide this, stick
> + * to a test-and-set lock.
> + *
> + * It also relies on atomic_fetch_add() being safe vs smp_store_release() on a
> + * sub-word of the value. This is generally true for anything LL/SC although
> + * you'd be hard pressed to find anything useful in architecture specifications
> + * about this. If your architecture cannot do this you might be better off with
> + * a test-and-set.
> + *
> + * It further assumes atomic_*_release() + atomic_*_acquire() is RCpc and hence
> + * uses atomic_fetch_add() which is SC to create an RCsc lock.
> + *

Probably it's better to use "fully-ordered" instead of "SC", because our
atomic documents never use "SC" or "Sequential Consisteny" to describe
the semantics, further I'm not sure our "fully-ordered" is equivalent to
SC, better not cause misunderstanding in the future here.

> + * The implementation uses smp_cond_load_acquire() to spin, so if the
> + * architecture has WFE like instructions to sleep instead of poll for word
> + * modifications be sure to implement that (see ARM64 for example).
> + *
> + */
> +
> +#ifndef __ASM_GENERIC_TICKET_LOCK_H
> +#define __ASM_GENERIC_TICKET_LOCK_H
> +
> +#include <linux/atomic.h>
> +#include <asm/ticket-lock-types.h>
> +
> +static __always_inline void ticket_lock(arch_spinlock_t *lock)
> +{
> +	u32 val = atomic_fetch_add(1<<16, lock); /* SC, gives us RCsc */
> +	u16 ticket = val >> 16;
> +
> +	if (ticket == (u16)val)
> +		return;
> +
> +	atomic_cond_read_acquire(lock, ticket == (u16)VAL);

If you want to make the lock RCsc, you will also need to make the above
atomic_cond_read_acquire() a RCsc acquire, now it's only RCpc.

Regards,
Boqun

> +}
> +
> +static __always_inline bool ticket_trylock(arch_spinlock_t *lock)
> +{
> +	u32 old = atomic_read(lock);
> +
> +	if ((old >> 16) != (old & 0xffff))
> +		return false;
> +
> +	return atomic_try_cmpxchg(lock, &old, old + (1<<16)); /* SC, for RCsc */
> +}
> +
> +static __always_inline void ticket_unlock(arch_spinlock_t *lock)
> +{
> +	u16 *ptr = (u16 *)lock + __is_defined(__BIG_ENDIAN);
> +	u32 val = atomic_read(lock);
> +
> +	smp_store_release(ptr, (u16)val + 1);
> +}
> +
> +static __always_inline int ticket_is_locked(arch_spinlock_t *lock)
> +{
> +	u32 val = atomic_read(lock);
> +
> +	return ((val >> 16) != (val & 0xffff));
> +}
> +
> +static __always_inline int ticket_is_contended(arch_spinlock_t *lock)
> +{
> +	u32 val = atomic_read(lock);
> +
> +	return (s16)((val >> 16) - (val & 0xffff)) > 1;
> +}
> +
> +static __always_inline int ticket_value_unlocked(arch_spinlock_t lock)
> +{
> +	return !ticket_is_locked(&lock);
> +}
> +
> +#define arch_spin_lock(l)		ticket_lock(l)
> +#define arch_spin_trylock(l)		ticket_trylock(l)
> +#define arch_spin_unlock(l)		ticket_unlock(l)
> +#define arch_spin_is_locked(l)		ticket_is_locked(l)
> +#define arch_spin_is_contended(l)	ticket_is_contended(l)
> +#define arch_spin_value_unlocked(l)	ticket_value_unlocked(l)
> +
> +#endif /* __ASM_GENERIC_TICKET_LOCK_H */
> -- 
> 2.34.1
>