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Message-ID: <63046420-264a-4e7c-b45c-17f0486ba4d9@infradead.org>
Date: Wed, 10 Jan 2024 10:15:49 -0800
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Subject: Re: [PATCH v3 2/4] membarrier: Create
 Documentation/scheduler/membarrier.rst
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To: Andrea Parri <parri.andrea@gmail.com>, paul.walmsley@sifive.com,
 palmer@dabbelt.com, aou@eecs.berkeley.edu, mathieu.desnoyers@efficios.com,
 paulmck@kernel.org, corbet@lwn.net
Cc: mmaas@google.com, hboehm@google.com, striker@us.ibm.com,
 charlie@rivosinc.com, rehn@rivosinc.com, linux-riscv@lists.infradead.org,
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References: <20240110145533.60234-1-parri.andrea@gmail.com>
 <20240110145533.60234-3-parri.andrea@gmail.com>
From: Randy Dunlap <rdunlap@infradead.org>
In-Reply-To: <20240110145533.60234-3-parri.andrea@gmail.com>
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Hi,

On 1/10/24 06:55, Andrea Parri wrote:
> To gather the architecture requirements of the "private/global
> expedited" membarrier commands.  The file will be expanded to
> integrate further information about the membarrier syscall (as
> needed/desired in the future).  While at it, amend some related
> inline comments in the membarrier codebase.
> 
> Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
> ---
>  Documentation/scheduler/index.rst      |  1 +
>  Documentation/scheduler/membarrier.rst | 37 ++++++++++++++++++++++++++
>  MAINTAINERS                            |  1 +
>  kernel/sched/core.c                    |  7 ++++-
>  kernel/sched/membarrier.c              |  8 +++---
>  5 files changed, 49 insertions(+), 5 deletions(-)
>  create mode 100644 Documentation/scheduler/membarrier.rst
> 
> diff --git a/Documentation/scheduler/index.rst b/Documentation/scheduler/index.rst
> index 3170747226f6d..43bd8a145b7a9 100644
> --- a/Documentation/scheduler/index.rst
> +++ b/Documentation/scheduler/index.rst
> @@ -7,6 +7,7 @@ Scheduler
>  
>  
>      completion
> +    membarrier
>      sched-arch
>      sched-bwc
>      sched-deadline
> diff --git a/Documentation/scheduler/membarrier.rst b/Documentation/scheduler/membarrier.rst
> new file mode 100644
> index 0000000000000..ab7ee3824b407
> --- /dev/null
> +++ b/Documentation/scheduler/membarrier.rst
> @@ -0,0 +1,37 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +========================
> +membarrier() System Call
> +========================
> +
> +MEMBARRIER_CMD_{PRIVATE,GLOBAL}_EXPEDITED - Architecture requirements
> +=====================================================================
> +
> +Memory barriers before updating rq->curr
> +----------------------------------------
> +
> +The command requires each architecture to have a full memory barrier after
> +coming from user-space, before updating rq->curr.  This barrier is implied
> +by the sequence rq_lock(); smp_mb__after_spinlock() in __schedule().  The
> +barrier matches a full barrier in the proximity of the membarrier system
> +call exit, cf. membarrier_{private,global}_expedited().
> +

What does "The command" refer to above and below, please?

Thanks.

> +Memory barriers after updating rq->curr
> +---------------------------------------
> +
> +The command requires each architecture to have a full memory barrier after
> +updating rq->curr, before returning to user-space.  The schemes providing
> +this barrier on the various architectures are as follows.
> +
> + - alpha, arc, arm, hexagon, mips rely on the full barrier implied by
> +   spin_unlock() in finish_lock_switch().
> +
> + - arm64 relies on the full barrier implied by switch_to().
> +
> + - powerpc, riscv, s390, sparc, x86 rely on the full barrier implied by
> +   switch_mm(), if mm is not NULL; they rely on the full barrier implied
> +   by mmdrop(), otherwise.  On powerpc and riscv, switch_mm() relies on
> +   membarrier_arch_switch_mm().
> +
> +The barrier matches a full barrier in the proximity of the membarrier system
> +call entry, cf. membarrier_{private,global}_expedited().


-- 
#Randy