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Message-ID: <ff2cf636-e0cf-422e-9ad3-4cd8fc51e3b2@efficios.com>
Date: Wed, 24 Jan 2024 11:13:16 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Andrea Parri <parri.andrea@...il.com>, paul.walmsley@...ive.com,
palmer@...belt.com, aou@...s.berkeley.edu, paulmck@...nel.org, corbet@....net
Cc: mmaas@...gle.com, hboehm@...gle.com, striker@...ibm.com,
charlie@...osinc.com, rehn@...osinc.com, linux-riscv@...ts.infradead.org,
linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/4] membarrier: Create
Documentation/scheduler/membarrier.rst
On 2024-01-10 09:55, 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.
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
>
> Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Signed-off-by: Andrea Parri <parri.andrea@...il.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().
> +
> +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().
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 0f8cec504b2ba..6bce0aeecb4f2 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -13815,6 +13815,7 @@ M: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> M: "Paul E. McKenney" <paulmck@...nel.org>
> L: linux-kernel@...r.kernel.org
> S: Supported
> +F: Documentation/scheduler/membarrier.rst
> F: arch/*/include/asm/membarrier.h
> F: include/uapi/linux/membarrier.h
> F: kernel/sched/membarrier.c
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 711dc753f7216..b51bc86f8340c 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6599,7 +6599,9 @@ static void __sched notrace __schedule(unsigned int sched_mode)
> * if (signal_pending_state()) if (p->state & @state)
> *
> * Also, the membarrier system call requires a full memory barrier
> - * after coming from user-space, before storing to rq->curr.
> + * after coming from user-space, before storing to rq->curr; this
> + * barrier matches a full barrier in the proximity of the membarrier
> + * system call exit.
> */
> rq_lock(rq, &rf);
> smp_mb__after_spinlock();
> @@ -6677,6 +6679,9 @@ static void __sched notrace __schedule(unsigned int sched_mode)
> * architectures where spin_unlock is a full barrier,
> * - switch_to() for arm64 (weakly-ordered, spin_unlock
> * is a RELEASE barrier),
> + *
> + * The barrier matches a full barrier in the proximity of
> + * the membarrier system call entry.
> */
> ++*switch_count;
>
> diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
> index 2ad881d07752c..f3d91628d6b8a 100644
> --- a/kernel/sched/membarrier.c
> +++ b/kernel/sched/membarrier.c
> @@ -251,7 +251,7 @@ static int membarrier_global_expedited(void)
> return 0;
>
> /*
> - * Matches memory barriers around rq->curr modification in
> + * Matches memory barriers after rq->curr modification in
> * scheduler.
> */
> smp_mb(); /* system call entry is not a mb. */
> @@ -300,7 +300,7 @@ static int membarrier_global_expedited(void)
>
> /*
> * Memory barrier on the caller thread _after_ we finished
> - * waiting for the last IPI. Matches memory barriers around
> + * waiting for the last IPI. Matches memory barriers before
> * rq->curr modification in scheduler.
> */
> smp_mb(); /* exit from system call is not a mb */
> @@ -339,7 +339,7 @@ static int membarrier_private_expedited(int flags, int cpu_id)
> return 0;
>
> /*
> - * Matches memory barriers around rq->curr modification in
> + * Matches memory barriers after rq->curr modification in
> * scheduler.
> */
> smp_mb(); /* system call entry is not a mb. */
> @@ -415,7 +415,7 @@ static int membarrier_private_expedited(int flags, int cpu_id)
>
> /*
> * Memory barrier on the caller thread _after_ we finished
> - * waiting for the last IPI. Matches memory barriers around
> + * waiting for the last IPI. Matches memory barriers before
> * rq->curr modification in scheduler.
> */
> smp_mb(); /* exit from system call is not a mb */
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
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