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@xxxxxxxxxxxx>
> Signed-off-by: Andrea Parri <parri.andrea@xxxxxxxxx>
> ---
> 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
