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Message-ID: <53aac2ac-46ae-46b8-9fdf-34527b79a63b@efficios.com>
Date: Wed, 29 Nov 2023 15:00:53 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Andrea Parri <parri.andrea@...il.com>
Cc: paulmck@...nel.org, palmer@...belt.com, paul.walmsley@...ive.com,
aou@...s.berkeley.edu, mmaas@...gle.com, hboehm@...gle.com,
striker@...ibm.com, charlie@...osinc.com, rehn@...osinc.com,
linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] membarrier: riscv: Provide core serializing command
On 2023-11-29 13:29, Andrea Parri wrote:
>>> diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
>>> index 217fd4de61342..f63222513076d 100644
>>> --- a/arch/riscv/mm/context.c
>>> +++ b/arch/riscv/mm/context.c
>>> @@ -323,6 +323,23 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next,
>>> if (unlikely(prev == next))
>>> return;
>>> +#if defined(CONFIG_MEMBARRIER) && defined(CONFIG_SMP)
>>> + /*
>>> + * The membarrier system call requires a full memory barrier
>>> + * after storing to rq->curr, before going back to user-space.
>>> + *
>>> + * Only need the full barrier when switching between processes:
>>> + * barrier when switching from kernel to userspace is not
>>> + * required here, given that it is implied by mmdrop(); barrier
>>> + * when switching from userspace to kernel is not needed after
>>> + * store to rq->curr.
>>> + */
>>> + if (unlikely(atomic_read(&next->membarrier_state) &
>>> + (MEMBARRIER_STATE_PRIVATE_EXPEDITED |
>>> + MEMBARRIER_STATE_GLOBAL_EXPEDITED)) && prev)
>>> + smp_mb();
>>> +#endif
>>
>> The approach looks good. Please implement it within a separate
>> membarrier_arch_switch_mm() as done on powerpc.
>
> Will do. Thanks.
>
> As regards the Fixes: tag, I guess it boils down to what we want or we
> need to take for commit "riscv: Support membarrier private cmd". :-)
I'm not seeing this commit in the Linux master branch, am I missing
something ?
> FWIW, a quick git-log search confirmed that MEMBARRIER has been around
> for quite some time in the RISC-V world (though I'm not familiar with
> any of its mainstream uses): commit 1464d00b27b2 says (at least) since
> 93917ad50972 ("RISC-V: Add support for restartable sequence"). I am
> currently inclined to pick the latter commit (and check it w/ Palmer),
> but other suggestions are welcome.
Supporting membarrier private expedited is not optional since Linux 4.14:
see kernel/sched/core.c:
membarrier_switch_mm(rq, prev->active_mm, next->mm);
/*
* sys_membarrier() requires an smp_mb() between setting
* rq->curr / membarrier_switch_mm() and returning to userspace.
*
* The below provides this either through switch_mm(), or in
* case 'prev->active_mm == next->mm' through
* finish_task_switch()'s mmdrop().
*/
switch_mm_irqs_off(prev->active_mm, next->mm, next);
Failure to provide the required barrier is a bug in the architecture's
switch_mm implementation when CONFIG_MEMBARRIER=y.
We should probably introduce a new
Documentation/features/sched/membarrier/arch-support.txt
to clarify this requirement.
Userspace code such as liburcu [1] heavily relies on membarrier private
expedited (when available) to speed up RCU read-side critical sections.
Various DNS servers, including BIND 9, use liburcu.
Thanks,
Mathieu
[1] https:/liburcu.org
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
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