Re: [PATCH] Document Linux's memory barriers

[Nick Piggin <nickpiggin@yahoo.com.au>]

David Howells wrote:

> The attached patch documents the Linux kernel's memory barriers.
>
> Signed-Off-By: David Howells <dhowells@redhat.com>
> ---

Good :)

> +==============================
> +IMPLIED KERNEL MEMORY BARRIERS
> +==============================
> +
> +Some of the other functions in the linux kernel imply memory barriers. For
> +instance all the following (pseudo-)locking functions imply barriers.
> +
> + (*) interrupt disablement and/or interrupts

Is this really the case? I mean interrupt disablement only synchronises with
the local CPU, so it probably should not _have_ to imply barriers (eg. some
architectures are playing around with "virtual" interrupt disablement).

[...]

> +
> +Either interrupt disablement (LOCK) and enablement (UNLOCK) will barrier
> +memory and I/O accesses individually, or interrupt handling will barrier
> +memory and I/O accesses on entry and on exit. This prevents an interrupt
> +routine interfering with accesses made in a disabled-interrupt section of code
> +and vice versa.
> +

But CPUs should always be consistent WRT themselves, so I'm not sure that
it is needed?

Thanks,
Nick

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