Re: [PATCH 2/2] arm64/sve: Eliminate data races on sve_default_vl

On Tue, Jun 16, 2020 at 04:04:51PM +0100, Will Deacon wrote:

On Tue, Jun 16, 2020 at 09:18:08AM -0400, Qian Cai wrote:

quoted

On Wed, Jun 10, 2020 at 06:03:10PM +0100, Dave Martin wrote:

quoted

sve_default_vl can be modified via the /proc/sys/abi/sve_default_vl
sysctl concurrently with use, and modified concurrently by multiple
threads.

Adding a lock for this seems overkill, and I don't want to think any
more than necessary, so just define wrappers using READ_ONCE()/
WRITE_ONCE().

This will avoid the possibility of torn accesses and repeated loads
and stores.

There's no evidence yet that this is going wrong in practice: this
is just hygiene.  For generic sysctl users, it would be better to
build this kind of thing into the sysctl common code somehow.

Reported-by: Will Deacon <will@kernel.org>
Signed-off-by: Dave Martin <Dave.Martin@arm.com>

While this original patch looks correct, linux-next has this,

[will: move set_sve_default_vl() inside #ifdef to squash allnoconfig warning]

1e570f512cbd ("arm64/sve: Eliminate data races on sve_default_vl")

which causes an error with CONFIG_ARM64_SVE=n,

This .config,
https://raw.githubusercontent.com/cailca/linux-mm/master/arm64.config

arch/arm64/kernel/fpsimd.c: In function ‘sve_proc_do_default_vl’:
arch/arm64/kernel/fpsimd.c:375:2: error: implicit declaration of
function ‘set_sve_default_vl’; did you mean ‘get_sve_default_vl’?
[-Werror=implicit-function-declaration]
  set_sve_default_vl(find_supported_vector_length(vl));
  ^~~~~~~~~~~~~~~~~~
  get_sve_default_vl

Thanks, I'll take a look.

I haven't looked in detail at this; I guess the new helpers just
need to be manually placed in the right #ifdef block.

Cheers
---Dave

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel