Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for read_count

Will Deacon
2020-09-15T22:32:26Z

On Tue, Sep 15, 2020 at 06:03:44PM +0200, peterz@infradead.org wrote:
> On Tue, Sep 15, 2020 at 05:51:50PM +0200, peterz@infradead.org wrote:
>
> > Anyway, I'll rewrite the Changelog and stuff it in locking/urgent.
>
> How's this?
>
> ---
> Subject: locking/percpu-rwsem: Use this_cpu_{inc,dec}() for read_count
> From: Hou Tao <houtao1@huawei.com>
> Date: Tue, 15 Sep 2020 22:07:50 +0800
>
> From: Hou Tao <houtao1@huawei.com>
>
> The __this_cpu*() accessors are (in general) IRQ-unsafe which, given
> that percpu-rwsem is a blocking primitive, should be just fine.
>
> However, file_end_write() is used from IRQ context and will cause
> load-store issues.

... on architectures where the per-cpu accessors are not atomic.

>
> Fixing it by using the IRQ-safe this_cpu_*() for operations on

Fixing => Fix ?

> read_count. This will generate more expensive code on a number of
> platforms, which might cause a performance regression for some of the
> other percpu-rwsem users.
>
> If any such is reported, we can consider alternative solutions.
>
> Fixes: 70fe2f48152e ("aio: fix freeze protection of aio writes")
> Signed-off-by: Hou Tao <houtao1@huawei.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Link: https://lkml.kernel.org/r/20200915140750.137881-1-houtao1@huawei.com
> ---
> include/linux/percpu-rwsem.h | 8 ++++----
> kernel/locking/percpu-rwsem.c | 4 ++--
> 2 files changed, 6 insertions(+), 6 deletions(-)

For the patch:

Acked-by: Will Deacon <will@kernel.org>

Will