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

Will Deacon
2020-09-16T08:21:12Z

On Tue, Sep 15, 2020 at 08:11:12PM +0200, peterz@infradead.org wrote:
> On Tue, Sep 15, 2020 at 05:11:23PM +0100, Will Deacon wrote:
> > 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.
>
> That's not entirely accurate, on x86 for example the per-cpu ops are not
> atomic, but they are not susceptible to this problem due to them being a
> single instruction from the point of interrupts -- either they wholly
> happen or they don't.

Hey, the implication is still correct though ;)

> So I'd reformulate it like: "... on architectures where the per-cpu
> accessors are not natively irq-safe" ?

But yeah, that's better. Thanks.

Will