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

[peterz@infradead.org]

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.

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