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

[peterz]

On Tue, Sep 15, 2020 at 05:31:14PM +0200, Oleg Nesterov wrote:

> > So yeah, fs/super totally abuses percpu_rwsem, and yes, using it from
> > IRQ context is totally out of spec. That said, we've (grudgingly)
> > accomodated them before.
> 
> Yes, I didn't expect percpu_up_ can be called from IRQ :/

Yeah, me neither. That's well out of spec for a blocking primitive in
general.

> > This seems to be a fairly long standing issue, and certainly not unique
> > to ARM64 either (Power, and anyone else using asm-gemeric/percpu.h,
> > should be similarly affected I think). The issue seems to stem from
> > Oleg's original rewrite:
> >
> >   a1fd3e24d8a4 ("percpu_rw_semaphore: reimplement to not block the readers unnecessarily")
> 
> Not really... I think it was 70fe2f48152e ("aio: fix freeze protection of aio writes").

Ah, that came later? Fair enough, I'll change the Fixes line.

> And iiuc io_uring does the same.

Indeed, I just went through a bunch of the file_end_write() callers.

> > and is certainly an understandable mistake.
> >
> > I'm torn on what to do, using this_cpu over __this_cpu is going to
> > adversely affect code-gen (and possibly performance) for all the
> > percpu-rwsem users that are not quite so 'creative'.
> 
> Yes, but what else can we do?

Well, I just talked about it with Will, there's a bunch of things we
could do, but they're all quite ugly.

My leading alternative was adding: percpu_down_read_irqsafe() /
percpu_up_read_irqsafe(), which use local_irq_save() instead of
preempt_disable().

But blergh.. Will also argued that by going with this patch, we'll get
an affected workload when someone reports a performance regression,
which I suppose is a bonus.

Anyway, I'll rewrite the Changelog and stuff it in locking/urgent.