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

[peterz]

On Tue, Sep 15, 2020 at 10:07:50PM +0800, Hou Tao wrote:
> Under aarch64, __this_cpu_inc() is neither IRQ-safe nor atomic, so
> when percpu_up_read() is invoked under IRQ-context (e.g. aio completion),
> and it interrupts the process on the same CPU which is invoking
> percpu_down_read(), the decreasement on read_count may lost and
> the final value of read_count on the CPU will be unexpected
> as shown below:

> Fixing it by using the IRQ-safe helper this_cpu_inc|dec() for
> operations on read_count.
> 
> Another plausible fix is to state that percpu-rwsem can NOT be
> used under IRQ context and convert all users which may
> use it under IRQ context.

*groan*...

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.

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")

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'.