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Date: Wed, 27 Feb 2013 20:25:51 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Michel Lespinasse <walken@google.com>
Subject: Re: [PATCH v6 04/46] percpu_rwlock: Implement the core design of
 Per-CPU Reader-Writer Locks
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On 02/27, Michel Lespinasse wrote:
>
> +void lg_rwlock_local_read_lock(struct lgrwlock *lgrw)
> +{
> +       preempt_disable();
> +
> +       if (__this_cpu_read(*lgrw->local_refcnt) ||
> +           arch_spin_trylock(this_cpu_ptr(lgrw->lglock->lock))) {
> +               __this_cpu_inc(*lgrw->local_refcnt);

Please look at __this_cpu_generic_to_op(). You need this_cpu_inc()
to avoid the race with irs. The same for _read_unlock.

But otherwise I agree, looks like a clever and working idea to me.
And simple!

> There is an interesting case where lg_rwlock_local_read_lock could be
> interrupted after getting the local lglock but before incrementing
> local_refcnt to 1; if that happens any nested readers within that
> interrupt will have to take the global rwlock read side. I think this
> is perfectly acceptable

Agreed.

Or interrupt can do spin_trylock(percpu-lock) after we take the global
->fallback_rwlock (if we race with write_lock + write_unlock), but I do
not see any possible deadlock in this case.

Oleg.