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Date: Thu, 24 Jan 2013 16:58:40 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Michel Lespinasse <walken@google.com>
Subject: Re: [PATCH v5 01/45] percpu_rwlock: Introduce the global
 reader-writer lock backend
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On 01/23, Michel Lespinasse wrote:
>
> On Tue, Jan 22, 2013 at 11:32 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
> >
> > I thought global locks are now fair. That is, a reader will block if a
> > writer is waiting. Hence, the above should deadlock on the current
> > rwlock_t types.
>
> I believe you are mistaken here. struct rw_semaphore is fair (and
> blocking), but rwlock_t is unfair. The reason we can't easily make
> rwlock_t fair is because tasklist_lock currently depends on the
> rwlock_t unfairness - tasklist_lock readers typically don't disable
> local interrupts, and tasklist_lock may be acquired again from within
> an interrupt, which would deadlock if rwlock_t was fair and a writer
> was queued by the time the interrupt is processed.

Yes.

And, iirc, it was even documented somewhere that while rwlock_t is not
really nice, it is good to share the locking with interrupts. You do
not need to disable irqs.

Oleg.