linux-kernel - Re: [PATCH v3] CPU hotplug: active_writer not woken up in some cases

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Message-ID: <20141210002619.GB516@redhat.com>
Date:	Wed, 10 Dec 2014 01:26:19 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	David Hildenbrand <dahi@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, heiko.carstens@...ibm.com,
	borntraeger@...ibm.com, rafael.j.wysocki@...el.com,
	peterz@...radead.org, bp@...e.de, jkosina@...e.cz
Subject: Re: [PATCH v3] CPU hotplug: active_writer not woken up in some
	cases - deadlock

On 12/09, Paul E. McKenney wrote:
>
> Would wait_event()/wake_up() work for the wakeup-writer case?

Yes, and in this case we could probably kill this puts_pending logic
and avoid cpu_hotplug.lock in put_online_cpus() altogether? Can't we
just make cpu_hotplug.refcount atomic_t?

Anyway, this makes me think again that this code should use percpu_rwsem.
Perhaps I'll try to make a patch next week...

(we need down_write_recursive_readers(), and probably rcusync patches).

Oleg.

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