linux-kernel - Re: [PATCH v3] lib: cpu_rmap: avoid flushing all workqueues

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Date:	Wed, 2 Jan 2013 20:21:37 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	David Decotigny <decot@...glers.com>
CC:	<linux-kernel@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Or Gerlitz <ogerlitz@...lanox.com>,
	Amir Vadai <amirv@...lanox.com>,
	"Paul E. McKenney" <paul.mckenney@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Josh Triplett <josh@...htriplett.org>,
	David Howells <dhowells@...hat.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: Re: [PATCH v3] lib: cpu_rmap: avoid flushing all workqueues

On Sat, 2012-12-29 at 11:57 -0800, David Decotigny wrote:
> In some cases, free_irq_cpu_rmap() is called while holding a lock
> (eg. rtnl).

I made fairly sure that it didn't get called while holding the RTNL
lock.  However it looks like some mlx4_en ethtool ops now call it
(indirectly).

> This can lead to deadlocks, because it invokes
> flush_scheduled_work() which ends up waiting for whole system
> workqueue to flush, but some pending works might try to acquire the
> lock we are already holding.

Which is why the kernel-doc says not to do that.  But realistically a
device driver may not easily be able to avoid holding either its
underlying device's lock or a susbsytem global lock... and presumably
there may sometimes be a work item that needs the device lock.

> This commit uses reference-counting to replace
> irq_run_affinity_notifiers(). It also removes
> irq_run_affinity_notifiers() altogether.
> 
> Signed-off-by: David Decotigny <decot@...glers.com>
[...]
> --- a/lib/cpu_rmap.c
> +++ b/lib/cpu_rmap.c
[...]
>  /**
>   * irq_cpu_rmap_add - add an IRQ to a CPU affinity reverse-map
> - * @rmap: The reverse-map
> + * @rmap: The per-IRQ reverse-map
[...]

Please drop this comment change; the only 'per-IRQ' thing is the IRQ
notifier structure which is private.

With that, you can add:

Reviewed-by: Ben Hutchings <bhutchings@...arflare.com>

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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