linux-kernel - Re: [RFC][PATCH 3/3] staging: ion: Avoid using rt

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Date:	Mon, 16 Dec 2013 17:34:56 -0800
From:	Colin Cross <ccross@...roid.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Android Kernel Team <kernel-team@...roid.com>,
	Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [RFC][PATCH 3/3] staging: ion: Avoid using rt_mutexes directly.

On Mon, Dec 16, 2013 at 5:22 PM, John Stultz <john.stultz@...aro.org> wrote:
> On 12/16/2013 04:17 PM, Colin Cross wrote:
>> The lock is only used to protect the heap's free_list and
>> free_list_size members, and is not held for any long or sleeping
>> operations.  Converting to a spinlock should prevent priority
>> inversion without using the rt_mutex.  I'd also rename it to free_lock
>> to so it doesn't get used as a general heap lock.
>
> Hrm.. So at least a trivial conversion to use spinlocks doesn't quite
> work out, as we call ion_buffer_destroy() in ion_heap_freelist_drain()
> while holding the lock, and that calls all sorts of not safe stuff.
>
> I'll spend some more time looking at it later tonight, but let me know
> if you have an approach for this case in mind.

Drop and re-grab the lock around ion_buffer_destroy, it's not
necessary during the destroy, and the list iteration is already using
list_for_each_entry_safe, so it doesn't matter if another caller
modifies the list during the destroy.
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