Re: [PATCH 1/3] vmevent: Should not grab mutex in the atomic context

[Pekka Enberg <penberg@kernel.org>]

> On Mon, Apr 09, 2012 at 11:40:31AM +0300, Pekka Enberg wrote:
>> On Mon, 2012-04-09 at 03:38 +0400, Anton Vorontsov wrote:
>> > vmevent grabs a mutex in the atomic context, and so this pops up:
>> >
>> > BUG: sleeping function called from invalid context at kernel/mutex.c:271
>> > in_atomic(): 1, irqs_disabled(): 0, pid: 0, name: swapper/0
> [...]
>> > This patch fixes the issue by removing the mutex and making the logic
>> > lock-free.
>> >
>> > Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
>>
>> What guarantees that there's only one thread writing to struct
>> vmevent_attr::value in vmevent_sample() now that the mutex is gone?

On Mon, Apr 9, 2012 at 3:29 PM, Anton Vorontsov
<anton.vorontsov@linaro.org> wrote:
> Well, it is called from the timer function, which has the same guaranties
> as an interrupt handler: it can have only one execution thread (unlike
> bare softirq handler), so we don't need to worry about racing w/
> ourselves?
>
> If you're concerned about several instances of timers accessing the
> same vmevent_watch, I don't really see how it is possible, as we
> allocate vmevent_watch together w/ the timer instance in vmevent_fd(),
> so there is always one timer per vmevent_watch.

Makes sense. A big fat comment on top of vmevent_sample() explaining
all this would be helpful... ;-)

                        Pekka

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