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

[Anton Vorontsov]

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@xxxxxxxxxx>
> 
> What guarantees that there's only one thread writing to struct
> vmevent_attr::value in vmevent_sample() now that the mutex is gone?

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.

Thanks,

-- 
Anton Vorontsov
Email: cbouatmailru@xxxxxxxxx
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