Re: [Qemu-devel] [PATCH] spapr_rng: fix race with main loop

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Mar 11, 2016 at 07:48:47PM +0100, Greg Kurz wrote:
> Since commit "60253ed1e6ec rng: add request queue support to rng-random",
> the use of a spapr_rng device may hang vCPU threads.
> 
> The following path is taken without holding the lock to the main loop mutex:
> 
> h_random()
>   rng_backend_request_entropy()
>     rng_random_request_entropy()
>       qemu_set_fd_handler()
> 
> The consequence is that entropy_available() may be called before the vCPU
> thread could even queue the request: depending on the scheduling, it may
> happen that entropy_available() does not call random_recv()->qemu_sem_post().
> The vCPU thread will then sleep forever in h_random()->qemu_sem_wait().
> 
> This could not happen before 60253ed1e6ec because entropy_available() used
> to call random_recv() unconditionally.
> 
> This patch ensures the lock is held to avoid the race.
> 
> Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>

Applied to ppc-for-2.6

> ---
> 
> Thomas,
> 
> This is the problem mentioned by Cedric in:
> 
> https://lists.nongnu.org/archive/html/qemu-devel/2016-03/msg02526.html
> 
> Cheers.
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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