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

From: David Gibson
Subject: Re: [Qemu-devel] [PATCH] spapr_rng: fix race with main loop
Date: Tue, 15 Mar 2016 11:30:39 +1100
User-agent: Mutt/1.5.24 (2015-08-30)

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 <address@hidden>

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_!

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