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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_!
http://www.ozlabs.org/~dgibson
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