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Re: [Qemu-devel] [PATCH] spapr_rng: fix race with main loop
From: |
Cédric Le Goater |
Subject: |
Re: [Qemu-devel] [PATCH] spapr_rng: fix race with main loop |
Date: |
Mon, 14 Mar 2016 11:03:53 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Icedove/38.6.0 |
On 03/11/2016 07:48 PM, 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>
Reviewed-by: Cédric Le Goater <address@hidden>
Thanks for finding this race,
C.
> ---
>
> Thomas,
>
> This is the problem mentioned by Cedric in:
>
> https://lists.nongnu.org/archive/html/qemu-devel/2016-03/msg02526.html
>
> Cheers.
>
> --
> Greg
>
> hw/ppc/spapr_rng.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/hw/ppc/spapr_rng.c b/hw/ppc/spapr_rng.c
> index a39d472b66fd..02d6be49f58e 100644
> --- a/hw/ppc/spapr_rng.c
> +++ b/hw/ppc/spapr_rng.c
> @@ -77,13 +77,13 @@ static target_ulong h_random(PowerPCCPU *cpu,
> sPAPRMachineState *spapr,
> hrdata.val.v64 = 0;
> hrdata.received = 0;
>
> - qemu_mutex_unlock_iothread();
> while (hrdata.received < 8) {
> rng_backend_request_entropy(rngstate->backend, 8 - hrdata.received,
> random_recv, &hrdata);
> + qemu_mutex_unlock_iothread();
> qemu_sem_wait(&hrdata.sem);
> + qemu_mutex_lock_iothread();
> }
> - qemu_mutex_lock_iothread();
>
> qemu_sem_destroy(&hrdata.sem);
> args[0] = hrdata.val.v64;
>