From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:34260) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ag3es-0001Ac-Fp for qemu-devel@nongnu.org; Wed, 16 Mar 2016 01:06:11 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ag3ep-0005RT-Ln for qemu-devel@nongnu.org; Wed, 16 Mar 2016 01:06:10 -0400 From: David Gibson Date: Wed, 16 Mar 2016 16:07:01 +1100 Message-Id: <1458104828-32541-10-git-send-email-david@gibson.dropbear.id.au> In-Reply-To: <1458104828-32541-1-git-send-email-david@gibson.dropbear.id.au> References: <1458104828-32541-1-git-send-email-david@gibson.dropbear.id.au> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Subject: [Qemu-devel] [PULL 09/16] spapr_rng: fix race with main loop List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: peter.maydell@linaro.org Cc: qemu-devel@nongnu.org, aik@ozlabs.ru, agraf@suse.de, mdroth@linux.vnet.ibm.com, alex.williamson@redhat.com, qemu-ppc@nongnu.org, David Gibson , Greg Kurz From: Greg Kurz 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 mut= ex: 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_pos= t(). The vCPU thread will then sleep forever in h_random()->qemu_sem_wait(). This could not happen before 60253ed1e6ec because entropy_available() use= d to call random_recv() unconditionally. This patch ensures the lock is held to avoid the race. Signed-off-by: Greg Kurz Reviewed-by: C=C3=A9dric Le Goater Reviewed-by: Thomas Huth Signed-off-by: David Gibson --- 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 a39d472..02d6be4 100644 --- a/hw/ppc/spapr_rng.c +++ b/hw/ppc/spapr_rng.c @@ -77,13 +77,13 @@ static target_ulong h_random(PowerPCCPU *cpu, sPAPRMa= chineState *spapr, hrdata.val.v64 =3D 0; hrdata.received =3D 0; =20 - qemu_mutex_unlock_iothread(); while (hrdata.received < 8) { rng_backend_request_entropy(rngstate->backend, 8 - hrdata.receiv= ed, random_recv, &hrdata); + qemu_mutex_unlock_iothread(); qemu_sem_wait(&hrdata.sem); + qemu_mutex_lock_iothread(); } - qemu_mutex_lock_iothread(); =20 qemu_sem_destroy(&hrdata.sem); args[0] =3D hrdata.val.v64; --=20 2.5.0