From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754115AbaIOQEo (ORCPT ); Mon, 15 Sep 2014 12:04:44 -0400 Received: from mx1.redhat.com ([209.132.183.28]:56935 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754051AbaIOQD5 (ORCPT ); Mon, 15 Sep 2014 12:03:57 -0400 From: Amos Kong To: virtualization@lists.linux-foundation.org Cc: kvm@vger.kernel.org, herbert@gondor.apana.org.au, m@bues.ch, mb@bu3sch.de, mpm@selenic.com, rusty@rustcorp.com.au, amit.shah@redhat.com, linux-kernel@vger.kernel.org Subject: [PATCH v2 2/3] hw_random: fix stuck in catting hwrng attributes Date: Tue, 16 Sep 2014 00:02:28 +0800 Message-Id: <1410796949-2221-3-git-send-email-akong@redhat.com> In-Reply-To: <1410796949-2221-1-git-send-email-akong@redhat.com> References: <1410796949-2221-1-git-send-email-akong@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org I started a QEMU (non-smp) guest with one virtio-rng device, and read random data from /dev/hwrng by dd: # dd if=/dev/hwrng of=/dev/null & In the same time, if I check hwrng attributes from sysfs by cat: # cat /sys/class/misc/hw_random/rng_* The cat process always gets stuck with slow backend (5 k/s), if we use a quick backend (1.2 M/s), the cat process will cost 1 to 2 minutes. The stuck doesn't exist for smp guest. Reading syscall enters kernel and call rng_dev_read(), it's user context. We used need_resched() to check if other tasks need to be run, but it almost always return false, and re-hold the mutex lock. The attributes accessing process always fails to hold the lock, so the cat gets stuck. User context doesn't allow other user contexts run on that CPU, unless the kernel code sleeps for some reason. This is why the need_reshed() always return false here. This patch removed need_resched() and always schedule other tasks then other tasks can have chance to hold the lock and execute protected code. Signed-off-by: Amos Kong --- drivers/char/hw_random/core.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c index c591d7e..263a370 100644 --- a/drivers/char/hw_random/core.c +++ b/drivers/char/hw_random/core.c @@ -195,8 +195,7 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf, mutex_unlock(&rng_mutex); - if (need_resched()) - schedule_timeout_interruptible(1); + schedule_timeout_interruptible(1); if (signal_pending(current)) { err = -ERESTARTSYS; -- 1.9.3