From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:43327) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1an5bj-0005V7-Fi for qemu-devel@nongnu.org; Mon, 04 Apr 2016 10:36:05 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1an5bd-0004Bc-DV for qemu-devel@nongnu.org; Mon, 04 Apr 2016 10:35:59 -0400 Received: from mail-lf0-x236.google.com ([2a00:1450:4010:c07::236]:36710) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1an5bd-0004BW-2t for qemu-devel@nongnu.org; Mon, 04 Apr 2016 10:35:53 -0400 Received: by mail-lf0-x236.google.com with SMTP id g184so99290639lfb.3 for ; Mon, 04 Apr 2016 07:35:52 -0700 (PDT) From: =?UTF-8?q?Alex=20Benn=C3=A9e?= Date: Mon, 4 Apr 2016 15:35:48 +0100 Message-Id: <1459780549-12942-2-git-send-email-alex.bennee@linaro.org> In-Reply-To: <1459780549-12942-1-git-send-email-alex.bennee@linaro.org> References: <1459780549-12942-1-git-send-email-alex.bennee@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Subject: [Qemu-devel] [PATCH v2 1/2] cpus: don't use atomic_read for vm_clock_warp_start List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: peter.maydell@linaro.org Cc: Peter Crosthwaite , qemu-devel@nongnu.org, sbruno@freebsd.org, pbonzini@redhat.com, =?UTF-8?q?Alex=20Benn=C3=A9e?= , Richard Henderson As vm_clock_warp_start is a 64 bit value this causes problems for the compiler trying to come up with a suitable atomic operation on 32 bit hosts. Because the variable is protected by vm_clock_seqlock, we check its value inside a seqlock critical section. Signed-off-by: Alex Bennée --- v2 - use seqlock read instead wrapping inside the write sequence lock --- cpus.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/cpus.c b/cpus.c index 8ae4777..cbeb1f6 100644 --- a/cpus.c +++ b/cpus.c @@ -338,10 +338,18 @@ static int64_t qemu_icount_round(int64_t count) static void icount_warp_rt(void) { + unsigned seq; + int64_t warp_start; + /* The icount_warp_timer is rescheduled soon after vm_clock_warp_start * changes from -1 to another value, so the race here is okay. */ - if (atomic_read(&vm_clock_warp_start) == -1) { + do { + seq = seqlock_read_begin(&timers_state.vm_clock_seqlock); + warp_start = vm_clock_warp_start; + } while (seqlock_read_retry(&timers_state.vm_clock_seqlock, seq)); + + if (warp_start == -1) { return; } -- 2.7.4