From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 439A9C00528 for ; Mon, 24 Jul 2023 01:26:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231459AbjGXB0n (ORCPT ); Sun, 23 Jul 2023 21:26:43 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53270 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231207AbjGXB0S (ORCPT ); Sun, 23 Jul 2023 21:26:18 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4DCB43C1B; Sun, 23 Jul 2023 18:24:11 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 258DF60F02; Mon, 24 Jul 2023 01:23:51 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id BB4C7C433C7; Mon, 24 Jul 2023 01:23:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1690161830; bh=qhh8Lf1XCfAF8rtfnerjBiHBNF3y9GF83zIN60JVDwI=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=NggVVS3CnqugiicMebbiSALrFr3emloe+Xc5QD18TMbxpUWQS0fEWHuVlhyw/2MU0 wKE0dk3M6q/kmhRsxLV6FMFSvRymRvr9pQ/Isva1qUvUrhjuc2RZLFzYHG2EGfFzun 6Jd9gCS6oWkFkP/wUsaZG7R9R8pziZKVafSU3pFvQV8cuTSnuDyUGkmoK0a84cVs2O JPmWOUGiNnUQHGAN+bpAUnyXl1XAsbWkqSqfFOoRbJE3Bskt5e87HvF6Ts0L2VaWfr xuyBlZzDfgUjuxCaYzzdzUn7IeP7b63QWAb5NEU8yIzORXKt8wg+g18EY2vQwKNPi7 JihsLlwEKlesQ== From: Sasha Levin To: linux-kernel@vger.kernel.org, stable@vger.kernel.org Cc: Geert Uytterhoeven , Peter Zijlstra , Arnd Bergmann , Tony Lindgren , Ulf Hansson , Sasha Levin Subject: [PATCH AUTOSEL 5.15 06/23] iopoll: Call cpu_relax() in busy loops Date: Sun, 23 Jul 2023 21:23:17 -0400 Message-Id: <20230724012334.2317140-6-sashal@kernel.org> X-Mailer: git-send-email 2.39.2 In-Reply-To: <20230724012334.2317140-1-sashal@kernel.org> References: <20230724012334.2317140-1-sashal@kernel.org> MIME-Version: 1.0 X-stable: review X-Patchwork-Hint: Ignore X-stable-base: Linux 5.15.121 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Geert Uytterhoeven [ Upstream commit b407460ee99033503993ac7437d593451fcdfe44 ] It is considered good practice to call cpu_relax() in busy loops, see Documentation/process/volatile-considered-harmful.rst. This can not only lower CPU power consumption or yield to a hyperthreaded twin processor, but also allows an architecture to mitigate hardware issues (e.g. ARM Erratum 754327 for Cortex-A9 prior to r2p0) in the architecture-specific cpu_relax() implementation. In addition, cpu_relax() is also a compiler barrier. It is not immediately obvious that the @op argument "function" will result in an actual function call (e.g. in case of inlining). Where a function call is a C sequence point, this is lost on inlining. Therefore, with agressive enough optimization it might be possible for the compiler to hoist the: (val) = op(args); "load" out of the loop because it doesn't see the value changing. The addition of cpu_relax() would inhibit this. As the iopoll helpers lack calls to cpu_relax(), people are sometimes reluctant to use them, and may fall back to open-coded polling loops (including cpu_relax() calls) instead. Fix this by adding calls to cpu_relax() to the iopoll helpers: - For the non-atomic case, it is sufficient to call cpu_relax() in case of a zero sleep-between-reads value, as a call to usleep_range() is a safe barrier otherwise. However, it doesn't hurt to add the call regardless, for simplicity, and for similarity with the atomic case below. - For the atomic case, cpu_relax() must be called regardless of the sleep-between-reads value, as there is no guarantee all architecture-specific implementations of udelay() handle this. Signed-off-by: Geert Uytterhoeven Acked-by: Peter Zijlstra (Intel) Acked-by: Arnd Bergmann Reviewed-by: Tony Lindgren Reviewed-by: Ulf Hansson Link: https://lore.kernel.org/r/45c87bec3397fdd704376807f0eec5cc71be440f.1685692810.git.geert+renesas@glider.be Signed-off-by: Sasha Levin --- include/linux/iopoll.h | 2 ++ 1 file changed, 2 insertions(+) diff --git a/include/linux/iopoll.h b/include/linux/iopoll.h index 2c8860e406bd8..0417360a6db9b 100644 --- a/include/linux/iopoll.h +++ b/include/linux/iopoll.h @@ -53,6 +53,7 @@ } \ if (__sleep_us) \ usleep_range((__sleep_us >> 2) + 1, __sleep_us); \ + cpu_relax(); \ } \ (cond) ? 0 : -ETIMEDOUT; \ }) @@ -95,6 +96,7 @@ } \ if (__delay_us) \ udelay(__delay_us); \ + cpu_relax(); \ } \ (cond) ? 0 : -ETIMEDOUT; \ }) -- 2.39.2