From owner-dev-commits-src-all@freebsd.org Wed Aug 11 18:49:35 2021 Return-Path: Delivered-To: dev-commits-src-all@mailman.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.nyi.freebsd.org (Postfix) with ESMTP id 0094A659041; Wed, 11 Aug 2021 18:49:34 +0000 (UTC) (envelope-from jhb@FreeBSD.org) Received: from smtp.freebsd.org (smtp.freebsd.org [96.47.72.83]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256 client-signature RSA-PSS (4096 bits) client-digest SHA256) (Client CN "smtp.freebsd.org", Issuer "R3" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id 4GlJlL6M6kz3sBk; Wed, 11 Aug 2021 18:49:34 +0000 (UTC) (envelope-from jhb@FreeBSD.org) Received: from John-Baldwins-MacBook-Pro.local (ralph.baldwin.cx [66.234.199.215]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (Client did not present a certificate) (Authenticated sender: jhb) by smtp.freebsd.org (Postfix) with ESMTPSA id 51C4FA473; Wed, 11 Aug 2021 18:49:34 +0000 (UTC) (envelope-from jhb@FreeBSD.org) To: Scott Long , src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-main@FreeBSD.org References: <202108102241.17AMfr63025604@gitrepo.freebsd.org> From: John Baldwin Subject: Re: git: 35547df5c786 - main - Call wakeup() with the lock held to avoid missed wakeup races. Message-ID: <9b1ec22c-1545-52ea-7bf9-a96140737711@FreeBSD.org> Date: Wed, 11 Aug 2021 11:49:33 -0700 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <202108102241.17AMfr63025604@gitrepo.freebsd.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-BeenThere: dev-commits-src-all@freebsd.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: Commit messages for all branches of the src repository List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 11 Aug 2021 18:49:35 -0000 On 8/10/21 3:41 PM, Scott Long wrote: > The branch main has been updated by scottl: > > URL: https://cgit.FreeBSD.org/src/commit/?id=35547df5c78653b2da030f920323c0357056099f > > commit 35547df5c78653b2da030f920323c0357056099f > Author: Scott Long > AuthorDate: 2021-08-10 22:36:38 +0000 > Commit: Scott Long > CommitDate: 2021-08-10 22:36:38 +0000 > > Call wakeup() with the lock held to avoid missed wakeup races. > > Submitted by: luiz > Sponsored by: Rubicon Communications, LLC ("Netgate") > --- > sys/dev/sdhci/sdhci.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/sys/dev/sdhci/sdhci.c b/sys/dev/sdhci/sdhci.c > index d075c2e05000..573e6949b57e 100644 > --- a/sys/dev/sdhci/sdhci.c > +++ b/sys/dev/sdhci/sdhci.c > @@ -2078,8 +2078,8 @@ sdhci_generic_release_host(device_t brdev __unused, device_t reqdev) > /* Deactivate led. */ > WR1(slot, SDHCI_HOST_CONTROL, slot->hostctrl &= ~SDHCI_CTRL_LED); > slot->bus_busy--; > - SDHCI_UNLOCK(slot); > wakeup(slot); > + SDHCI_UNLOCK(slot); > return (0); > } Hmm, how does this avoid a race? The sleep is checking bus_busy under the lock and should never see a stale value and go back to sleep after the wakeup has occurred: SDHCI_LOCK(slot); while (slot->bus_busy) msleep(slot, &slot->mtx, 0, "sdhciah", 0); slot->bus_busy++; /* Activate led. */ WR1(slot, SDHCI_HOST_CONTROL, slot->hostctrl |= SDHCI_CTRL_LED); SDHCI_UNLOCK(slot); Dropping the lock before wakeup() is a tiny optimization that avoids having the second thread wakeup and immediately block on the lock before it has been released by the first thread. -- John Baldwin