From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9A6483B42F7; Wed, 1 Jul 2026 08:05:58 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.92.199 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782893162; cv=none; b=n7uz8SKuaBOH7FklemLV9isfS46VhRgDWohyMYsPq6zMoLRhG1njuNUc5HPg2zluK53d1RVJZvybtRnry12YcVfERRaIBgiG/o/57530nFbZPeE+qDb+gRnYUGDq34btqA7UJPyvZfP5peJsEctQVZrnPJVqVNJ4m/iL3mnH/NE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782893162; c=relaxed/simple; bh=CEd7/7n0vxbLFyarXrnlKBjTXa6ydc/2mERAV+dHrCk=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=BdssmYTvAntDeN2s9Qgs+egXTwAVCzwxogT0DT6AC/yBKh7bXOfvSp3f0iRhL0CYB/jkZfW04cWNPjTlrmVeQNK1xEzAMevI2iorv1VH5t6gyvDjEciyybXG+Wd+Yqps0bmpLdT77H2hpAF13EO2gLmTJllULwtC7HhuWJL4cEk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=pass smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=bl/EuA1u; arc=none smtp.client-ip=90.155.92.199 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="bl/EuA1u" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=cuhOHofr2+cEtj6vEWuMudbNE67MdM7Lx2mNe4+82/c=; b=bl/EuA1uopl2R2wTxjJMCXG7+L 0yhs3lTNImMVX94W03UHLFtXhLUPGYx1JJ6NqCVe0VcUQzh5qpeqCeVsZZ4p2Wj5az8c0NIi5OtDe vAIsYkhAZKgFdK6Z1V+KLIWIEL6GtCv58F3PT1yFaw8wdBNb5Qhw42Tf7E76mCX8ea5sND2X+krSj jwqQP73dmVuZU73Vc7weBY1ObBl83zIGTEhP01Ij18YtT20Ix9XaUSSnYVRn9k6oMg6NKEx9vI4yv sO+QJxKOhdUMsw05nPRNVYe3YFQOfza+8fDF/M/dMpb5iamyEuVrcmx/1GIRV9NYk6ZKMAk+4clZ5 9a+Uly7A==; Received: from 77-249-17-252.cable.dynamic.v4.ziggo.nl ([77.249.17.252] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.99.2 #2 (Red Hat Linux)) id 1wepxC-00000002Unr-3NzK; Wed, 01 Jul 2026 08:05:55 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id A166230035C; Wed, 01 Jul 2026 10:05:53 +0200 (CEST) Date: Wed, 1 Jul 2026 10:05:53 +0200 From: Peter Zijlstra To: linux-kernel@vger.kernel.org Cc: linux-tip-commits@vger.kernel.org, Usama Arif , Thomas Gleixner , Dmitry Ilvokhin , x86@kernel.org Subject: Re: [tip: smp/core] smp: Use release stores for csd_lock_record() state Message-ID: <20260701080553.GQ48970@noisy.programming.kicks-ass.net> References: <20260629105745.1696683-1-usama.arif@linux.dev> <178274146231.3843924.7768569808457911760.tip-bot2@tip-bot2> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <178274146231.3843924.7768569808457911760.tip-bot2@tip-bot2> On Mon, Jun 29, 2026 at 01:57:42PM -0000, tip-bot2 for Usama Arif wrote: > The following commit has been merged into the smp/core branch of tip: > > Commit-ID: ec9f57e6fefbc0497b9f047ebe60ef9adaae7480 > Gitweb: https://git.kernel.org/tip/ec9f57e6fefbc0497b9f047ebe60ef9adaae7480 > Author: Usama Arif > AuthorDate: Mon, 29 Jun 2026 03:57:45 -07:00 > Committer: Thomas Gleixner > CommitterDate: Mon, 29 Jun 2026 15:53:14 +02:00 > > smp: Use release stores for csd_lock_record() state > > __csd_lock_record() publishes per-CPU diagnostic state (cur_csd, > cur_csd_func, cur_csd_info) that is consumed from a remote CPU by > csd_lock_wait_toolong() via smp_load_acquire(&cur_csd). To order the > matching cur_csd_func/cur_csd_info stores before the cur_csd > publication, the producer issues smp_wmb() before writing cur_csd; > to order the publication before the subsequent callback execution or > CSD unlock, it issues smp_mb() after the write. The clear path > mirrors this with smp_mb() before storing NULL into cur_csd so the > preceding callback/unlock is observed first. > > The smp_mb() pair is heavier than what the consumer actually > requires (on x86 each emits a locked full barrier). The consumer > only needs to observe the matching cur_csd_func/cur_csd_info when it > sees a non-NULL cur_csd, and to observe the preceding callback/unlock > when it sees NULL -- both of which a release/acquire pair provides. > The extra two-way ordering enforced by smp_mb() -- that cur_csd > publication be observed before callback execution or unlock becomes > visible -- would only matter if cur_csd were an exact live-state > marker. csd_lock_wait_toolong() does not treat it that way: it > snapshots cur_csd via smp_load_acquire() and then prints / dumps / > re-IPIs without an RCU-style stall-ended recheck, so the diagnostic > already tolerates the remote CPU completing its work between snapshot > and report. cur_csd is best-effort context, not a precise stall > boundary. > > Replace the smp_wmb() + plain store + smp_mb() in the publish path, > and the smp_mb() + plain store in the clear path, with > smp_store_release(). This pairs with the smp_load_acquire() in > csd_lock_wait_toolong(): preceding cur_csd_func/cur_csd_info stores > become visible before a remote reader observes the non-NULL > publication, and any preceding callback/unlock becomes visible before > a reader observes the NULL clear. > > Signed-off-by: Usama Arif > Signed-off-by: Thomas Gleixner > Reviewed-by: Dmitry Ilvokhin > Link: https://patch.msgid.link/20260629105745.1696683-1-usama.arif@linux.dev > --- > kernel/smp.c | 26 +++++++++++++++++++------- > 1 file changed, 19 insertions(+), 7 deletions(-) > > diff --git a/kernel/smp.c b/kernel/smp.c > index a0bb56b..8a847a3 100644 > --- a/kernel/smp.c > +++ b/kernel/smp.c > @@ -182,16 +182,22 @@ static atomic_t csd_bug_count = ATOMIC_INIT(0); > static void __csd_lock_record(call_single_data_t *csd) > { > if (!csd) { > - smp_mb(); /* NULL cur_csd after unlock. */ > - __this_cpu_write(cur_csd, NULL); > + /* > + * Pairs with smp_load_acquire() of cur_csd in > + * csd_lock_wait_toolong(): orders any preceding CSD > + * callback/unlock before a remote reader observes NULL. > + */ > + smp_store_release(this_cpu_ptr(&cur_csd), NULL); So at some point I suppose we should have *this_cpu_store_release() I suppose. As is this generates suboptimal code on x86. But since I don't typically run with CSD_LOCK_WAIT_DEBUG=y, I'll pretend not to care at this exact moment. > return; > } > __this_cpu_write(cur_csd_func, csd->func); > __this_cpu_write(cur_csd_info, csd->info); > - smp_wmb(); /* func and info before csd. */ > - __this_cpu_write(cur_csd, csd); > - smp_mb(); /* Update cur_csd before function call. */ > - /* Or before unlock, as the case may be. */ > + /* > + * Pairs with smp_load_acquire() of cur_csd in > + * csd_lock_wait_toolong(): publishes cur_csd_func and > + * cur_csd_info before the non-NULL pointer becomes visible. > + */ > + smp_store_release(this_cpu_ptr(&cur_csd), csd); Let me say that RELEASE+ACQUIRE does not a full barrier make, specifically smp_store_release() and smp_load_acquire() do not have RCsc, they're RCtso or possibly weaker still (I don't remember). The thing that saves your bacon here is the address dependency. > } > static __always_inline void csd_lock_record(call_single_data_t *csd) > @@ -272,7 +278,13 @@ static bool csd_lock_wait_toolong(call_single_data_t *csd, u64 ts0, u64 *ts1, in > cpux = 0; > else > cpux = cpu; > - cpu_cur_csd = smp_load_acquire(&per_cpu(cur_csd, cpux)); /* Before func and info. */ > + /* > + * Pairs with smp_store_release() of cur_csd in __csd_lock_record(): > + * a non-NULL cur_csd here implies cur_csd_func and cur_csd_info > + * are the matching publication; a NULL value is ordered after any > + * preceding CSD callback/unlock on the remote CPU. > + */ > + cpu_cur_csd = smp_load_acquire(&per_cpu(cur_csd, cpux)); > /* How long since this CSD lock was stuck. */ > ts_delta = ts2 - ts0; > pr_alert("csd: %s non-responsive CSD lock (#%d) on CPU#%d, waiting %lld ns for CPU#%02d %pS(%ps).\n",