From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail.ilvokhin.com (mail.ilvokhin.com [178.62.254.231]) (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 2448E3DA5CF for ; Thu, 25 Jun 2026 14:42:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=178.62.254.231 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782398550; cv=none; b=gXORM9r5pIWdSIktuUeQzP29CqyVr1u4BM9ybQmIx8OZGj6LZKTbAmrcbDnQNU9jTzmH8U0k5VbPG25O6fsg4Ouctx9OqEhZ++j5npGkSCuE8rw2UEoRqDSALaZlprUsjKMQ0oXnU1Kjw1XyDzGYhwWAnmfVQRYd5rVM5xd+aTY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782398550; c=relaxed/simple; bh=JGVTRb5DqYe0LpDZ1wZvuP+94/ev4LoITOfgIh5CyTA=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=NwTyifPLbzlbXsPKdB7LmBPPoMvP46CLN8ubSokg3MdjfCT5yj1GYnQT/3AIgXbP/R3hvu4GfBbRTRYgFFSeFo3+mxIlpMqwPscdB3697UDyA6mUyVW07mjxypRYqCCr22Yvqcp5jDMfIwZrEZH79ru/1E08fI8yufZg07y0liU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=ilvokhin.com; spf=pass smtp.mailfrom=ilvokhin.com; dkim=pass (1024-bit key) header.d=ilvokhin.com header.i=@ilvokhin.com header.b=X2SLTx2L; arc=none smtp.client-ip=178.62.254.231 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=ilvokhin.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=ilvokhin.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=ilvokhin.com header.i=@ilvokhin.com header.b="X2SLTx2L" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ilvokhin.com; s=mail; t=1782398541; bh=uWaBoqYfbNoWhVcIPdsvXePepaUpdOqRtpnHcvJcJbc=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=X2SLTx2LiaRZ9qppN1ShUfMj+uhI1Os05qSVFcd9vm6Ea1SCyYamf0qO9z5HKUVhh 3Rq9uRniaDIQrxqxa/b76AqymMEL2Fkc9wzlMhFat/hGVi3LsQIEFDJM790iOegHOU H4rfRwiWsUEq8xHap0znlhRnZA81JjEeGy0kJnQo= Received: from shell.ilvokhin.com (shell.ilvokhin.com [138.68.190.75]) (Authenticated sender: d@ilvokhin.com) by mail.ilvokhin.com (Postfix) with ESMTPSA id 37A1BDAFEA; Thu, 25 Jun 2026 14:42:21 +0000 (UTC) Date: Thu, 25 Jun 2026 14:42:17 +0000 From: Dmitry Ilvokhin To: Usama Arif Cc: lkmm@lists.linux.dev, joelagnelf@nvidia.com, linux-kernel@vger.kernel.org, marco.crivellari@suse.com, paulmck@kernel.org, rafael.j.wysocki@intel.com, rdunlap@infradead.org, riel@surriel.com, sshegde@linux.ibm.com, tglx@kernel.org, ulfh@kernel.org, yury.norov@gmail.com, rcu@vger.kernel.org, shakeel.butt@linux.dev, hannes@cmpxchg.org, kernel-team@meta.com Subject: Re: [PATCH v2] smp: Use release stores for csd_lock_record() state Message-ID: References: <20260622163807.4187558-1-usama.arif@linux.dev> Precedence: bulk X-Mailing-List: lkmm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260622163807.4187558-1-usama.arif@linux.dev> On Mon, Jun 22, 2026 at 09:38:07AM -0700, Usama Arif wrote: > __csd_lock_record() publishes per-CPU CSD debug state that is read by > csd_lock_wait_toolong() on another CPU. The remote side first reads > cur_csd with smp_load_acquire() and, when non-NULL, may then read the > matching cur_csd_func and cur_csd_info fields. > > Use smp_store_release() when publishing cur_csd so that the preceding > cur_csd_func and cur_csd_info stores are ordered before the pointer > that csd_lock_wait_toolong() acquires. This replaces the open-coded > smp_wmb() plus plain cur_csd store with the release operation that > matches the smp_load_acquire() in csd_lock_wait_toolong(). > > For the clear path, use smp_store_release(&cur_csd, NULL) so that > clearing the diagnostic state remains ordered after the preceding > callback/unlock work, without requiring a full barrier before the > store. On x86 this removes the locked full barrier from the clear > path; on weaker memory models it uses the release operation needed by > the smp_load_acquire() in csd_lock_wait_toolong(). The changelog only calls out the clear path here, but the publish path also drops its trailing smp_mb() (plus the smp_wmb()), so on x86 both paths lose a locked full barrier. Worth describing symmetrically. > > The old code also had smp_mb() calls around cur_csd updates. Those would > only be needed if cur_csd were treated as an exact live-state marker whose > publication had to be observed before callback execution or CSD unlock. > CSD stall warnings do not currently have RCU-style stall-ended checks, so > they already allow the stall to end while diagnostics are being assembled. > The cur_csd record is therefore best-effort diagnostic context, not a > precise completion/stall boundary. > > Signed-off-by: Usama Arif > --- > v1 -> v2: https://lore.kernel.org/all/01437928-ff79-4d8e-823b-7f20146946f6@linux.dev/ > - Document where the smp_store_release() synchronizes with (Alan Stern, > Randy Dunlap and Paul McKenney). > --- > kernel/smp.c | 18 ++++++++++++------ > 1 file changed, 12 insertions(+), 6 deletions(-) > > diff --git a/kernel/smp.c b/kernel/smp.c > index a0bb56bd8dda..685829875a3e 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); > 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); > } Since v2 is specifically about documenting the pairing, it would be good to make it symmetric and add the comment on the acquire side in csd_lock_wait_toolong(). > > static __always_inline void csd_lock_record(call_single_data_t *csd) > -- > 2.53.0-Meta >