From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-184.mta0.migadu.com (out-184.mta0.migadu.com [91.218.175.184]) (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 3706F3CCFD0 for ; Wed, 24 Jun 2026 15:27:43 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.184 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782314865; cv=none; b=mtKj0l4TPftoRhjgt9cxFo/Qx06LQNk70L7UIcriaiXa0E31hAYQ16Uzi24FTUUz7p7gUmHufBAFlUfLyddAe3eDSizd1/C59s+MszCT1S7tqLU9SO4OrgsoDR8sW5bVHqCsldZIC9dzFV3TpOASJEGepUyluC9QMa6TeTg5HVw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782314865; c=relaxed/simple; bh=l/w1BUMQ8nW/GqlL6vjm8tb+sRH+Bkhz6ZNlKu/gL4o=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=EtpEPuAZ2nrsYgaY1UseSMdIgJYEQh8V+kDEfri2lL8up4J8CrVNxBMUjWuKDDyfITj3ix3rEfKGye6YmjIol3IGdm/TvisU+d+HHEb0xmCR2uqB0ApVeJbmP5EXLmcxf0GnfTn3XM9Tj5j888/ybRJrUY8J33O0kp1YiLIrgxE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=O/FKJt46; arc=none smtp.client-ip=91.218.175.184 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="O/FKJt46" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1782314860; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=TO+JQz2CsGaSGPeub/M5ejcrVEttO5GzNLxhrt7apS4=; b=O/FKJt469VtIbSe4MNcQ5/5ggZST+FVaXiNQNdXptyQyxmHKV6avtbkudNne00bFbIYsn5 M4gw9YSiq9dyDLIVum7ji3YgQCou0xcpN51uoZmmNwuVOcJiuJTCiqBo93Syib/5WN/C4N RCb6LID893kqir4qY1pixyQGyVUzbN0= Date: Wed, 24 Jun 2026 16:27:38 +0100 Precedence: bulk X-Mailing-List: lkmm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v2] smp: Use release stores for csd_lock_record() state To: Kunwu Chan , 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 Cc: shakeel.butt@linux.dev, hannes@cmpxchg.org, kernel-team@meta.com References: <20260622163807.4187558-1-usama.arif@linux.dev> <1030b968-4184-4818-99dc-aeb0956ef6a3@linux.dev> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Usama Arif In-Reply-To: <1030b968-4184-4818-99dc-aeb0956ef6a3@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 24/06/2026 15:15, Kunwu Chan wrote: > On 6/23/26 00:38, 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 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. >  The original comments around those updates seem to suggest a stronger > relationship: >   /* Update cur_csd before function call. */ >   /* NULL cur_csd after unlock. */ > > The changelog characterizes cur_csd as best-effort diagnostic context.  > > Is there prior consensus that cur_csd carries no ordering relationship  > to callback execution / unlock state,  > > or is this patch effectively relaxing that historical assumption? > > > Thanks,  Kunwu Thanks for the review Kunwu! The patch preserves the ordering that reader actually needs: csd_lock_wait_toolong() observes non-NULL cur_csd, the matching func/info have been published. If it observes NULL, the prior callback/unlock work is ordered before the clear. So yes, this relaxes the historical implementation ordering around the diagnostic marker, but not the CSD execution/reuse ordering. > >> 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); >> } >> >> static __always_inline void csd_lock_record(call_single_data_t *csd)