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 0124A3BFE5C; Wed, 24 Jun 2026 14:16:09 +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=1782310571; cv=none; b=cOkrw8uv6raPPC6+uuh9RlycHGeC+fBng7A+QQlDHmz8aPWXNGbTdimxualC/5frdA0lzayeCxKnl7MOuHRH+44+H4IKsC62NJUP4mMPdbdErcnV6X3+aFfaRHBN3CzHz1T3PhlUqz/J//ShNXjjgolXt5vczzZ/Fx3PyjFOavY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782310571; c=relaxed/simple; bh=UI2NNA2hb1Ih3IR06qE170SNFF1aFICiLHj70kD4Y2M=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=QjYBwawY3ydcttyYgoCnXPGNJM6A9dXflkD2nOk1LBvNrbeoRKoiBGkmU8AY/SwRk3RhfOE8gYJn8ERum+qIsGiTWssZELzt8m0K1aBbNTlOXMM3cpQ6aQoCD4igVR90XxKIrbxmLo/CrgQeqAlGTY3hEkNeb778fgoKwpWFMbQ= 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=YGeN61UR; 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="YGeN61UR" Message-ID: <1030b968-4184-4818-99dc-aeb0956ef6a3@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1782310566; 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=dWM7/K0F3C5pxQFwSgsZm5S7ELhGilSGA5hWWaaRFNI=; b=YGeN61URMed8FpXNsXYxLJfqOAStpoVJ5VV3vvz8ElP4F+Nbh8QvaWpMTXkba3YhZjdty+ X+rLtuGt36ajhdUfdgvzVJcQsyEVGssupxitTGSraomwG6NLuh7Nfc6/rolwGtBYJxpzm/ oQzB1gybTiKUQI+oEnwNtgC27a+bocA= Date: Wed, 24 Jun 2026 22:15:03 +0800 Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v2] smp: Use release stores for csd_lock_record() state To: Usama Arif , 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> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Kunwu Chan In-Reply-To: <20260622163807.4187558-1-usama.arif@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 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 > 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)