From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 0E6604756B6; Mon, 31 Aug 2026 16:04:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788192267; cv=none; b=hxnmyIfziKqlEdqlDUa6KQwfjJsIBHuAgXrof88HCgzIjtAPVUuhYPiPcYSbje4On3zISmN+ecodlCpoKBL0Q9B8SRqEOnXyGVcQM7nGqG6uNjUP7eAv7Cny72Pe8LWsGyzSB5x7VhF+g0t5F0SdHZQS9KuKL6etfDu8m92ebUA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788192267; c=relaxed/simple; bh=YtBIQydOwiDa9z0YXBYl9v06WxleGdRGUOLPwSPpXlk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=VrIjPUJyDaQxKbwczvJPuHhUFjvbS7Bryqf1EjZf23goap150e5pthGhRLlicawzbR8ERaDtNfNyaTxdVemPRL7+Af2T0kLMK/38aavH1tcGFmduc8gfrv3Sqy3vOFMZGBAZkkKI8/9d3yITOtP9ozF6SQ1NwbBSKflhVmzwI40= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Ppunhs5P; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Ppunhs5P" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D1E641F000E9; Mon, 31 Aug 2026 16:04:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788192265; bh=Uq7YyNnWah9USSxVq9LR8zCXv+SqDnnoFlu6Hbu1WXQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Ppunhs5PKu5CdT+XvNVUL7Rb5Z2CPCQLTrH/hyEU6ZLrfpnaoIxBlFJ8zdDIqRVZo SvxYhhrw9qHo6yB0Fw0wo98g5QjKkN3tbOqS88Obb4g8lWANkTd77KODHvL1gJAlIb pRPCjJaLUNt57zOR5HdQo3g9ZH6POfvt1ihde/LOIyCPAwe6VABEbvnHGNlwFeJ/j/ VdvyAQIYifTdJBPgeVSC52vQqQ0MbX22IyhMLKEXyUwsB09o/+12jvj1Wtry+GKxzX 9xQvTOWg/VrGEOEYHAQr/mgb41JarJb33o+FN9vIfLTxgPfWIG0leWcatULj0ORPjk fKk6KXw7nqMsA== Message-ID: Date: Mon, 31 Aug 2026 18:04:21 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] mm/slab: don't use kfree_rcu sheaves on PREEMPT_RT in kvfree_call_rcu() Content-Language: en-US To: Sebastian Andrzej Siewior , ThangNN99 Cc: Harry Yoo , Andrew Morton , Clark Williams , Steven Rostedt , Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, syzbot+acf142088e0182172e58@syzkaller.appspotmail.com References: <20260831130057.HLukQ-zm@linutronix.de> <20260831133222.8637-1-ngocthang2710.1999@gmail.com> <20260831143500.x-saxdAs@linutronix.de> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 8/31/26 16:35, Sebastian Andrzej Siewior wrote: > On 2026-08-31 20:32:22 [+0700], ThangNN99 wrote: >> --- a/mm/slab_common.c >> +++ b/mm/slab_common.c >> @@ -2034,7 +2027,14 @@ void kvfree_call_rcu(struct kvfree_rcu_head *head, void *ptr) >> if (!head) >> might_sleep(); >> >> - if (kfree_rcu_sheaf(ptr)) >> + /* >> + * Callers may hold a raw_spinlock_t here on PREEMPT_RT (e.g. >> + * set_cpus_allowed_force(), whose callers all hold >> + * task_struct::pi_lock), and the sheaf/barn locks are also taken >> + * as blocking locks elsewhere, so trying them here creates a >> + * lockdep-visible ordering conflict. Skip sheaves on PREEMPT_RT. >> + */ > > You mix up things. A raw_spinlock_t should work in general and should > not cause a problem. The task_struct::pi_lock is special: It used during > wakes and the inner waitlock of the rtmutex acquires it even during a > trylock. For PREEMPT_RT we don't want to acquire any locks while the > pi_lock is held. > > What about > > kvfree_rcu() is called by set_cpus_allowed_force() with > task_struct::pi_lock acquired. On PREEMPT_RT the local_trylock() > usage below will acquire the waitlock which must be avoided. > Therefore avoid it on PREEMPT_RT. Thanks. > Vlastimil? IMHO it's not useful to iterate a fix through emails with apparently a LLM agent, so let's continue here: https://lore.kernel.org/all/20260831-b4-kfree_rcu_hotfix-v1-1-4f0fb882638b@kernel.org/ > >> + if (!IS_ENABLED(CONFIG_PREEMPT_RT) && kfree_rcu_sheaf(ptr)) >> return; >> >> // Queue the object but don't yet schedule the batch. > > Sebastian