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 52EF142BC56; Thu, 23 Jul 2026 09:46:12 +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=1784799974; cv=none; b=a5LnstnXTJwziqPP9ozi+aO5RtqfDIW4tSkyc0xceROQ6bs9L2DKV8Xa0n+/yLFURoelc7IIaszISwrWQAumaO5Gf12c6NTxqgRM2B7JNP60XKVdtI+aX0HYg/iKaq/xdpUon1EH4F0jU63Yi1mueEq1KbllrXbNF98HE9p8gZw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784799974; c=relaxed/simple; bh=NitLufdE/4i0EaywdulcGJUyybW1WobriDxx0kW0zuw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=aWCtqND6MYdYJU5O6xOgtC8PI+EIjX5zXM+oTBo29RMMiiNuHPIezvxDkdNS3uFnYXyQ99ceJVeIkeh31sCnQfUr+T8hzNV4GyyLrNsaQhEJMxUcFRmcQrZOKKRW3+UUVBlfN3MgZ0eJpAxeGFqfWckE18YgDqqbRUahICPsLlk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=a8lBic8b; 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="a8lBic8b" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 928321F000E9; Thu, 23 Jul 2026 09:46:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784799971; bh=aQoLo96zw0hxxBrNcRicMtUhLwVl+yi3CsLLfhJfpPY=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=a8lBic8br0xTW38lbZuPJ1DFrHJ8Y8LS6+VHPO0N36kvB/eF5J14iY2vjWqpj+Daf gYHlVlYLJ7kXnczCN0bXXoGs5as9D7H3Sqb6jQxicxySwAlbFvY0vmJoFtdNLY0D1o RvphI1iVHFattz/PKtKSHR4jtKRorsH2ODMNLLsUg7/dRIH7fcGl4tYudyFmT0WiqJ GLOheMJRIrOi3zM1pH+xTgIx3AbNzQ8IJ1307cIFEU2dDpkKmTQVLO0olOpus/cmO6 plcsQ3lU426dgBkTclS3U6r1GAc+O/IzzS4H3ihJBWCwr9TVSo2S6AsveSrExeCpzx nAy5I/c6mkhhA== Message-ID: Date: Thu, 23 Jul 2026 11:46:08 +0200 Precedence: bulk X-Mailing-List: linux-rt-devel@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH slab/for-next v4 5/8] mm/slab: allow kfree_rcu_sheaf() on PREEMPT_RT Content-Language: en-US To: Harry Yoo , sashiko-reviews@lists.linux.dev Cc: bpf@vger.kernel.org, linux-rt-devel@lists.linux.dev, linux-kernel@vger.kernel.org, hao.li@linux.dev References: <20260720-kfree_rcu_nolock-v4-0-964e03c41a4e@kernel.org> <20260720-kfree_rcu_nolock-v4-5-964e03c41a4e@kernel.org> <20260720125637.129D01F00A3A@smtp.kernel.org> <1c7ce106-c5b0-4689-8c24-b32a0966eb02@kernel.org> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/23/26 07:39, Harry Yoo wrote: > On 7/20/26 9:56 PM, sashiko-bot@kernel.org wrote: >> >>> diff --git a/mm/slub.c b/mm/slub.c >>> index 8afa6b47b1f2b..deac315d0f23e 100644 >>> --- a/mm/slub.c >>> +++ b/mm/slub.c >> [ ... ] >>> @@ -6080,10 +6081,10 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj, unsigned int free_flags) >>> struct slab_sheaf *rcu_sheaf; >>> bool allow_spin = free_flags_allow_spinning(free_flags); >>> >>> - if (WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_RT))) >>> - return false; >>> + VM_WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_RT) && allow_spin); >>> >>> - lock_map_acquire_try(&kfree_rcu_sheaf_map); >>> + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) >>> + lock_map_acquire_try(&kfree_rcu_sheaf_map); >>> >>> if (!local_trylock(&s->cpu_sheaves->lock)) >> ^^^^^^^^^^^^^ >> [Severity: High] >> This is a pre-existing issue, but since we are enabling this path for >> PREEMPT_RT, does the use of GFP_NOWAIT later in this function cause >> lock recursion deadlocks? >> >> When the fast path fails, __kfree_rcu_sheaf() calls: >> >> empty = alloc_empty_sheaf(s, GFP_NOWAIT, alloc_flags); >> >> GFP_NOWAIT includes __GFP_KSWAPD_RECLAIM, which invokes wakeup_kswapd(). >> The wake-up path enters the scheduler via try_to_wake_up() and attempts to >> acquire scheduler locks like pi_lock and the runqueue lock. > > This indeed looks like a real issue as kfree_rcu() can be called under > scheduler locks, Are there any paths that actually do that? > but not sure how it's been undiscovered for this long? With kfree_rcu_sheaf_map we prevent the raw spinlock nesting detection in general. But I guess if there was an actual kfree_rcu() call under any scheduler lock involved in wakeup_kswapd(), lockdep would still trigger, right? >> If kvfree_call_rcu() was called while the CPU already holds these scheduler >> locks, will this wake-up attempt cause a lock recursion self-deadlock? >> >