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t=1788271154; bh=e4NZGL9sW0yTZysi364rSyA0YFBhdTM0YG9uCeDwPSE=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Ug+tFPfl+VDaefaVU/4BHgnPy4Ce554caP+BlVg3VmC2CafOtgwHuapsFUIbT00AM JhBz78XPBoE1K7hRx8/zC063NUrEq5FGMqx6wpPftXQthG2CT9G0g3URZMrE4U0TIY e3zCkZDksY2EmW26k9Gyy6Z5pr54TGS3qGDEduTJMHYJF/pL4zzJX8h82ZWL7H2SYd zj0S2+Rm1ob2cjq2uV3T3jXfn+GY2AORNNkiA/y9hy2O4Bu7U0lfSAYSM8FapRpaHM BKqjJ4dChKTHB63GV2bJjWfzK31GlWjYVs3pb8wPL2DheszV97Ty3+skGtidZzaHYM J5jlsijPojrWw== Message-ID: <47aa459f-27d5-4b2f-9eb9-36ebca63d890@kernel.org> Date: Tue, 1 Sep 2026 15:59:09 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/slab: disallow kfree_rcu_sheaf() on PREEMPT_RT again Content-Language: en-US To: Sebastian Andrzej Siewior Cc: Harry Yoo , Clark Williams , Steven Rostedt , Andrew Morton , Peter Zijlstra , Alexei Starovoitov , 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, ThangNN99 References: <20260831-b4-kfree_rcu_hotfix-v1-1-4f0fb882638b@kernel.org> <20260901073339.uyKHXWCX@linutronix.de> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Stat-Signature: o3h1qsu6ehoe6jcsodoe3fxqxky7zt1p X-Rspamd-Queue-Id: 658A920009 X-Rspamd-Server: rspam06 X-HE-Tag: 1788271155-243163 X-HE-Meta: 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 +d2Jib6P vcDskQslDxbxu5CoHzzDHZQQVndnBdUlFuQGUJxhnVr5SJvLB4PRPdCQKJgzoMXHNWYsF9WG72FDKxIbH1JRHV5BVYW0McsC/oFZJfP7ouJ/gF5PhdQC4ETbw36MraVBk7Ahvu4pwFRNBv6J9ETakqgLm7vwkAd0HEtCAlLNaZBxljses52K/m1oqQ4H3QHvmSG90oj/1Budzoj5C66jy+0d7RwElZwzWaTvfyhz+9OJaliw2Es6AV6mynCtaa3EHzFlsavmvM/AUUY94Iwm1yUv0Gv6uZX+IeHZ85njBZO0XnBro3HVcgEC0UIkCvY93JqNV2LFdr4Hycdca6XqEyZncbD367V0hUzGl0/n452G9UCa+b5ksQwV9sLnDEQq+YZJnmF3UdQnIxWpE3oLsG3ludXw/FzVKxGL1vwNGUtB8o7mBjM5VQmx9uHp7/DYsK6IuTAYSy5qx89Vto/nRCvnIwX5U2KEO9waqVUarjSM0gJ8= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/1/26 09:33, Sebastian Andrzej Siewior wrote: > On 2026-08-31 18:02:38 [+0200], Vlastimil Babka (SUSE) wrote: >> This partially reverts commit 2a8bb29ec9b2 ("mm/slab: allow >> kfree_rcu_sheaf() on PREEMPT_RT"). It was based on an assumption that >> local_trylock() is safe on PREEMPT_RT from any context. > … >> Signed-off-by: Vlastimil Babka (SUSE) > Reviewed-by: Sebastian Andrzej Siewior > >> --- >> Incidentally I have posted a RFC [1] that leads to replacing that >> kfree_rcu() from set_cpus_allowed_force() but now after back from >> vacation I need to check the feedback and based on this bug report I can >> already see it makes the same bad assumption that trylock is fine. > > free_to_pcs() has still this trylock. > > What I am not so sure how good is that kfree_rcu_nolock() may allocate > memory for the sheaf if there is none around. Per sashiko review it's actually bad too under the pi_lock, because GFP_NOWAIT means __GFP_KSWAPD_RECLAIM which can mean wakeup_kswapd() and thus also need scheduler locks. And it's not a PREEMPT_RT-only issue... > It could have a pool of X > and if it runs out, it runs out and waits until the clean up process > feeds the used sheafs back. There is fallback and the run out is not the > usual case. I'd rather not invent new pools, since there's fallback and the sheaf+barn is already a pool. Could be enough to make sure the allocation attempt is safe, i.e. use only __GFP_NOWARN. > I do remember RCU tried the same thing but then it got to the case where > HEAD had to be supplied or it had to be preemptible so could wait for > grace period and free it. I just don't remember if it had a pool pages > to fill pointers to or allocated pages if it run out. And I am too lazy > to look atm. Yeah. > Sebastian