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b=HXl+95NGCkRn8wpoJGvYycHo6AFaZsZMhDw8m68jymnrceifFIzGIlqfOs8JNnCNS ndprzaCRxDNXYtrz2A2B0DjKXBH6/qTAXIJl/SoPyxrZwwkHwXB6h3YjDS0vQmpJbw FU64XqtGrRI0dTQ3q9MqUVCe8TPxCHSJpBuPjXmESI8ommzpz4g9wPhnMMD2eQW5wi YP0XLHJug5Crdf2Cd2Ewyc4Mfb4/a5hWceLXfL049a1IlD39ID5VMR9f4n5sg7k9KV KxpQtTMAYWOFgv0i20IDqwo0HOTgaE20eL3Pp1t+/HSyooz1pVXv/BSvyCbPv/CWYg blDgD6ygsVJgQ== Date: Fri, 4 Sep 2026 10:57:56 +0100 From: Harry Yoo To: "Vlastimil Babka (SUSE)" Cc: Sebastian Andrzej Siewior , 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 Subject: Re: [PATCH] mm/slab: disallow kfree_rcu_sheaf() on PREEMPT_RT again Message-ID: References: <20260831-b4-kfree_rcu_hotfix-v1-1-4f0fb882638b@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260831-b4-kfree_rcu_hotfix-v1-1-4f0fb882638b@kernel.org> X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: E63B81C0003 X-Stat-Signature: o41t1kx6thkg6z19kr7tj8u7cnr6ytx8 X-Rspam-User: X-HE-Tag: 1788515882-936351 X-HE-Meta: U2FsdGVkX1/4eNcMSh0VWO7uj7iw3apWmKeF03Be6dh+wIwNdtlwMzySb3xaGWgC3DD3uDAeUolRNW7gS77b/ifcHpRmRxsEZSu9hvtxutFXDagqUEM1A0eM9bv1ALzUjfM6gks+/vWh9U2NNXa6JrjsBOruzIg3vyKKHfuLeltMoVaPBsS6pITzh4StdzvH9gqAJ3eudFL0ueu89KZsTzlnFwtI5hUBC1ap81upy5SXyMIZulJaFGMCsPO9lFg4tWF/JAP/RBuNBSXDSHaWYsit2x2S/UAurBBMP1lLF+v6yL8TIPQQlFzs2KEWiBAL0lDdbVGyD7BAFuBiQBXvts5TMupS2gkCk96VBQEvO/E4nnkliV51Injee5372vGR5Tb86LAi+g6JACS3/wzCxTFJjjW1hl7EvzMM0Uyo4QMUQpK2hRcHLUyRgX3MspN76fIbYfq8SnLKTd8HknnXpyHRConONMEoobIKkPLDz3Ix9t7Ta5fy8q6QiuUPba4oDNAPg0nKUVrJV6mvcg71O6g5+kprjiCv/2Vgf8Fk5qwYi6VU/JfjypiO78jhulS0FQVZKuFCYS5OiG3uF0gT0yG13xTh43o+wfe6fRTiRUV0UIHEXmR6056xF/N/ePwkrATLj86jGbC169R80ijvPSAdvvIUuVZJbL+KoDf8NymkySuwb0IuahmrWcoiXN1CtM0BPX29/FFHAaXekZGZU93NPhRqOxMNCbC4s1HveGf+FiMp5ra3PLU/Givv6vVw0ckSFKqRXrxeMJ8fduxeFantWbi9HUagM2Q73kdfjXsrpZ21ii+/NABWU51B9k8sop/4GPQuMZ1NGVuNSORi/xRYXyy4OhD2neQYnr+V17qj+U58FhMMNtDpsPk45no7MyRuhVZKYNBrcD0dlCzoKLTVomAQsfO3fj7zbLBtsF+IlP8TkVglwMU8Gwz0RT29k8T9Ma5DEj8pwBc0Htd rOUriIFI 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Aug 31, 2026 at 06:02:38PM +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. > > However kvfree_rcu() is also called by set_cpus_allowed_force() with > task_struct::pi_lock acquired and there it's not safe, as syzbot > has reported. > > For the immediate fix, skip kfree_rcu_sheaf() on PREEMPT_RT again from > kvfree_call_rcu(). In theory, kfree_rcu_nolock() would have the same > problem when called from under pi_lock on PREEMPT_RT but that can > be addressed if such a caller is proposed. > > Add an explanation comment, courtesy of Sebastian. > > Reported-by: syzbot+acf142088e0182172e58@syzkaller.appspotmail.com > Closes: https://syzkaller.appspot.com/bug?extid=acf142088e0182172e58 > Reported-by: ThangNN99 > Fixes: 2a8bb29ec9b2 ("mm/slab: allow kfree_rcu_sheaf() on PREEMPT_RT") > Signed-off-by: Vlastimil Babka (SUSE) > --- Looks good to me, Reviewed-by: Harry Yoo (Meta) -- Cheers, Harry / Hyeonggon