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McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Suren Baghdasaryan , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, rcu@vger.kernel.org, bpf@vger.kernel.org Subject: Re: [PATCH for-next v3 4/9] mm/slab: use call_rcu() in unknown context if irqs are enabled Message-ID: References: <20260615-kfree_rcu_nolock-v3-0-70a54f3775bb@kernel.org> <20260615-kfree_rcu_nolock-v3-4-70a54f3775bb@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260615-kfree_rcu_nolock-v3-4-70a54f3775bb@kernel.org> X-Rspamd-Action: no action X-Rspam-User: X-Stat-Signature: 9qm7h6gqmswh6au3fkqwmptpqmndwig6 X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 6001440012 X-HE-Tag: 1782311421-142870 X-HE-Meta: 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 TZqrtAXS UOrp4nOxWxG59VTs79MHyUVc2NRCSja+r4RHYU1FDeLmZFhhltg4pQTHp6k5oCec2ylGMCc4LJWLED4QlPIpfqKOVcpPeqa2M3jKZyd1E5vgfockkMvdDD6ZXiddZQYhOJb2pSwmuzt+9g0RABAk2wGpt7hwVIm6/O4+Tcfz0oicYna8f86rJSM+lc5LsOqZ2VispzUNbN3XrSD8LI3NOxMfpNU96Gdhx3N0uTu4GEmBkF+WIVGG47Uz8dqjQWGz65B2Uo6dkDwmakUfx+1oF/AWNtOH5WqK7ua/0q7SVwtSQRt2G8klUDeEjLNl9i9sWGPjFYZFe7p3hl+R6nLFFYm2N7lBuleouj1ds3z9skeN/SwwRtBzTvKw7CewMyGE7iaS0U9dND+LRb7XrNY8PfN3EyQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Jun 15, 2026 at 08:05:58PM +0900, Harry Yoo (Oracle) wrote: > call_rcu() disables IRQs with local_irq_save() to protect its per-cpu > data structures. Therefore, if IRQs are not disabled, they cannot be > corrupted by reentrance into call_rcu(). So fall back to the deferred > path only when !allow_spin && irqs_disabled(). > > The RCU subsystem does not guarantee this contractually, and this > optimization relies on RCU's implementation details. Ideally, it should > be removed once call_rcu_nolock() is supported by the RCU subsystem. > > Link: https://lore.kernel.org/linux-mm/CAADnVQKRVD5ZSnEKbZZU7w86gHbGHUug2pvzpgZTngNS+fg4rw@mail.gmail.com > Suggested-by: Alexei Starovoitov > Signed-off-by: Harry Yoo (Oracle) > --- > mm/slub.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/mm/slub.c b/mm/slub.c > index b0d38d515386..6a3552b70683 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -6158,8 +6158,8 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj, bool allow_spin) > if (likely(rcu_sheaf->size < s->sheaf_capacity)) { > rcu_sheaf = NULL; > } else { > - if (unlikely(!allow_spin)) { > - /* call_rcu() cannot be called in an unknown context */ > + /* call_rcu() disables IRQs to protect percpu data structures */ > + if (unlikely(!allow_spin && irqs_disabled())) { Perhaps this could be more neatly exposed by RCU? something like maybe_in_call_rcu(). Also, it's a bit odd that you're introducing code and then subsequently patching over it with an improvement. Maybe this could be squashed into patch 3? > rcu_sheaf->size--; > local_unlock(&s->cpu_sheaves->lock); > goto fail; > > -- > 2.53.0 > -- Pedro