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Peter Anvin" , Andrew Morton , Dave Chinner , Qi Zheng , Roman Gushchin , Muchun Song , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , K Prateek Nayak , Uladzislau Rezki , David Hildenbrand , Lorenzo Stoakes , "Liam R . Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Kees Cook , Clark Williams , suleiman@google.com, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-rt-devel@lists.linux.dev Subject: Re: [RFC 06/10] Reclaim memory from blocked kernel stacks Message-ID: <20260831073619.Ssq-wt7s@linutronix.de> References: <20260827232948.2520558-1-stevensd@google.com> <20260827232948.2520558-7-stevensd@google.com> <20260828133620._x2XfJR_@linutronix.de> <20260828135947.GU776954@noisy.programming.kicks-ass.net> <20260829084901.GA776954@noisy.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: X-Stat-Signature: 4gth31ifenq5qo1uhkicbe95369msni9 X-Rspamd-Queue-Id: EBD2B100003 X-Rspamd-Server: rspam02 X-Rspam-User: X-HE-Tag: 1788161782-160474 X-HE-Meta: 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 OR5x00R/ OrXf9yjqw7vdVFh0UfKqES4Dci9RUgnkHZbtz6Hhpb8RRPLjDHcbsyhIfV/AW6yfyBgdbx6oeKk+XlRktP/fjHCanwGP6l08iRNzf7bSjQLYteimdrDlzOeGgSagum1Ew9WTc3xgCKE+Qd4wDQIspug4FkbWeL6plm+qk3jJqJL0tJ0anVDx2yRVC8BZrZqzDv3Lx Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2026-08-29 15:49:22 [+0100], Matthew Wilcox wrote: > On Sat, Aug 29, 2026 at 10:49:01AM +0200, Peter Zijlstra wrote: > > > On PREEMPT_RT, it is necessary to skip the call to > > > alloc_pages_nolock_noprof() from under pi_lock, since with that > > > configuration spin_trylock can end up needing to take pi_lock. But at > > > least on !PREEMPT_RT, there is no risk of deadlock. > > > > Yeah, which puts the lie to that horrific hack Alexei did. We should > > probably teach lockdep about spin_trylock() not being safe and see the > > house of cards crumble. > > Why is spin_trylock() unsafe on PREEMPT_RT? It's not intuitive since > one can mutex_trylock() in interrupt context or under spinlock. no, you shouldn't do mutex_trylock() in IRQ. There was something, I don't remember but that is the reason people use semaphores because it is okay to do down_trylock() in IRQ. It might be unlock path. Sebastian