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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" , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Kees Cook , Clark Williams , suleiman@google.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-rt-devel@lists.linux.dev Subject: Re: [RFC 06/10] Reclaim memory from blocked kernel stacks Message-ID: <20260831141909.WrGjMogw@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> <20260831073619.Ssq-wt7s@linutronix.de> <0849e5ca-6dec-4cf9-83ac-89ab08364044@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <0849e5ca-6dec-4cf9-83ac-89ab08364044@kernel.org> X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 22ED2100003 X-Stat-Signature: tgffrmommm1c1z67p7nu89aa7jwbpgab X-Rspam-User: X-HE-Tag: 1788185954-817220 X-HE-Meta: U2FsdGVkX1/ZQMrMcYxxwNY5habDBtY6KvwNxds/HyFzQ/R1tAnNGegWofX8LvAZShdOG7tyB4F9As6+uFkaybK60Rb6gHmdRkKVfRkvBAd3Ud/sNyASFrz4igQQk17KMx3jUpmccGKIp0yOCKJhUqvw43OsH2sLl31u0JV4ZZ4Ky/7MLg5IF8tPWV9dZntZgiZQWOsrrJwce5nxCqpq6kDaOHUk5va1LYfqDnfObuuHsiNuPtUw9hGvWuU3qB6xEg+J1/jJhFvBuXclAGkREY14N1wkpnbNnXzu78IKMnDtZA7Zpd/5sJGDyWuzKKn2B9V2gqFTYO8DU4KKVVt6KlJJYNsU1rS655vwyJ6+w/518WAD+jatUbknb550SXs3rbLJRWzM6YIVPVEBdL7NhL7c6WuSRsALrSnuLkvuPMNon4+FPKsk6yfs4cMCm7v4wQZYMPuSO3jrWOBiKVNh2ZhS6t3S+qzw285WVgtBiWvqCNNXAlB/klH7Y3xGHufjZAdBF3ZKYlncbftKgJ7z6RaspmjZg4CqRrrpVbWvhcIXus8gQd5x7L4uJt1DMdh4xOmAviCTqza1ExbnvNPX9ao9VmsH/TQWnthF8bg61tZsxXfexmpcyj1DYcxFOlFgN7lQ9PUZdRZk39iDdP8H955AP5O8YvUDgpz7XK4VQHxvx9Fye9q1/aO8G/JyBqi+LoieHj0Gj+DF9asDiMjY80cOHtrjAfRw7AkzbIF/79u0Nmz4UMc1K25mhg6PWYUnKcbeYZDSclOr6I5E4jr/Oa/yS4A7WJlT39AVW3ZASrSXOwSZuwyQ2dIFVzkYXYiHvdzE+uwD68LX9cEWPy8doeSBibxv6+dYfmWgSc5zhHG+bBZDD/QoRR6aw7FM326ifotBV4icceSxrboWLIjyNBbUgu3qTVScPGH/FireKGxlbfxoGGqXrD3+3ISv17Zwd5vv/dp/APFg5Xxwe/U hmHHSkyb S+mDPyYZU7wb/ipbk8q8JnH6Vu+jB0DhbsvVV4+bU9+VeDRljymxT9v349CsGXj6R6cYxBzXebZSnIVUehhgSySedoJkEnjddIn2uis9sUWKZuQk0vseokc+QJZ8uvh6Qlz1QmNhVipLbyZVYE9uI+DvmpEZ6ZOlwQGMwO6Zz+8AQLQHiGP7UcAxJCH7N9f/pLtym30c7/teQECjz5tzt6QJLlAR9vjfNol3vnmenNl8QMLQ= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2026-08-31 16:03:38 [+0200], Vlastimil Babka (SUSE) wrote: > On 8/31/26 09:36, Sebastian Andrzej Siewior wrote: > > 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. > > Is it already happening then? > > https://lore.kernel.org/all/20260831053846.107974-1-ngocthang2710.1999@gmail.com/ here I would strip kfree_rcu_sheaf() from the kfree_rcu(). > >> 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. > > Is it what this comment (recently moved to can_spin_trylock()) explains? > > /* > * In PREEMPT_RT spin_trylock() will call raw_spin_lock() which is > * unsafe in NMI. If spin_trylock() is called from hard IRQ the current > * task may be waiting for one rt_spin_lock, but rt_spin_trylock() will > * mark the task as the owner of another rt_spin_lock which will > * confuse PI logic, so return immediately if called from hard IRQ or > * NMI. > * > * Note, irqs_disabled() case is ok. spin_trylock() can be called > * from raw_spin_lock_irqsave region. > */ > if (IS_ENABLED(CONFIG_PREEMPT_RT) && (in_nmi() || in_hardirq())) > return false; > > Of course checking for being under raw_spinlock_t can't be added like this, > hm. Yes. Sebastian