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b=Ppunhs5PKu5CdT+XvNVUL7Rb5Z2CPCQLTrH/hyEU6ZLrfpnaoIxBlFJ8zdDIqRVZo SvxYhhrw9qHo6yB0Fw0wo98g5QjKkN3tbOqS88Obb4g8lWANkTd77KODHvL1gJAlIb pRPCjJaLUNt57zOR5HdQo3g9ZH6POfvt1ihde/LOIyCPAwe6VABEbvnHGNlwFeJ/j/ VdvyAQIYifTdJBPgeVSC52vQqQ0MbX22IyhMLKEXyUwsB09o/+12jvj1Wtry+GKxzX 9xQvTOWg/VrGEOEYHAQr/mgb41JarJb33o+FN9vIfLTxgPfWIG0leWcatULj0ORPjk fKk6KXw7nqMsA== Message-ID: Date: Mon, 31 Aug 2026 18:04:21 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] mm/slab: don't use kfree_rcu sheaves on PREEMPT_RT in kvfree_call_rcu() Content-Language: en-US To: Sebastian Andrzej Siewior , ThangNN99 Cc: Harry Yoo , Andrew Morton , Clark Williams , Steven Rostedt , 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 References: <20260831130057.HLukQ-zm@linutronix.de> <20260831133222.8637-1-ngocthang2710.1999@gmail.com> <20260831143500.x-saxdAs@linutronix.de> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: A28444000E X-Stat-Signature: poj7jtgabbqj4gmdfndg5hadm78mdfyg X-Rspam-User: X-HE-Tag: 1788192266-787089 X-HE-Meta: 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 wNO0FFTi cgtUdvcFbRq0tHWg6vSB8ZNgPnrYxJxIcGLm9tKi5moliiXekOnNC0Dep88kHPN7T5hk2ve3pf4FVRZnxFZTns7I7EAkilb6tv7P5x4FSwJx1S43FGwtqxwnAoKyCi7Bmh1sZc2aJYaQZ4Q2CYla22AFHI3UtbTmLj1BEOe51oe+dnBegKve9T43F/R9heIviVobAu9tfhSqzf8nyAqNby3x3Uyc4+y3Spyjq1CQcBmcJiFaq1HAokdp0JOTN1fqV35bQZ6Tnl17Gwmni1wX6Jq677CseWRHQXJgNRyAqyaBU8D4qgcBoInBu+p4maGUkoTQoCCb80CZTMQ1CiidLoVqVDiiPYNDmotDK0+m3WC2Caul7bsXSQhW6QPW8kPcbA+g0kCoun65JgSso7oSIHUhYlKhJsSqFp/hG5ujabgTHqDJTKzt6mbtFFLg+h59j6CQ2dYMS0VLMU7f62fLnbkp/VA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/31/26 16:35, Sebastian Andrzej Siewior wrote: > On 2026-08-31 20:32:22 [+0700], ThangNN99 wrote: >> --- a/mm/slab_common.c >> +++ b/mm/slab_common.c >> @@ -2034,7 +2027,14 @@ void kvfree_call_rcu(struct kvfree_rcu_head *head, void *ptr) >> if (!head) >> might_sleep(); >> >> - if (kfree_rcu_sheaf(ptr)) >> + /* >> + * Callers may hold a raw_spinlock_t here on PREEMPT_RT (e.g. >> + * set_cpus_allowed_force(), whose callers all hold >> + * task_struct::pi_lock), and the sheaf/barn locks are also taken >> + * as blocking locks elsewhere, so trying them here creates a >> + * lockdep-visible ordering conflict. Skip sheaves on PREEMPT_RT. >> + */ > > You mix up things. A raw_spinlock_t should work in general and should > not cause a problem. The task_struct::pi_lock is special: It used during > wakes and the inner waitlock of the rtmutex acquires it even during a > trylock. For PREEMPT_RT we don't want to acquire any locks while the > pi_lock is held. > > What about > > kvfree_rcu() is called by set_cpus_allowed_force() with > task_struct::pi_lock acquired. On PREEMPT_RT the local_trylock() > usage below will acquire the waitlock which must be avoided. > Therefore avoid it on PREEMPT_RT. Thanks. > Vlastimil? IMHO it's not useful to iterate a fix through emails with apparently a LLM agent, so let's continue here: https://lore.kernel.org/all/20260831-b4-kfree_rcu_hotfix-v1-1-4f0fb882638b@kernel.org/ > >> + if (!IS_ENABLED(CONFIG_PREEMPT_RT) && kfree_rcu_sheaf(ptr)) >> return; >> >> // Queue the object but don't yet schedule the batch. > > Sebastian