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b=XuUW4AFUy2dwHWVZf12krJbN9Ni/p6KdoXKlSOpUg42W5KpGAQwTxfimPF3TPllJ0 6uQbZkh5TYAx0j228akBJyGcEcPjzkmp2GTnDZHYNqte5BnL/vW/TMKAlYjGHiWbEp WhDPsiiQ5uSkyp4oHeUlhrwstVsEpTthDvbp0zO8pH78F88UsyGBxMtb+W2PUD2+0V 9A65BzsNs8wr7gvoFG/bUken5QZPbXpZtnQSwSokJZ1IEHFG4E4IvIkCIJa6FyV8LL tAvxNarFvgv6cuOrMyxT8vzFGOmEegIyQCAA3s1DR5VmOrZqwYu4GFmf9KOIemEDsV y9R2aBMcHi6BA== Date: Wed, 2 Sep 2026 11:05:18 +0100 From: Harry Yoo To: Sebastian Andrzej Siewior Cc: ThangNN99 , Vlastimil Babka , 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, ast@kernel.org, puranjay@kernel.org Subject: Re: [PATCH v3] mm/slab: don't use kfree_rcu sheaves on PREEMPT_RT in kvfree_call_rcu() Message-ID: References: <20260831130057.HLukQ-zm@linutronix.de> <20260831133222.8637-1-ngocthang2710.1999@gmail.com> <20260831143500.x-saxdAs@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260831143500.x-saxdAs@linutronix.de> X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 73B6D4000E X-Stat-Signature: 87eiohhe1cwx1h9jje7roz5ncq7w4t98 X-Rspam-User: X-HE-Tag: 1788343522-43663 X-HE-Meta: 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 /ru6/t2C 1Nr/CEU8DBfUVPqKTfDgHBUw+DAue2BpNPW4BYWtMSa0Mz2TxTbJTcdePh2AokGA5XqvuKe1U9SDJe7J/grM8fSoIzYWtMfKl583qvypHmhzj/RSNgeQCaPvudHRJSTDaByaeySecI8WUtwp22uglzrvRX8ov2sQh2PGPg4IGaW9nXI3VSl+ipQbY0kjqLtmmRjfieS1IUktViNQN7DhkV44dvo6B02F34yMpBHscNmN6Ko+veNZOrR8J8pWR17ak5viI3Gg9haJRMVxl1lW7KYcFnqQvgEI8wWAMWllxChPT8eMH7wiJcVeRnuDxr85db2DUxdTYHDiRdTR2rZQiLXdbt+OlPn3py//k7qPe4R+U3vzEzQ1jzhXLm61mmi5CuM861pluhjvQZ30mwrnNQCyIHVYu9zfoyWin1C349BtAYjhSCm5GbPSlPw== 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 04:35:00PM +0200, 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. So... even can_spin_trylock() is not enough handle the pi_lock's special case. Probably we should make can_spin_trylock() check if current tasks's pi_lock is acquired, to determine if it's safe to call _trylock()? > 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. -- Cheers, Harry / Hyeonggon