linux-kernel - Re: [PATCH v2] rcu: Remove redundant rcu_read_lock/unlock() in spin

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Message-ID: <69af79b6-a4f2-4007-aef4-fcaeeadd3f99@redhat.com>
Date: Mon, 15 Sep 2025 10:13:26 -0400
From: Waiman Long <llong@...hat.com>
To: pengdonglin <dolinux.peng@...il.com>, tj@...nel.org, tony.luck@...el.com,
 jani.nikula@...ux.intel.com, ap420073@...il.com, jv@...sburgh.net,
 freude@...ux.ibm.com, bcrl@...ck.org, trondmy@...nel.org, kees@...nel.org
Cc: bigeasy@...utronix.de, linux-kernel@...r.kernel.org,
 linux-rt-devel@...ts.linux.dev, linux-nfs@...r.kernel.org,
 linux-aio@...ck.org, linux-fsdevel@...r.kernel.org,
 linux-security-module@...r.kernel.org, netdev@...r.kernel.org,
 intel-gfx@...ts.freedesktop.org, linux-acpi@...r.kernel.org,
 linux-s390@...r.kernel.org, cgroups@...r.kernel.org,
 Hillf Danton <hdanton@...a.com>, "Paul E . McKenney" <paulmck@...nel.org>,
 pengdonglin <pengdonglin@...omi.com>
Subject: Re: [PATCH v2] rcu: Remove redundant rcu_read_lock/unlock() in
 spin_lock critical sections


On 9/15/25 9:47 AM, pengdonglin wrote:
> From: pengdonglin <pengdonglin@...omi.com>
>
> Per Documentation/RCU/rcu_dereference.rst [1], since Linux 4.20's RCU
> consolidation [2][3], RCU read-side critical sections include:
>    - Explicit rcu_read_lock()
>    - BH/interrupt/preemption-disabling regions
>    - Spinlock critical sections (including CONFIG_PREEMPT_RT kernels [4])
>
> Thus, explicit rcu_read_lock()/unlock() calls within spin_lock*() regions are redundant.
> This patch removes them, simplifying locking semantics while preserving RCU protection.
>
> [1] https://elixir.bootlin.com/linux/v6.17-rc5/source/Documentation/RCU/rcu_dereference.rst#L407
> [2] https://lore.kernel.org/lkml/20180829222021.GA29944@linux.vnet.ibm.com/
> [3] https://lwn.net/Articles/777036/
> [4] https://lore.kernel.org/lkml/6435833a-bdcb-4114-b29d-28b7f436d47d@paulmck-laptop/
>
> Cc: Waiman Long <longman@...hat.com>
> Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Cc: Hillf Danton <hdanton@...a.com>
> Cc: Paul E. McKenney <paulmck@...nel.org>
> Signed-off-by: pengdonglin <pengdonglin@...omi.com>
> Signed-off-by: pengdonglin <dolinux.peng@...il.com>
> ---
> Changes in v2:
>    - Clarified commit message to prevent accidental backport to older kernels
>    - Added lockdep_is_held() to avoid false positives
> ---
>   drivers/acpi/apei/ghes.c                        | 2 --
>   drivers/gpu/drm/i915/gt/intel_ring_submission.c | 2 --
>   drivers/net/amt.c                               | 8 --------
>   drivers/net/bonding/bond_3ad.c                  | 2 --
>   drivers/net/wireless/ath/ath9k/xmit.c           | 2 --
>   drivers/s390/crypto/pkey_base.c                 | 3 ---
>   fs/aio.c                                        | 6 ++----
>   fs/nfs/callback_proc.c                          | 2 --
>   fs/nfs/nfs4state.c                              | 2 --
>   fs/nfs/pnfs.c                                   | 9 ---------
>   fs/nfs/pnfs_dev.c                               | 4 ----
>   ipc/msg.c                                       | 1 -
>   ipc/sem.c                                       | 1 -
>   ipc/shm.c                                       | 1 -
>   ipc/util.c                                      | 2 --
>   kernel/cgroup/cgroup.c                          | 2 --
>   kernel/cgroup/cpuset.c                          | 6 ------
>   kernel/cgroup/debug.c                           | 4 ----
>   net/mac80211/cfg.c                              | 2 --
>   net/mac80211/debugfs.c                          | 2 --
>   net/mac80211/debugfs_netdev.c                   | 2 --
>   net/mac80211/debugfs_sta.c                      | 2 --
>   net/mac80211/sta_info.c                         | 2 --
>   net/ncsi/ncsi-manage.c                          | 2 --
>   security/yama/yama_lsm.c                        | 4 ----
>   25 files changed, 2 insertions(+), 73 deletions(-)
>
  ...
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 27adb04df675..9b7e8e8e9411 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -4073,7 +4073,6 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
>   	struct cpuset *cs;
>   
>   	spin_lock_irqsave(&callback_lock, flags);
> -	rcu_read_lock();
>   
>   	cs = task_cs(tsk);
>   	if (cs != &top_cpuset)
> @@ -4095,7 +4094,6 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
>   			cpumask_copy(pmask, possible_mask);
>   	}
>   
> -	rcu_read_unlock();
>   	spin_unlock_irqrestore(&callback_lock, flags);
>   }
>   
> @@ -4168,9 +4166,7 @@ nodemask_t cpuset_mems_allowed(struct task_struct *tsk)
>   	unsigned long flags;
>   
>   	spin_lock_irqsave(&callback_lock, flags);
> -	rcu_read_lock();
>   	guarantee_online_mems(task_cs(tsk), &mask);
> -	rcu_read_unlock();
>   	spin_unlock_irqrestore(&callback_lock, flags);
>   
>   	return mask;
> @@ -4265,10 +4261,8 @@ bool cpuset_current_node_allowed(int node, gfp_t gfp_mask)
>   	/* Not hardwall and node outside mems_allowed: scan up cpusets */
>   	spin_lock_irqsave(&callback_lock, flags);
>   
> -	rcu_read_lock();
>   	cs = nearest_hardwall_ancestor(task_cs(current));
>   	allowed = node_isset(node, cs->mems_allowed);
> -	rcu_read_unlock();
>   
>   	spin_unlock_irqrestore(&callback_lock, flags);
>   	return allowed;

For cpuset,

Acked-by: Waiman Long <longman@...hat.com>