linux-kernel - Re: [PATCH] cpufreq: drop redundant cpus_read_lock() from store_local

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Message-ID: <CAJZ5v0gS72-yJ2=EwEb4qz2m47RAhU1Va+UnGNJAjTsT1AJiQQ@mail.gmail.com>
Date: Wed, 14 May 2025 14:43:42 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Seyediman Seyedarab <imandevel@...il.com>
Cc: rafael@...nel.org, viresh.kumar@...aro.org, linux-pm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, skhan@...uxfoundation.org, 
	linux-kernel-mentees@...ts.linux.dev
Subject: Re: [PATCH] cpufreq: drop redundant cpus_read_lock() from store_local_boost()

On Tue, May 13, 2025 at 3:55 AM Seyediman Seyedarab <imandevel@...il.com> wrote:
>
> Lockdep reports a possible circular locking dependency[1] when
> cpu_hotplug_lock is acquired inside store_local_boost(), after
> policy->rwsem has already been taken by store().
>
> However, the boost update is strictly per-policy and does not
> access shared state or iterate over all policies. Since policy->rwsem
> is already held, this is enough to serialize against concurrent
> topology changes for the current policy.
>
> Remove the cpus_read_lock() to resolve the lockdep warning and
> avoid unnecessary locking.
>
>  [1]
>  ======================================================
>  WARNING: possible circular locking dependency detected
>  6.15.0-rc6-debug-gb01fc4eca73c #1 Not tainted
>  ------------------------------------------------------
>  power-profiles-/588 is trying to acquire lock:
>  ffffffffb3a7d910 (cpu_hotplug_lock){++++}-{0:0}, at: store_local_boost+0x56/0xd0
>
>  but task is already holding lock:
>  ffff8b6e5a12c380 (&policy->rwsem){++++}-{4:4}, at: store+0x37/0x90
>
>  which lock already depends on the new lock.
>
>  the existing dependency chain (in reverse order) is:
>
>  -> #2 (&policy->rwsem){++++}-{4:4}:
>         down_write+0x29/0xb0
>         cpufreq_online+0x7e8/0xa40
>         cpufreq_add_dev+0x82/0xa0
>         subsys_interface_register+0x148/0x160
>         cpufreq_register_driver+0x15d/0x260
>         amd_pstate_register_driver+0x36/0x90
>         amd_pstate_init+0x1e7/0x270
>         do_one_initcall+0x68/0x2b0
>         kernel_init_freeable+0x231/0x270
>         kernel_init+0x15/0x130
>         ret_from_fork+0x2c/0x50
>         ret_from_fork_asm+0x11/0x20
>
>  -> #1 (subsys mutex#3){+.+.}-{4:4}:
>         __mutex_lock+0xc2/0x930
>         subsys_interface_register+0x7f/0x160
>         cpufreq_register_driver+0x15d/0x260
>         amd_pstate_register_driver+0x36/0x90
>         amd_pstate_init+0x1e7/0x270
>         do_one_initcall+0x68/0x2b0
>         kernel_init_freeable+0x231/0x270
>         kernel_init+0x15/0x130
>         ret_from_fork+0x2c/0x50
>         ret_from_fork_asm+0x11/0x20
>
>  -> #0 (cpu_hotplug_lock){++++}-{0:0}:
>         __lock_acquire+0x10ed/0x1850
>         lock_acquire.part.0+0x69/0x1b0
>         cpus_read_lock+0x2a/0xc0
>         store_local_boost+0x56/0xd0
>         store+0x50/0x90
>         kernfs_fop_write_iter+0x132/0x200
>         vfs_write+0x2b3/0x590
>         ksys_write+0x74/0xf0
>         do_syscall_64+0xbb/0x1d0
>         entry_SYSCALL_64_after_hwframe+0x56/0x5e
>
> Signed-off-by: Seyediman Seyedarab <ImanDevel@...il.com>
> ---
>  drivers/cpufreq/cpufreq.c | 3 ---
>  1 file changed, 3 deletions(-)
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 731ecfc17..759dd178a 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -622,10 +622,7 @@ static ssize_t store_local_boost(struct cpufreq_policy *policy,
>         if (!policy->boost_supported)
>                 return -EINVAL;
>
> -       cpus_read_lock();
>         ret = policy_set_boost(policy, enable);
> -       cpus_read_unlock();
> -
>         if (!ret)
>                 return count;
>
> --

Applied as 6.16 material, thanks!