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Message-ID: <CAJZ5v0hFBAi7dNhQC1DJG1Fg44LaR-j-vsxO5=G8VbDj15Kr5Q@mail.gmail.com>
Date: Sat, 10 May 2025 13:41:29 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Seyediman Seyedarab <imandevel@...il.com>, Viresh Kumar <viresh.kumar@...aro.org>
Cc: linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
skhan@...uxfoundation.org, linux-kernel-mentees@...ts.linux.dev
Subject: Re: [PATCH v3] cpufreq: fix locking order in store_local_boost to
prevent deadlock
On Fri, May 9, 2025 at 7:03 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Fri, May 9, 2025 at 7:29 AM Seyediman Seyedarab <imandevel@...il.com> wrote:
> >
> > On 25/05/02 10:36AM, Viresh Kumar wrote:
> > > On 30-04-25, 12:09, Seyediman Seyedarab wrote:
> > > > Lockdep reports a possible circular locking dependency[1] when
> > > > writing to /sys/devices/system/cpu/cpufreq/policyN/boost,
> > > > triggered by power-profiles-daemon at boot.
> > > >
> > > > store_local_boost() used to acquire cpu_hotplug_lock *after*
> > > > the policy lock had already been taken by the store() handler.
> > > > However, the expected locking hierarchy is to acquire
> > > > cpu_hotplug_lock before the policy guard. This inverted lock order
> > > > creates a *theoretical* deadlock possibility.
> > > >
> > > > Acquire cpu_hotplug_lock in the store() handler *only* for the
> > > > local_boost attribute, before entering the policy guard block,
> > > > and remove the cpus_read_lock/unlock() calls from store_local_boost().
> > > > Also switch from guard() to scoped_guard() to allow explicitly wrapping
> > > > the policy guard inside the cpu_hotplug_lock critical section.
> > > >
> > > > [1]
> > > > ======================================================
> > > > WARNING: possible circular locking dependency detected
> > > > 6.15.0-rc4-debug #28 Not tainted
> > > > ------------------------------------------------------
> > > > power-profiles-/596 is trying to acquire lock:
> > > > ffffffffb147e910 (cpu_hotplug_lock){++++}-{0:0}, at: store_local_boost+0x6a/0xd0
> > > >
> > > > but task is already holding lock:
> > > > ffff9eaa48377b80 (&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+0x841/0xa00
> > > > cpufreq_add_dev+0x71/0x80
> > > > subsys_interface_register+0x14b/0x170
> > > > cpufreq_register_driver+0x154/0x250
> > > > amd_pstate_register_driver+0x36/0x70
> > > > amd_pstate_init+0x1e7/0x270
> > > > do_one_initcall+0x67/0x2c0
> > > > kernel_init_freeable+0x230/0x270
> > > > 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+0x83/0x170
> > > > cpufreq_register_driver+0x154/0x250
> > > > amd_pstate_register_driver+0x36/0x70
> > > > amd_pstate_init+0x1e7/0x270
> > > > do_one_initcall+0x67/0x2c0
> > > > kernel_init_freeable+0x230/0x270
> > > > kernel_init+0x15/0x130
> > > > ret_from_fork+0x2c/0x50
> > > > ret_from_fork_asm+0x11/0x20
> > > >
> > > > -> #0 (cpu_hotplug_lock){++++}-{0:0}:
> > > > __lock_acquire+0x1087/0x17e0
> > > > lock_acquire.part.0+0x66/0x1b0
> > > > cpus_read_lock+0x2a/0xc0
> > > > store_local_boost+0x6a/0xd0
> > > > store+0x50/0x90
> > > > kernfs_fop_write_iter+0x135/0x200
> > > > vfs_write+0x2ab/0x540
> > > > ksys_write+0x6c/0xe0
> > > > do_syscall_64+0xbb/0x1d0
> > > > entry_SYSCALL_64_after_hwframe+0x56/0x5e
> > > >
> > > > Signed-off-by: Seyediman Seyedarab <ImanDevel@...il.com>
> > > > ---
> > > > Changes in v3:
> > > > - Rebased over PM tree's linux-next branch
> > > > - Added a comment to explain why this piece of code is required
> > > > - Switched from guard() to scoped_guard() to allow explicitly wrapping
> > > > the policy guard inside the cpu_hotplug_lock critical section.
> > > >
> > > > Changes in v2:
> > > > - Restrict cpu_hotplug_lock acquisition to only
> > > > the local_boost attribute in store() handler.
> > > >
> > > > Regards,
> > > > Seyediman
> > > >
> > > > drivers/cpufreq/cpufreq.c | 23 ++++++++++++++++-------
> > > > 1 file changed, 16 insertions(+), 7 deletions(-)
> > > >
> > > > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> > > > index 21fa733a2..b349adbeb 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;
> > > >
> > > > @@ -1006,16 +1003,28 @@ static ssize_t store(struct kobject *kobj, struct attribute *attr,
> > > > {
> > > > struct cpufreq_policy *policy = to_policy(kobj);
> > > > struct freq_attr *fattr = to_attr(attr);
> > > > + int ret = -EBUSY;
> > > >
> > > > if (!fattr->store)
> > > > return -EIO;
> > > >
> > > > - guard(cpufreq_policy_write)(policy);
> > > > + /*
> > > > + * store_local_boost() requires cpu_hotplug_lock to be held, and must be
> > > > + * called with that lock acquired *before* taking policy->rwsem to avoid
> > > > + * lock ordering violations.
> > > > + */
> > > > + if (fattr == &local_boost)
> > > > + cpus_read_lock();
> > > >
> > > > - if (likely(!policy_is_inactive(policy)))
> > > > - return fattr->store(policy, buf, count);
> > > > + scoped_guard(cpufreq_policy_write, policy) {
> > > > + if (likely(!policy_is_inactive(policy)))
> > > > + ret = fattr->store(policy, buf, count);
> > > > + }
> > > >
> > > > - return -EBUSY;
> > > > + if (fattr == &local_boost)
> > > > + cpus_read_unlock();
> > > > +
> > > > + return ret;
> > > > }
> > >
> > > Acked-by: Viresh Kumar <viresh.kumar@...aro.org>
> > >
> > > --
> > > viresh
> >
> > Hi there,
> >
> > Just following up to see if there's anything you'd like me to
> > change or address in the patch before it can move forward.
> > Please let me know if any updates are needed.
>
> I'm kind of wondering why local_boost needs cpus_read_lock() at all.
> Holding the policy rwsem blocks CPU online/offline already for this
> policy.
>
> Is that because ->set_boost() may need to synchronize with the other policies?
IOW, what can go wrong if the cpus_read_lock() locking is dropped from
there altogether?
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