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([2601:600:947f:f020:85dc:d2b2:c5ee:e3c4]) by smtp.gmail.com with ESMTPSA id af79cd13be357-8cb8d0614fbsm288082385a.17.2026.02.21.11.20.30 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 21 Feb 2026 11:20:32 -0800 (PST) From: Waiman Long X-Google-Original-From: Waiman Long Message-ID: <19ed2ed3-908e-421a-89a9-773e7f98524f@redhat.com> Date: Sat, 21 Feb 2026 14:20:29 -0500 Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 5/6] cgroup/cpuset: Call housekeeping_update() without holding cpus_read_lock To: Chen Ridong , Tejun Heo , Johannes Weiner , =?UTF-8?Q?Michal_Koutn=C3=BD?= , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , Frederic Weisbecker , Thomas Gleixner , Shuah Khan Cc: cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org References: <20260212164640.2408295-1-longman@redhat.com> <20260212164640.2408295-6-longman@redhat.com> Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2/13/26 2:47 AM, Chen Ridong wrote: > Hi Longman: > > On 2026/2/13 0:46, Waiman Long wrote: >> The current cpuset partition code is able to dynamically update >> the sched domains of a running system and the corresponding >> HK_TYPE_DOMAIN housekeeping cpumask to perform what is essentally the >> "isolcpus=domain,..." boot command line feature at run time. >> >> The housekeeping cpumask update requires flushing a number of different >> workqueues which may not be safe with cpus_read_lock() held as the >> workqueue flushing code may acquire cpus_read_lock() or acquiring locks >> which have locking dependency with cpus_read_lock() down the chain. Below >> is an example of such circular locking problem. >> >> ====================================================== >> WARNING: possible circular locking dependency detected >> 6.18.0-test+ #2 Tainted: G S >> ------------------------------------------------------ >> test_cpuset_prs/10971 is trying to acquire lock: >> ffff888112ba4958 ((wq_completion)sync_wq){+.+.}-{0:0}, at: touch_wq_lockdep_map+0x7a/0x180 >> >> but task is already holding lock: >> ffffffffae47f450 (cpuset_mutex){+.+.}-{4:4}, at: cpuset_partition_write+0x85/0x130 >> >> which lock already depends on the new lock. >> >> the existing dependency chain (in reverse order) is: >> -> #4 (cpuset_mutex){+.+.}-{4:4}: >> -> #3 (cpu_hotplug_lock){++++}-{0:0}: >> -> #2 (rtnl_mutex){+.+.}-{4:4}: >> -> #1 ((work_completion)(&arg.work)){+.+.}-{0:0}: >> -> #0 ((wq_completion)sync_wq){+.+.}-{0:0}: >> >> Chain exists of: >> (wq_completion)sync_wq --> cpu_hotplug_lock --> cpuset_mutex >> >> 5 locks held by test_cpuset_prs/10971: >> #0: ffff88816810e440 (sb_writers#7){.+.+}-{0:0}, at: ksys_write+0xf9/0x1d0 >> #1: ffff8891ab620890 (&of->mutex#2){+.+.}-{4:4}, at: kernfs_fop_write_iter+0x260/0x5f0 >> #2: ffff8890a78b83e8 (kn->active#187){.+.+}-{0:0}, at: kernfs_fop_write_iter+0x2b6/0x5f0 >> #3: ffffffffadf32900 (cpu_hotplug_lock){++++}-{0:0}, at: cpuset_partition_write+0x77/0x130 >> #4: ffffffffae47f450 (cpuset_mutex){+.+.}-{4:4}, at: cpuset_partition_write+0x85/0x130 >> >> Call Trace: >> >> : >> touch_wq_lockdep_map+0x93/0x180 >> __flush_workqueue+0x111/0x10b0 >> housekeeping_update+0x12d/0x2d0 >> update_parent_effective_cpumask+0x595/0x2440 >> update_prstate+0x89d/0xce0 >> cpuset_partition_write+0xc5/0x130 >> cgroup_file_write+0x1a5/0x680 >> kernfs_fop_write_iter+0x3df/0x5f0 >> vfs_write+0x525/0xfd0 >> ksys_write+0xf9/0x1d0 >> do_syscall_64+0x95/0x520 >> entry_SYSCALL_64_after_hwframe+0x76/0x7e >> >> To avoid such a circular locking dependency problem, we have to >> call housekeeping_update() without holding the cpus_read_lock() and >> cpuset_mutex. The current set of wq's flushed by housekeeping_update() >> may not have work functions that call cpus_read_lock() directly, >> but we are likely to extend the list of wq's that are flushed in the >> future. Moreover, the current set of work functions may hold locks that >> may have cpu_hotplug_lock down the dependency chain. >> >> One way to do that is to defer the housekeeping_update() call after >> the current cpuset critical section has finished without holding >> cpus_read_lock. For cpuset control file write, this can be done by >> deferring it using task_work right before returning to userspace. >> >> To enable mutual exclusion between the housekeeping_update() call and >> other cpuset control file write actions, a new top level cpuset_top_mutex >> is introduced. This new mutex will be acquired first to allow sharing >> variables used by both code paths. However, cpuset update from CPU >> hotplug can still happen in parallel with the housekeeping_update() >> call, though that should be rare in production environment. >> >> As cpus_read_lock() is now no longer held when >> tmigr_isolated_exclude_cpumask() is called, it needs to acquire it >> directly. >> >> The lockdep_is_cpuset_held() is also updated to return true if either >> cpuset_top_mutex or cpuset_mutex is held. >> >> Signed-off-by: Waiman Long >> --- >> kernel/cgroup/cpuset.c | 99 ++++++++++++++++++++++++++++++++--- >> kernel/sched/isolation.c | 4 +- >> kernel/time/timer_migration.c | 4 +- >> 3 files changed, 93 insertions(+), 14 deletions(-) >> >> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c >> index 48b7f275085b..c6a97956a991 100644 >> --- a/kernel/cgroup/cpuset.c >> +++ b/kernel/cgroup/cpuset.c >> @@ -65,14 +65,28 @@ static const char * const perr_strings[] = { >> * CPUSET Locking Convention >> * ------------------------- >> * >> - * Below are the three global locks guarding cpuset structures in lock >> + * Below are the four global/local locks guarding cpuset structures in lock >> * acquisition order: >> + * - cpuset_top_mutex >> * - cpu_hotplug_lock (cpus_read_lock/cpus_write_lock) >> * - cpuset_mutex >> * - callback_lock (raw spinlock) >> * >> - * A task must hold all the three locks to modify externally visible or >> - * used fields of cpusets, though some of the internally used cpuset fields >> + * As cpuset will now indirectly flush a number of different workqueues in >> + * housekeeping_update() to update housekeeping cpumasks when the set of >> + * isolated CPUs is going to be changed, it may be vulnerable to deadlock >> + * if we hold cpus_read_lock while calling into housekeeping_update(). >> + * >> + * The first cpuset_top_mutex will be held except when calling into >> + * cpuset_handle_hotplug() from the CPU hotplug code where cpus_write_lock >> + * and cpuset_mutex will be held instead. The main purpose of this mutex >> + * is to prevent regular cpuset control file write actions from interfering >> + * with the call to housekeeping_update(), though CPU hotplug operation can >> + * still happen in parallel. This mutex also provides protection for some >> + * internal variables. >> + * >> + * A task must hold all the remaining three locks to modify externally visible >> + * or used fields of cpusets, though some of the internally used cpuset fields >> * and internal variables can be modified without holding callback_lock. If only >> * reliable read access of the externally used fields are needed, a task can >> * hold either cpuset_mutex or callback_lock which are exposed to other >> @@ -100,6 +114,7 @@ static const char * const perr_strings[] = { >> * cpumasks and nodemasks. >> */ >> >> +static DEFINE_MUTEX(cpuset_top_mutex); >> static DEFINE_MUTEX(cpuset_mutex); >> >> /* >> @@ -111,6 +126,8 @@ static DEFINE_MUTEX(cpuset_mutex); >> * >> * CSCB: Readable by holding either cpuset_mutex or callback_lock. Writable >> * by holding both cpuset_mutex and callback_lock. >> + * >> + * T: Read/write-able by holding the cpuset_top_mutex. >> */ >> >> /* >> @@ -135,6 +152,18 @@ static cpumask_var_t isolated_cpus; /* CSCB */ >> */ >> static bool isolated_cpus_updating; /* RWCS */ >> >> +/* >> + * Copy of isolated_cpus to be passed to housekeeping_update() >> + */ >> +static cpumask_var_t isolated_hk_cpus; /* T */ >> + >> +/* >> + * Flag to prevent queuing more than one task_work to the same cpuset_top_mutex >> + * critical section. >> + */ >> +static bool isolcpus_twork_queued; /* T */ >> + >> + >> /* >> * A flag to force sched domain rebuild at the end of an operation. >> * It can be set in >> @@ -301,20 +330,24 @@ void lockdep_assert_cpuset_lock_held(void) >> */ >> void cpuset_full_lock(void) >> { >> + mutex_lock(&cpuset_top_mutex); >> cpus_read_lock(); >> mutex_lock(&cpuset_mutex); >> } >> >> void cpuset_full_unlock(void) >> { >> + isolcpus_twork_queued = false; > This is odd. > >> mutex_unlock(&cpuset_mutex); >> cpus_read_unlock(); >> + mutex_unlock(&cpuset_top_mutex); >> } >> >> #ifdef CONFIG_LOCKDEP >> bool lockdep_is_cpuset_held(void) >> { >> - return lockdep_is_held(&cpuset_mutex); >> + return lockdep_is_held(&cpuset_mutex) || >> + lockdep_is_held(&cpuset_top_mutex); >> } >> #endif >> >> @@ -1338,6 +1371,28 @@ static bool prstate_housekeeping_conflict(int prstate, struct cpumask *new_cpus) >> return false; >> } >> >> +/* >> + * housekeeping_update() will only be called if isolated_cpus differs >> + * from isolated_hk_cpus. To be safe, rebuild_sched_domains() will always >> + * be called just in case there are still pending sched domains changes. >> + */ >> +static void isolcpus_tworkfn(struct callback_head *cb) >> +{ >> + bool update_hk = true; >> + >> + guard(mutex)(&cpuset_top_mutex); >> + scoped_guard(spinlock_irq, &callback_lock) { >> + if (cpumask_equal(isolated_hk_cpus, isolated_cpus)) >> + update_hk = false; >> + else >> + cpumask_copy(isolated_hk_cpus, isolated_cpus); >> + } >> + if (update_hk) >> + WARN_ON_ONCE(housekeeping_update(isolated_hk_cpus) < 0); >> + rebuild_sched_domains(); >> + kfree(cb); >> +} >> + >> /* >> * update_isolation_cpumasks - Update external isolation related CPU masks >> * >> @@ -1346,15 +1401,42 @@ static bool prstate_housekeeping_conflict(int prstate, struct cpumask *new_cpus) >> */ >> static void update_isolation_cpumasks(void) >> { >> - int ret; >> + struct callback_head *twork_cb; >> >> if (!isolated_cpus_updating) >> return; >> + else >> + isolated_cpus_updating = false; >> + >> + /* >> + * CPU hotplug shouldn't set isolated_cpus_updating. >> + * >> + * To have better flexibility and prevent the possibility of deadlock, >> + * we defer the housekeeping_update() call to after the current cpuset >> + * critical section has finished. This is done via the synchronous >> + * task_work which will be executed right before returning to userspace. >> + * >> + * update_isolation_cpumasks() may be called more than once in the >> + * same cpuset_mutex critical section. >> + */ >> + lockdep_assert_held(&cpuset_top_mutex); >> + if (isolcpus_twork_queued) >> + return; >> >> - ret = housekeeping_update(isolated_cpus); >> - WARN_ON_ONCE(ret < 0); >> + twork_cb = kzalloc(sizeof(struct callback_head), GFP_KERNEL); >> + if (!twork_cb) >> + return; >> >> - isolated_cpus_updating = false; >> + /* >> + * isolcpus_tworkfn() will be invoked before returning to userspace >> + */ >> + init_task_work(twork_cb, isolcpus_tworkfn); >> + if (task_work_add(current, twork_cb, TWA_RESUME)) { >> + kfree(twork_cb); >> + WARN_ON_ONCE(1); /* Current task shouldn't be exiting */ >> + } else { >> + isolcpus_twork_queued = true; >> + } >> } >> > Actually, I find this function quite complex, with numerous global > variables to maintain. > > I'm considering whether we can simplify it. Could we just call > update_isolation_cpumasks() at the end of update_prstate(), > update_cpumask(), and update_exclusive_cpumask()? > > i.e. > > static void update_isolation_cpumasks(void) > { > struct callback_head twork_cb > > if (!isolated_cpus_updating) > return; > task_work_add(...) > isolated_cpus_updating = false; > } > > static int update_prstate(struct cpuset *cs, int new_prs) > { > ... > free_tmpmasks(&tmpmask); > update_isolation_cpumasks(); > return 0; > } > > For rebuilding scheduling domains, we could rebuild them during the > set operation only when force_sd_rebuild = true and > !isolated_cpus_updating. Otherwise, the rebuild would be deferred > and performed only once in isolcpus_tworkfn(). Yes, the v5 series make the code more complex, so now I am reverting back to a more simple way. Cheers, Longman