From a84097e625f2b9e7f273161c004f34b7be63b348 Mon Sep 17 00:00:00 2001 From: Waiman Long Date: Sat, 21 Feb 2026 13:54:18 -0500 Subject: [PATCH] cgroup/cpuset: Call housekeeping_update() without holding cpus_read_lock 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 essentially 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. So housekeeping_update() is now called after releasing cpus_read_lock and cpuset_mutex at the end of a cpuset operation. These two locks are then re-acquired later before calling rebuild_sched_domains_locked(). 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. Fixes: 03ff73510169 ("cpuset: Update HK_TYPE_DOMAIN cpumask from cpuset") Signed-off-by: Waiman Long Signed-off-by: Tejun Heo --- kernel/cgroup/cpuset.c | 47 +++++++++++++++++++++++++++++++---- kernel/sched/isolation.c | 4 +-- kernel/time/timer_migration.c | 4 +-- 3 files changed, 44 insertions(+), 11 deletions(-) diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c index aa915e9b588f..aa45351c6f38 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. */ /* @@ -134,6 +151,11 @@ static cpumask_var_t isolated_cpus; /* CSCB */ */ static bool update_housekeeping; /* RWCS */ +/* + * Copy of isolated_cpus to be passed to housekeeping_update() + */ +static cpumask_var_t isolated_hk_cpus; /* T */ + /* * A flag to force sched domain rebuild at the end of an operation. * It can be set in @@ -297,6 +319,7 @@ void lockdep_assert_cpuset_lock_held(void) */ void cpuset_full_lock(void) { + mutex_lock(&cpuset_top_mutex); cpus_read_lock(); mutex_lock(&cpuset_mutex); } @@ -305,12 +328,14 @@ void cpuset_full_unlock(void) { 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 @@ -1315,9 +1340,20 @@ static void update_hk_sched_domains(void) { if (update_housekeeping) { /* Updating HK cpumasks implies rebuild sched domains */ - WARN_ON_ONCE(housekeeping_update(isolated_cpus)); update_housekeeping = false; force_sd_rebuild = true; + cpumask_copy(isolated_hk_cpus, isolated_cpus); + + /* + * housekeeping_update() is now called without holding + * cpus_read_lock and cpuset_mutex. Only cpuset_top_mutex + * is still being held for mutual exclusion. + */ + mutex_unlock(&cpuset_mutex); + cpus_read_unlock(); + WARN_ON_ONCE(housekeeping_update(isolated_hk_cpus)); + cpus_read_lock(); + mutex_lock(&cpuset_mutex); } /* force_sd_rebuild will be cleared in rebuild_sched_domains_locked() */ if (force_sd_rebuild) @@ -3635,6 +3671,7 @@ int __init cpuset_init(void) BUG_ON(!alloc_cpumask_var(&top_cpuset.exclusive_cpus, GFP_KERNEL)); BUG_ON(!zalloc_cpumask_var(&subpartitions_cpus, GFP_KERNEL)); BUG_ON(!zalloc_cpumask_var(&isolated_cpus, GFP_KERNEL)); + BUG_ON(!zalloc_cpumask_var(&isolated_hk_cpus, GFP_KERNEL)); cpumask_setall(top_cpuset.cpus_allowed); nodes_setall(top_cpuset.mems_allowed); diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c index 3b725d39c06e..ef152d401fe2 100644 --- a/kernel/sched/isolation.c +++ b/kernel/sched/isolation.c @@ -123,8 +123,6 @@ int housekeeping_update(struct cpumask *isol_mask) struct cpumask *trial, *old = NULL; int err; - lockdep_assert_cpus_held(); - trial = kmalloc(cpumask_size(), GFP_KERNEL); if (!trial) return -ENOMEM; @@ -136,7 +134,7 @@ int housekeeping_update(struct cpumask *isol_mask) } if (!housekeeping.flags) - static_branch_enable_cpuslocked(&housekeeping_overridden); + static_branch_enable(&housekeeping_overridden); if (housekeeping.flags & HK_FLAG_DOMAIN) old = housekeeping_cpumask_dereference(HK_TYPE_DOMAIN); diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c index 6da9cd562b20..83428aa03aef 100644 --- a/kernel/time/timer_migration.c +++ b/kernel/time/timer_migration.c @@ -1559,8 +1559,6 @@ int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask) cpumask_var_t cpumask __free(free_cpumask_var) = CPUMASK_VAR_NULL; int cpu; - lockdep_assert_cpus_held(); - if (!works) return -ENOMEM; if (!alloc_cpumask_var(&cpumask, GFP_KERNEL)) @@ -1570,6 +1568,7 @@ int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask) * First set previously isolated CPUs as available (unisolate). * This cpumask contains only CPUs that switched to available now. */ + guard(cpus_read_lock)(); cpumask_andnot(cpumask, cpu_online_mask, exclude_cpumask); cpumask_andnot(cpumask, cpumask, tmigr_available_cpumask); @@ -1626,7 +1625,6 @@ static int __init tmigr_init_isolation(void) cpumask_andnot(cpumask, cpu_possible_mask, housekeeping_cpumask(HK_TYPE_DOMAIN)); /* Protect against RCU torture hotplug testing */ - guard(cpus_read_lock)(); return tmigr_isolated_exclude_cpumask(cpumask); } late_initcall(tmigr_init_isolation);