On 2/3/26 8:55 PM, Chen Ridong wrote:
On 2026/2/3 2:29, Waiman Long wrote:
On 2/1/26 8:11 PM, Chen Ridong wrote:
On 2026/2/1 7:13, Waiman Long wrote:
On 1/30/26 9:53 PM, Chen Ridong wrote:
On 2026/1/30 23:42, 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:
<TASK>
:
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 introduce a new top level cpuset_top_mutex
which will be acquired first. This new cpuset_top_mutex will provide
the need mutual exclusion without the need to hold cpus_read_lock().
Introducing a new global lock warrants careful consideration. I wonder if we
could make all updates to isolated_cpus asynchronous. If that is feasible, we
could avoid adding a global lock altogether. If not, we need to clarify which
updates must remain synchronous and which ones can be handled asynchronously.
Almost all the cpuset code are run with cpuset_mutex held with either
cpus_read_lock or cpus_write_lock. So there is no concurrent access/update to
any of the cpuset internal data. The new cpuset_top_mutex is aded to resolve the
possible deadlock scenarios with the new housekeeping_update() call without
breaking this model. Allow parallel concurrent access/update to cpuset data will
greatly complicate the code and we will likely missed some corner cases that we
I agree with that point. However, we already have paths where isolated_cpus is
updated asynchronously, meaning parallel concurrent access/update is already
happening. Therefore, we cannot entirely avoid such scenarios, so why not keep
the locking simple(make all updates to isolated_cpus asynchronous)?
isolated_cpus should only be updated in isolated_cpus_update() where both
cpuset_mutex and callback_lock are held. It can be read asynchronously if either
cpuset_mutex or callback_lock is held. Can you show me the places where this
rule isn't followed?
I was considering that since the hotplug path calls update_isolation_cpumasks
asynchronously, could other cpuset paths (such as setting CPUs or partitions)
also call update_isolation_cpumasks asynchronously? If so, the global
cpuset_top_mutex lock might be unnecessary. Note that isolated_cpus is updated
synchronously, while housekeeping_update is invoked asynchronously.
update_isolation_cpumasks() is always called synchronously as
cpuset_mutex will always be held. With the current patchset, the only
asynchronous piece is CPU hotplug vs the the housekeeping_update() call
as it is being called without holding cpus_read_lock(). AFASICS, it
should not be a problem. Please let me if you are aware of some
potential hazard with the current setup.
Cheers,
Longman
