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Since commit a5b98009f16d ("sched/psi: > fix race between file release and pressure write"), pressure_write() > holds cgroup_mutex across psi_trigger_create(), which may call > kthread_create() for the psimon kthread. kthreadd's fork then enters > scx_pre_fork() and waits for the read side of scx_fork_rwsem. > > This results in a deadlock. The enable worker holds scx_fork_rwsem and > waits for cgroup_mutex, while the PSI writer holds cgroup_mutex and > waits for psimon creation to complete. Any concurrent fork blocks on > scx_pre_fork() behind the enable worker. > > The hung-task detector captured all three sides of the deadlock: > > scx_enable_help: > __mutex_lock > scx_enable_workfn > kthread_worker_fn > > systemd: > wait_for_completion_killable > __kthread_create_on_node > kthread_create_on_node > psi_trigger_create > pressure_write > kernfs_fop_write_iter > > python3: > percpu_rwsem_wait > __percpu_down_read > scx_pre_fork > sched_fork > copy_process > kernel_clone > > It also identified systemd as the likely owner of the mutex on which > scx_enable_help was blocked. > > We reproduced this on a 128-CPU AMD EPYC 7713 by enabling scx_lavd > concurrently with writes to cgroup PSI trigger files. Unrelated tasks > piled up in scx_pre_fork() and process creation on the box stopped. > > Fix the inversion by acquiring cgroup_mutex before scx_fork_rwsem in > scx_root_enable_workfn() and releasing them in reverse order, while > preserving the existing exclusion around cgroup and task initialisation. > > Fixes: a5b98009f16d ("sched/psi: fix race between file release and pressure write") > Cc: stable@vger.kernel.org > Signed-off-by: Matt Fleming This seems to introduce the following (running the sched_ext kselftests): [ 28.963575] ====================================================== [ 28.963670] WARNING: possible circular locking dependency detected [ 28.963752] 7.1.0-virtme #1 Not tainted [ 28.963804] ------------------------------------------------------ [ 28.963887] sched_ext_helpe/2619 is trying to acquire lock: [ 28.963954] ffffffff83685240 (scx_cgroup_ops_rwsem){+.+.}-{0:0}, at: scx_root_disable+0x45d/0x840 [ 28.964071] [ 28.964071] but task is already holding lock: [ 28.964151] ffffffff83682910 (scx_fork_rwsem){++++}-{0:0}, at: scx_root_disable+0x160/0x840 [ 28.964260] [ 28.964260] which lock already depends on the new lock. [ 28.964260] [ 28.964355] [ 28.964355] the existing dependency chain (in reverse order) is: [ 28.964455] [ 28.964455] -> #2 (scx_fork_rwsem){++++}-{0:0}: [ 28.964539] percpu_down_write+0x49/0x150 [ 28.964610] scx_root_enable_workfn+0x5d0/0xd30 [ 28.964679] kthread_worker_fn+0x121/0x360 [ 28.964749] kthread+0x10c/0x140 [ 28.964802] ret_from_fork+0x189/0x330 [ 28.964872] ret_from_fork_asm+0x1a/0x30 [ 28.964941] [ 28.964941] -> #1 (cgroup_mutex){+.+.}-{4:4}: [ 28.965025] __mutex_lock+0xbe/0xd80 [ 28.965070] scx_root_enable_workfn+0x5c4/0xd30 [ 28.965138] kthread_worker_fn+0x121/0x360 [ 28.965210] kthread+0x10c/0x140 [ 28.965263] ret_from_fork+0x189/0x330 [ 28.965331] ret_from_fork_asm+0x1a/0x30 [ 28.965402] [ 28.965402] -> #0 (scx_cgroup_ops_rwsem){+.+.}-{0:0}: [ 28.965485] __lock_acquire+0x14c5/0x2a40 [ 28.965554] lock_acquire+0xd3/0x280 [ 28.965608] percpu_down_write+0x49/0x150 [ 28.965677] scx_root_disable+0x45d/0x840 [ 28.965745] kthread_worker_fn+0x121/0x360 [ 28.965815] kthread+0x10c/0x140 [ 28.965867] ret_from_fork+0x189/0x330 [ 28.965935] ret_from_fork_asm+0x1a/0x30 [ 28.966002] [ 28.966002] other info that might help us debug this: [ 28.966002] [ 28.966097] Chain exists of: [ 28.966097] scx_cgroup_ops_rwsem --> cgroup_mutex --> scx_fork_rwsem [ 28.966097] [ 28.966233] Possible unsafe locking scenario: [ 28.966233] [ 28.966314] CPU0 CPU1 [ 28.966379] ---- ---- [ 28.966444] lock(scx_fork_rwsem); [ 28.966496] lock(cgroup_mutex); [ 28.966579] lock(scx_fork_rwsem); [ 28.966661] lock(scx_cgroup_ops_rwsem); [ 28.966713] [ 28.966713] *** DEADLOCK *** [ 28.966713] [ 28.966794] 2 locks held by sched_ext_helpe/2619: [ 28.966861] #0: ffffffff83682818 (scx_enable_mutex){+.+.}-{4:4}, at: scx_root_disable+0xba/0x840 [ 28.966974] #1: ffffffff83682910 (scx_fork_rwsem){++++}-{0:0}, at: scx_root_disable+0x160/0x840 [ 28.967090] [ 28.967090] stack backtrace: [ 28.967158] CPU: 8 UID: 0 PID: 2619 Comm: sched_ext_helpe Not tainted 7.1.0-virtme #1 PREEMPT(full) [ 28.967164] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011 [ 28.967166] Call Trace: [ 28.967169] [ 28.967172] dump_stack_lvl+0x6d/0xa0 [ 28.967175] print_circular_bug+0x2e1/0x300 [ 28.967179] check_noncircular+0x144/0x170 [ 28.967182] __lock_acquire+0x14c5/0x2a40 [ 28.967202] ? scx_root_disable+0x45d/0x840 [ 28.967204] lock_acquire+0xd3/0x280 [ 28.967207] ? scx_root_disable+0x45d/0x840 [ 28.967209] percpu_down_write+0x49/0x150 [ 28.967212] ? scx_root_disable+0x45d/0x840 [ 28.967213] scx_root_disable+0x45d/0x840 [ 28.967219] ? kthread_worker_fn+0x51/0x360 [ 28.967221] kthread_worker_fn+0x121/0x360 [ 28.967223] ? __pfx_scx_disable_workfn+0x10/0x10 [ 28.967224] ? __pfx_kthread_worker_fn+0x10/0x10 [ 28.967227] kthread+0x10c/0x140 [ 28.967228] ? __pfx_kthread+0x10/0x10 [ 28.967229] ret_from_fork+0x189/0x330 [ 28.967231] ? __pfx_kthread+0x10/0x10 [ 28.967232] ret_from_fork_asm+0x1a/0x30 [ 28.967235] Thanks, -Andrea > --- > kernel/sched/ext/ext.c | 8 +++++--- > 1 file changed, 5 insertions(+), 3 deletions(-) > > diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c > index 691d53fe0f64..ba89eafe7964 100644 > --- a/kernel/sched/ext/ext.c > +++ b/kernel/sched/ext/ext.c > @@ -7193,7 +7193,10 @@ static void scx_root_enable_workfn(struct kthread_work *work) > /* > * Lock out forks, cgroup on/offlining and moves before opening the > * floodgate so that they don't wander into the operations prematurely. > + * cgroup_mutex must nest outside scx_fork_rwsem because cgroup file > + * operations may create kthreads while holding cgroup_mutex. > */ > + scx_cgroup_lock(); > percpu_down_write(&scx_fork_rwsem); > > WARN_ON_ONCE(scx_init_task_enabled); > @@ -7216,7 +7219,6 @@ static void scx_root_enable_workfn(struct kthread_work *work) > * while tasks are being initialized so that scx_cgroup_can_attach() > * never sees uninitialized tasks. > */ > - scx_cgroup_lock(); > set_cgroup_sched(sch_cgroup(sch), sch); > ret = scx_cgroup_init(sch); > if (ret) > @@ -7283,8 +7285,8 @@ static void scx_root_enable_workfn(struct kthread_work *work) > put_task_struct(p); > } > scx_task_iter_stop(&sti); > - scx_cgroup_unlock(); > percpu_up_write(&scx_fork_rwsem); > + scx_cgroup_unlock(); > > /* > * All tasks are READY. It's safe to turn on scx_enabled() and switch > @@ -7369,8 +7371,8 @@ static void scx_root_enable_workfn(struct kthread_work *work) > return; > > err_disable_unlock_all: > - scx_cgroup_unlock(); > percpu_up_write(&scx_fork_rwsem); > + scx_cgroup_unlock(); > /* we'll soon enter disable path, keep bypass on */ > err_disable: > mutex_unlock(&scx_enable_mutex); > -- > 2.43.0 >