[PATCH] sched_ext: Fix deadlock with PSI trigger creation

[PATCH] sched_ext: Fix deadlock with PSI trigger creation

[Matt Fleming]

From: Matt Fleming <mfleming@cloudflare.com>

scx_root_enable_workfn() currently takes scx_fork_rwsem for writing
before acquiring cgroup_mutex. 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 <mfleming@cloudflare.com>
---
 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

Re: [PATCH] sched_ext: Fix deadlock with PSI trigger creation

[Andrea Righi]

Hi Matt,

On Fri, Jul 10, 2026 at 11:04:41AM +0100, Matt Fleming wrote:
> From: Matt Fleming <mfleming@cloudflare.com>
> 
> scx_root_enable_workfn() currently takes scx_fork_rwsem for writing
> before acquiring cgroup_mutex. 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 <mfleming@cloudflare.com>

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]  <TASK>
[   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]  </TASK>

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
> 

Re: [PATCH] sched_ext: Fix deadlock with PSI trigger creation

[Andrea Righi]

On Fri, Jul 10, 2026 at 07:46:39PM +0200, Andrea Righi wrote:
> Hi Matt,
> 
> On Fri, Jul 10, 2026 at 11:04:41AM +0100, Matt Fleming wrote:
> > From: Matt Fleming <mfleming@cloudflare.com>
> > 
> > scx_root_enable_workfn() currently takes scx_fork_rwsem for writing
> > before acquiring cgroup_mutex. 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 <mfleming@cloudflare.com>
> 
> 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.

Looking more at this, I think we should fix the PSI path instead, sched_ext
locking order is consistently using:

  scx_fork_rwsem -> scx_cgroup_ops_rwsem -> cgroup_mutex

Maybe we can rework pressure_write() to pin the cgroup and file context, then
drop both cgroup_mutex and kernfs active protection before creating the PSI
kthread?

Thanks,
-Andrea

> [   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]  <TASK>
> [   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]  </TASK>
> 
> 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
> > 

[PATCH cgroup/for-7.2-fixes] cgroup: Create the psimon kthread outside of cgroup_mutex

[Tejun Heo]

a5b98009f16d ("sched/psi: fix race between file release and pressure write")
made pressure_write() hold cgroup_mutex across psi_trigger_create(), which
forks the psimon kthread for the first rtpoll trigger. As kthread creation
depends on the whole fork path, the commit inadvertently created a lot of
unwanted locking dependencies from cgroup_mutex.

sched_ext got hit by one: its enable path blocks forks and then grabs
cgroup_mutex, so a pressure write racing a scheduler enable deadlocks, with
every other fork piling up behind.

Fix it by splitting trigger creation so that the worker is forked with
cgroup_mutex dropped and the kernfs active reference left broken. The latter
matters because rmdir and cgroup.pressure writes drain active references
under cgroup_mutex. Publishing the trigger last keeps error reporting
synchronous and preserves the of->priv lifetime rules.

The trigger registered in the first stage pins the group's rtpoll machinery
across the unlocked window, leaving only creation races to resolve. The
catch-up poll on installation covers scheduling attempts dropped while there
was no worker.

Fixes: a5b98009f16d ("sched/psi: fix race between file release and pressure write")
Cc: stable@vger.kernel.org
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Edward Adam Davis <eadavis@qq.com>
Cc: Chen Ridong <chenridong@huaweicloud.com>
Reported-by: Matt Fleming <mfleming@cloudflare.com>
Closes: https://lore.kernel.org/all/20260710100441.2653477-1-matt@readmodwrite.com/
Signed-off-by: Tejun Heo <tj@kernel.org>
---
Matt, your reordering trades one deadlock for another: CLONE_INTO_CGROUP
forks grab cgroup_mutex inside the scx_fork_rwsem read section, so an
enable racing such a clone deadlocks the other way around. The fork has to
move out of the locked sections instead. Can you verify this fixes the
deadlock in your setup?

 include/linux/psi.h    |    4 ++
 kernel/cgroup/cgroup.c |   23 +++++++++++++++-
 kernel/sched/psi.c     |   69 +++++++++++++++++++++++++++++++++++++------------
 3 files changed, 78 insertions(+), 18 deletions(-)

--- a/include/linux/psi.h
+++ b/include/linux/psi.h
@@ -25,7 +25,9 @@ void psi_memstall_leave(unsigned long *f
 int psi_show(struct seq_file *s, struct psi_group *group, enum psi_res res);
 struct psi_trigger *psi_trigger_create(struct psi_group *group, char *buf,
 				       enum psi_res res, struct file *file,
-				       struct kernfs_open_file *of);
+				       struct kernfs_open_file *of,
+				       bool *need_rtpoll_worker);
+int psi_trigger_create_rtpoll_worker(struct psi_group *group);
 void psi_trigger_destroy(struct psi_trigger *t);
 
 __poll_t psi_trigger_poll(void **trigger_ptr, struct file *file,
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -3996,6 +3996,7 @@ static ssize_t pressure_write(struct ker
 	struct psi_trigger *new;
 	struct cgroup *cgrp;
 	struct psi_group *psi;
+	bool need_rtpoll_worker;
 	ssize_t ret = 0;
 
 	cgrp = cgroup_kn_lock_live(of->kn, false);
@@ -4015,12 +4016,32 @@ static ssize_t pressure_write(struct ker
 	}
 
 	psi = cgroup_psi(cgrp);
-	new = psi_trigger_create(psi, buf, res, of->file, of);
+	new = psi_trigger_create(psi, buf, res, of->file, of,
+				 &need_rtpoll_worker);
 	if (IS_ERR(new)) {
 		ret = PTR_ERR(new);
 		goto out_unlock;
 	}
 
+	/*
+	 * The worker fork must run with neither cgroup_mutex nor the file's
+	 * kernfs active reference held. The latter is broken since
+	 * cgroup_kn_lock_live(). @of->priv may be released while unlocked, so
+	 * recheck before publishing @new.
+	 */
+	if (need_rtpoll_worker) {
+		cgroup_unlock();
+		ret = psi_trigger_create_rtpoll_worker(psi);
+		cgroup_lock();
+
+		if (!ret && !of->priv)
+			ret = -ENODEV;
+		if (ret) {
+			psi_trigger_destroy(new);
+			goto out_unlock;
+		}
+	}
+
 	smp_store_release(&ctx->psi.trigger, new);
 
 out_unlock:
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -1292,9 +1292,44 @@ int psi_show(struct seq_file *m, struct
 	return 0;
 }
 
+/*
+ * Create @group's rtpoll worker after psi_trigger_create() reported the need
+ * for one. kthread creation depends on the whole fork path and we don't want
+ * all of that nested inside cgroup_mutex, so the caller must drop it and any
+ * other lock that forks can wait behind. If two callers race, the loser stops
+ * its never-woken kthread.
+ */
+int psi_trigger_create_rtpoll_worker(struct psi_group *group)
+{
+	struct task_struct *task;
+
+	task = kthread_create(psi_rtpoll_worker, group, "psimon");
+	if (IS_ERR(task))
+		return PTR_ERR(task);
+
+	scoped_guard(mutex, &group->rtpoll_trigger_lock) {
+		if (!rcu_access_pointer(group->rtpoll_task)) {
+			atomic_set(&group->rtpoll_wakeup, 0);
+			wake_up_process(task);
+			rcu_assign_pointer(group->rtpoll_task, task);
+
+			/*
+			 * Poll once to catch up on scheduling attempts dropped
+			 * while there was no rtpoll worker.
+			 */
+			psi_schedule_rtpoll_work(group, 1, true);
+			return 0;
+		}
+	}
+
+	kthread_stop(task);
+	return 0;
+}
+
 struct psi_trigger *psi_trigger_create(struct psi_group *group, char *buf,
 				       enum psi_res res, struct file *file,
-				       struct kernfs_open_file *of)
+				       struct kernfs_open_file *of,
+				       bool *need_rtpoll_worker)
 {
 	struct psi_trigger *t;
 	enum psi_states state;
@@ -1302,6 +1337,8 @@ struct psi_trigger *psi_trigger_create(s
 	bool privileged;
 	u32 window_us;
 
+	*need_rtpoll_worker = false;
+
 	if (static_branch_likely(&psi_disabled))
 		return ERR_PTR(-EOPNOTSUPP);
 
@@ -1362,26 +1399,14 @@ struct psi_trigger *psi_trigger_create(s
 	if (privileged) {
 		mutex_lock(&group->rtpoll_trigger_lock);
 
-		if (!rcu_access_pointer(group->rtpoll_task)) {
-			struct task_struct *task;
-
-			task = kthread_create(psi_rtpoll_worker, group, "psimon");
-			if (IS_ERR(task)) {
-				kfree(t);
-				mutex_unlock(&group->rtpoll_trigger_lock);
-				return ERR_CAST(task);
-			}
-			atomic_set(&group->rtpoll_wakeup, 0);
-			wake_up_process(task);
-			rcu_assign_pointer(group->rtpoll_task, task);
-		}
-
 		list_add(&t->node, &group->rtpoll_triggers);
 		group->rtpoll_min_period = min(group->rtpoll_min_period,
 			div_u64(t->win.size, UPDATES_PER_WINDOW));
 		group->rtpoll_nr_triggers[t->state]++;
 		group->rtpoll_states |= (1 << t->state);
 
+		*need_rtpoll_worker = !rcu_access_pointer(group->rtpoll_task);
+
 		mutex_unlock(&group->rtpoll_trigger_lock);
 	} else {
 		mutex_lock(&group->avgs_lock);
@@ -1541,6 +1566,8 @@ static ssize_t psi_write(struct file *fi
 	size_t buf_size;
 	struct seq_file *seq;
 	struct psi_trigger *new;
+	bool need_rtpoll_worker;
+	int ret;
 
 	if (static_branch_likely(&psi_disabled))
 		return -EOPNOTSUPP;
@@ -1565,12 +1592,22 @@ static ssize_t psi_write(struct file *fi
 		return -EBUSY;
 	}
 
-	new = psi_trigger_create(&psi_system, buf, res, file, NULL);
+	new = psi_trigger_create(&psi_system, buf, res, file, NULL,
+				 &need_rtpoll_worker);
 	if (IS_ERR(new)) {
 		mutex_unlock(&seq->lock);
 		return PTR_ERR(new);
 	}
 
+	if (need_rtpoll_worker) {
+		ret = psi_trigger_create_rtpoll_worker(&psi_system);
+		if (ret) {
+			psi_trigger_destroy(new);
+			mutex_unlock(&seq->lock);
+			return ret;
+		}
+	}
+
 	smp_store_release(&seq->private, new);
 	mutex_unlock(&seq->lock);
 

Re: [PATCH cgroup/for-7.2-fixes] cgroup: Create the psimon kthread outside of cgroup_mutex

[Matt Fleming]

On Fri, Jul 10, 2026 at 01:49:45PM -1000, Tejun Heo wrote:
> Matt, your reordering trades one deadlock for another: CLONE_INTO_CGROUP
> forks grab cgroup_mutex inside the scx_fork_rwsem read section, so an
> enable racing such a clone deadlocks the other way around. The fork has to
> move out of the locked sections instead. Can you verify this fixes the
> deadlock in your setup?

Thanks for fixing this. I'll test this out ASAP and report back.

Re: [PATCH cgroup/for-7.2-fixes] cgroup: Create the psimon kthread outside of cgroup_mutex

[Tejun Heo]

Hello,

Superseded by the v2 posting, now a two patch series with an additional
rtpoll_timer UAF fix:

  https://lore.kernel.org/r/20260712174619.3553231-1-tj@kernel.org

Thanks.

--
tejun