sched/psi: Create the psimon kthread outside of cgroup_mutex

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.

v2: Retagged sched/psi (was cgroup).

Fixes: a5b98009f16d ("sched/psi: fix race between file release and pressure write")
Cc: stable@vger.kernel.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>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Tested-by: Matt Fleming <mfleming@cloudflare.com>
Acked-by: Suren Baghdasaryan <surenb@google.com>

1 files changed, 22 insertions, 1 deletions

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 38f8d9df8fbc..b5b461d4418b 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -3996,6 +3996,7 @@ static ssize_t pressure_write(struct kernfs_open_file *of, char *buf,
 	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 kernfs_open_file *of, char *buf,
 	}
 
 	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: