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Waiman Long
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JIRA: https://redhat.atlassian.net/browse/RHEL-232556 Conflicts: A context diff with the first kernel/sched/psi.c hunk due to missing upstream commit 44585f7bc0cb ("psi: fix "defined but not used" warnings when CONFIG_PROC_FS=n"). commit fadeedd7cfc5d73d33fa3d7ac54b9b27aabd09d2 Author: Tejun Heo <tj@kernel.org> Date: Sun, 12 Jul 2026 07:23:55 -1000 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> Signed-off-by: Waiman Long <longman@redhat.com>