From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	Tejun Heo <tj@kernel.org>,
	Andrea Righi <arighi@nvidia.com>
Subject: [PATCH 7.1 260/271] sched_ext: Take cgroup_lock() first in scx_cgroup_lock()
Date: Mon, 17 Aug 2026 15:33:05 +0200
Message-ID: <20260817132547.425654772@linuxfoundation.org>
In-Reply-To: <20260817132536.752504388@linuxfoundation.org>
References: <20260817132536.752504388@linuxfoundation.org>
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7.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Tejun Heo <tj@kernel.org>

commit 5f8b69642d18e1f3e11996707842ac530444e959 upstream.

scx_cgroup_lock() write-locks scx_cgroup_ops_rwsem and then takes
cgroup_lock(), which can deadlock through kernfs:

  scx enable/disable         cgroup rmdir           cpu.weight write
  ------------------         ------------           ----------------
                             cgroup_lock()
  percpu_down_write(rwsem)
  cgroup_lock()
                                                    kernfs_get_active()
                                                    percpu_down_read(rwsem)
                             kernfs_drain()

The enable path waits for the rmdir to release cgroup_mutex. The rmdir,
deactivating the cpu controller's files, waits in kernfs_drain() for the
write's active reference. The write, in scx_group_set_weight(), waits for
the rwsem behind the pending writer.

Take cgroup_lock() first. The set_* paths take no cgroup locks inside the
read side, so a pending write-lock then only waits for read sections that
always run to completion, and no dependency from the rwsem back to
cgroup_mutex remains.

Fixes: a5bd6ba30b33 ("sched_ext: Use cgroup_lock/unlock() to synchronize against cgroup operations")
Cc: stable@vger.kernel.org # v6.18+
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 kernel/sched/ext.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -4515,20 +4515,25 @@ static struct cgroup *root_cgroup(void)
 	return &cgrp_dfl_root.cgrp;
 }
 
+/*
+ * cgroup_lock() must nest outside the rwsem write side: a writer waiting
+ * for cgroup_mutex deadlocks with cgroup teardown, which holds it while
+ * draining a set_* file write blocked on the rwsem behind the writer.
+ */
 static void scx_cgroup_lock(void)
 {
+	cgroup_lock();
 #ifdef CONFIG_EXT_GROUP_SCHED
 	percpu_down_write(&scx_cgroup_ops_rwsem);
 #endif
-	cgroup_lock();
 }
 
 static void scx_cgroup_unlock(void)
 {
-	cgroup_unlock();
 #ifdef CONFIG_EXT_GROUP_SCHED
 	percpu_up_write(&scx_cgroup_ops_rwsem);
 #endif
+	cgroup_unlock();
 }
 #else	/* CONFIG_EXT_GROUP_SCHED || CONFIG_EXT_SUB_SCHED */
 static struct cgroup *root_cgroup(void) { return NULL; }