[PATCH V2] x86/intel_rdt: Ensure usage of CPUs are locked while needed

[Reinette Chatre]

The user triggers the creation of a pseudo-locked region when writing
the requested schemata to the schemata resctrl file. The pseudo-locking
of a region is required to be done on a CPU that is associated with the
cache on which the pseudo-locked region will reside. In order to run the
locking code on a specific CPU the needed CPU has to be selected and
ensured to remain online during the entire locking sequence.

At this time the cpu_hotplug_lock is not taken during the pseudo-lock
region creation and it is thus possible for a CPU to be selected to run
the pseudo-locking code and then that CPU to go offline before the
thread is able to run on it.

Fix this by ensuring that the cpu_hotplug_lock is taken while the CPU on
which code has to run needs to be controlled. Since the cpu_hotplug_lock
is always taken before rdtgroup_mutex the lock order is maintained.

Fixes: e0bdfe8e36f3 ("x86/intel_rdt: Support creation/removal of pseudo-locked region")
Signed-off-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
V2:
- Rebase against tip/x86/urgent
- Modify subject from x86/resctrl to x86/intel_rdt to match subject used
  before the code reorganization.

 arch/x86/kernel/cpu/intel_rdt_ctrlmondata.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kernel/cpu/intel_rdt_ctrlmondata.c b/arch/x86/kernel/cpu/intel_rdt_ctrlmondata.c
index 27937458c231..efa4a519f5e5 100644
--- a/arch/x86/kernel/cpu/intel_rdt_ctrlmondata.c
+++ b/arch/x86/kernel/cpu/intel_rdt_ctrlmondata.c
@@ -23,6 +23,7 @@
 
 #define pr_fmt(fmt)	KBUILD_MODNAME ": " fmt
 
+#include <linux/cpu.h>
 #include <linux/kernfs.h>
 #include <linux/seq_file.h>
 #include <linux/slab.h>
@@ -310,9 +311,11 @@ ssize_t rdtgroup_schemata_write(struct kernfs_open_file *of,
 		return -EINVAL;
 	buf[nbytes - 1] = '\0';
 
+	cpus_read_lock();
 	rdtgrp = rdtgroup_kn_lock_live(of->kn);
 	if (!rdtgrp) {
 		rdtgroup_kn_unlock(of->kn);
+		cpus_read_unlock();
 		return -ENOENT;
 	}
 	rdt_last_cmd_clear();
@@ -367,6 +370,7 @@ ssize_t rdtgroup_schemata_write(struct kernfs_open_file *of,
 
 out:
 	rdtgroup_kn_unlock(of->kn);
+	cpus_read_unlock();
 	return ret ?: nbytes;
 }
 
-- 
2.17.0