The patch titled
Subject: kernel/cpu.c: document clear_tasks_mm_cpumask()
has been added to the -mm tree. Its filename is
kernel-cpuc-document-clear_tasks_mm_cpumask.patch
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From: Anton Vorontsov <anton.vorontsov@xxxxxxxxxx>
Subject: kernel/cpu.c: document clear_tasks_mm_cpumask()
Add more comments on clear_tasks_mm_cpumask, plus adds a runtime check:
the function is only suitable for offlined CPUs, and if called
inappropriately, the kernel should scream aloud.
Suggested-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Suggested-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Signed-off-by: Anton Vorontsov <anton.vorontsov@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/cpu.c | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)
diff -puN kernel/cpu.c~kernel-cpuc-document-clear_tasks_mm_cpumask kernel/cpu.c
--- a/kernel/cpu.c~kernel-cpuc-document-clear_tasks_mm_cpumask
+++ a/kernel/cpu.c
@@ -13,6 +13,7 @@
#include <linux/oom.h>
#include <linux/rcupdate.h>
#include <linux/export.h>
+#include <linux/bug.h>
#include <linux/kthread.h>
#include <linux/stop_machine.h>
#include <linux/mutex.h>
@@ -173,6 +174,18 @@ void __ref unregister_cpu_notifier(struc
}
EXPORT_SYMBOL(unregister_cpu_notifier);
+/**
+ * clear_tasks_mm_cpumask - Safely clear tasks' mm_cpumask for a CPU
+ * @cpu: a CPU id
+ *
+ * This function walks up all processes, finds a valid mm struct for
+ * each one and then clears a corresponding bit in mm's cpumask. While
+ * this all sounds trivial, there are various non-obvious corner cases,
+ * which this function tries to solve in a safe manner.
+ *
+ * Also note that the function uses a somewhat relaxed locking scheme,
+ * so it may be called only for an already offlined CPU.
+ */
void clear_tasks_mm_cpumask(int cpu)
{
struct task_struct *p;
@@ -184,10 +197,15 @@ void clear_tasks_mm_cpumask(int cpu)
* Thus, we may use rcu_read_lock() here, instead of grabbing
* full-fledged tasklist_lock.
*/
+ WARN_ON(cpu_online(cpu));
rcu_read_lock();
for_each_process(p) {
struct task_struct *t;
+ /*
+ * Main thread might exit, but other threads may still have
+ * a valid mm. Find one.
+ */
t = find_lock_task_mm(p);
if (!t)
continue;
_
Subject: Subject: kernel/cpu.c: document clear_tasks_mm_cpumask()
Patches currently in -mm which might be from anton.vorontsov@xxxxxxxxxx are
linux-next.patch
cpu-introduce-clear_tasks_mm_cpumask-helper.patch
arm-use-clear_tasks_mm_cpumask.patch
powerpc-use-clear_tasks_mm_cpumask.patch
sh-use-clear_tasks_mm_cpumask.patch
blackfin-a-couple-of-task-mm-handling-fixes.patch
blackfin-fix-possible-deadlock-in-decode_address.patch
um-should-hold-tasklist_lock-while-traversing-processes.patch
um-fix-possible-race-on-task-mm.patch
um-properly-check-all-process-threads-for-a-live-mm.patch
kernel-cpuc-document-clear_tasks_mm_cpumask.patch
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