work_on_cpu(): rewrite it to create a kernel thread on demand

Impact: circular locking bugfix

The various implemetnations and proposed implemetnations of work_on_cpu()
are vulnerable to various deadlocks because they all used queues of some
form.

Unrelated pieces of kernel code thus gained dependencies wherein if one
work_on_cpu() caller holds a lock which some other work_on_cpu() callback
also takes, the kernel could rarely deadlock.

Fix this by creating a short-lived kernel thread for each work_on_cpu()
invokation.

This is not terribly fast, but the only current caller of work_on_cpu() is
pci_call_probe().

It would be nice to find some other way of doing the node-local
allocations in the PCI probe code so that we can zap work_on_cpu()
altogether.  The code there is rather nasty.  I can't think of anything
simple at this time...

Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 19 insertions, 17 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index b6b966ce1451..f71fb2a08950 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -966,20 +966,20 @@ undo:
 }
 
 #ifdef CONFIG_SMP
-static struct workqueue_struct *work_on_cpu_wq __read_mostly;
 
 struct work_for_cpu {
-        struct work_struct work;
+        struct completion completion;
         long (*fn)(void *);
         void *arg;
         long ret;
 };
 
-static void do_work_for_cpu(struct work_struct *w)
+static int do_work_for_cpu(void *_wfc)
 {
-        struct work_for_cpu *wfc = container_of(w, struct work_for_cpu, work);
-
+        struct work_for_cpu *wfc = _wfc;
         wfc->ret = wfc->fn(wfc->arg);
+        complete(&wfc->completion);
+        return 0;
 }
 
 /**
@@ -990,17 +990,23 @@ static void do_work_for_cpu(struct work_struct *w)
  *
  * This will return the value @fn returns.
  * It is up to the caller to ensure that the cpu doesn't go offline.
+ * The caller must not hold any locks which would prevent @fn from completing.
  */
 long work_on_cpu(unsigned int cpu, long (*fn)(void *), void *arg)
 {
-        struct work_for_cpu wfc;
-
-        INIT_WORK(&wfc.work, do_work_for_cpu);
-        wfc.fn = fn;
-        wfc.arg = arg;
-        queue_work_on(cpu, work_on_cpu_wq, &wfc.work);
-        flush_work(&wfc.work);
-
+        struct task_struct *sub_thread;
+        struct work_for_cpu wfc = {
+                .completion = COMPLETION_INITIALIZER_ONSTACK(wfc.completion),
+                .fn = fn,
+                .arg = arg,
+        };
+
+        sub_thread = kthread_create(do_work_for_cpu, &wfc, "work_for_cpu");
+        if (IS_ERR(sub_thread))
+                return PTR_ERR(sub_thread);
+        kthread_bind(sub_thread, cpu);
+        wake_up_process(sub_thread);
+        wait_for_completion(&wfc.completion);
         return wfc.ret;
 }
 EXPORT_SYMBOL_GPL(work_on_cpu);
@@ -1016,8 +1022,4 @@ void __init init_workqueues(void)
         hotcpu_notifier(workqueue_cpu_callback, 0);
         keventd_wq = create_workqueue("events");
         BUG_ON(!keventd_wq);
-#ifdef CONFIG_SMP
-        work_on_cpu_wq = create_workqueue("work_on_cpu");
-        BUG_ON(!work_on_cpu_wq);
-#endif
 }