Re: [Patch] restore the RCU callback to defer put_task_struct() Re: Problems with 2.6.17-rt8

Bill Huey
2006-08-08T03:05:50Z

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On Mon, Aug 07, 2006 at 07:56:15PM -0700, Bill Huey wrote:
> On Thu, Aug 03, 2006 at 10:27:41AM -0400, Steven Rostedt wrote:
>
> ...(output and commentary a log deleted)...
>
> > This could also have a side effect that messes things up.
> >
> > Unfortunately, right now I'm assigned to other tasks and I cant spend
> > much more time on this at the moment. So hopefully, Ingo, Thomas or
> > Bill, or someone else can help you find the reason for this problem.
>
> Steve and company,
>
> Speaking of which, after talking to Steve about this and confirming this
> with a revert of changes. put_task_struct() can't deallocated memory from
> either the zone or SLAB cache without taking a sleeping lock. It can't
> be called directly from finish_task_switch to reap the thread because of
> that (violation in atomic).
>
> It is for this reason the RCU call back to delay processing was put into
> place to reap threads and was, seemingly by accident, missing from
> patch-2.6.17-rt7 to -rt8. That is what broke it in the first place.
>
> I tested it with a "make -j4" which triggers the warning and it they all
> go away now.
>
> Reverse patch attached:

Resend with instrumentation code removed:

bill

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============================================================
--- include/linux/sched.h 0ed8993484be9c13728f4ebdaa51fc0f0c229018
+++ include/linux/sched.h db79c6b458b0776d3768141ff993a7c9e64d5794
@@ -1105,6 +1110,15 @@
extern void free_task(struct task_struct *tsk);
#define get_task_struct(tsk) do { atomic_inc(&(tsk)->usage); } while(0)

+#ifdef CONFIG_PREEMPT_RT
+extern void __put_task_struct_cb(struct rcu_head *rhp);
+
+static inline void put_task_struct(struct task_struct *t)
+{
+ if (atomic_dec_and_test(&t->usage))
+ call_rcu(&t->rcu, __put_task_struct_cb);
+}
+#else
extern void __put_task_struct(struct task_struct *t);

static inline void put_task_struct(struct task_struct *t)
@@ -1112,6 +1126,7 @@
if (atomic_dec_and_test(&t->usage))
__put_task_struct(t);
}
+#endif

/*
* Per process flags
============================================================
--- kernel/fork.c 506dabd42d242f78e0321594c7723481e0cd87dc
+++ kernel/fork.c d07df07ac627dd27933b4bfb83768461ef28731c
@@ -54,6 +54,8 @@
#include <asm/cacheflush.h>
#include <asm/tlbflush.h>

+#include <linux/dobject.h>
+
/*
* Protected counters by write_lock_irq(&tasklist_lock)
*/
@@ -120,6 +122,26 @@
}
EXPORT_SYMBOL(free_task);

+#ifdef CONFIG_PREEMPT_RT
+void __put_task_struct_cb(struct rcu_head *rhp)
+{
+ struct task_struct *tsk = container_of(rhp, struct task_struct, rcu);
+
+ BUG_ON(atomic_read(&tsk->usage));
+ WARN_ON(!(tsk->flags & PF_DEAD));
+ WARN_ON(!(tsk->exit_state & (EXIT_DEAD | EXIT_ZOMBIE)));
+ WARN_ON(tsk == current);
+
+ security_task_free(tsk);
+ free_uid(tsk->user);
+ put_group_info(tsk->group_info);
+
+ if (!profile_handoff_task(tsk))
+ free_task(tsk);
+}
+
+#else
+
void __put_task_struct(struct task_struct *tsk)
{
WARN_ON(!(tsk->exit_state & (EXIT_DEAD | EXIT_ZOMBIE)));
@@ -134,6 +156,7 @@
if (!profile_handoff_task(tsk))
free_task(tsk);
}
+#endif

void __init fork_init(unsigned long mempages)
{