Re: [ANNOUNCE] 3.0.4-rt13

Peter Zijlstra
2011-09-13T13:37:15Z

On Mon, 2011-09-12 at 16:53 +0200, Mike Galbraith wrote:
> On Mon, 2011-09-12 at 15:52 +0200, Mike Galbraith wrote:
>
> > 36717 <...>-6266 [002] 242.543129: sys_semop <-system_call_fastpath
> > 36718 <...>-6266 [002] 242.543129: sys_semtimedop <-sys_semop
> > 36719 <...>-6266 [002] 242.543131: ipc_lock_check <-sys_semtimedop
> > 36720 <...>-6266 [002] 242.543131: ipc_lock <-ipc_lock_check
> > 36721 <...>-6266 [002] 242.543132: __rcu_read_lock <-ipc_lock
> > 36722 <...>-6266 [002] 242.543133: migrate_disable <-ipc_lock
> > 36723 <...>-6266 [002] 242.543134: migrate_disable: migrate_disable: in_atomic:0 p->migrate_disable_atomic:0 p->migrate_disable:0
> > 36724 <...>-6266 [002] 242.543134: pin_current_cpu <-migrate_disable
> > 36725 <...>-6266 [002] 242.543134: _raw_spin_lock_irqsave <-migrate_disable
> > 36726 <...>-6266 [002] 242.543135: _raw_spin_unlock_irqrestore <-migrate_disable
> > 36727 <...>-6266 [002] 242.543135: rt_spin_lock <-ipc_lock
> > 36728 <...>-6266 [002] 242.543136: ipcperms <-sys_semtimedop
> > 36729 <...>-6266 [002] 242.543137: ns_capable <-ipcperms
> > 36730 <...>-6266 [002] 242.543138: cap_capable <-ns_capable
> > 36731 <...>-6266 [002] 242.543138: pid_vnr <-sys_semtimedop
> > 36732 <...>-6266 [002] 242.543139: try_atomic_semop <-sys_semtimedop
> > 36733 <...>-6266 [002] 242.543140: do_smart_update <-sys_semtimedop
> > 36734 <...>-6266 [002] 242.543140: update_queue <-do_smart_update
> > 36735 <...>-6266 [002] 242.543141: try_atomic_semop <-update_queue
> > 36736 <...>-6266 [002] 242.543142: update_queue <-do_smart_update
> > 36737 <...>-6266 [002] 242.543142: try_atomic_semop <-update_queue
> > 36738 <...>-6266 [002] 242.543143: update_queue <-do_smart_update
> > 36739 <...>-6266 [002] 242.543143: try_atomic_semop <-update_queue
> > 36740 <...>-6266 [002] 242.543144: get_seconds <-do_smart_update
> > 36741 <...>-6266 [002] 242.543144: rt_spin_unlock <-sys_semtimedop
> > 36742 <...>-6266 [002] 242.543144: migrate_enable <-sys_semtimedop
> > 36743 <...>-6266 [002] 242.543145: migrate_enable: migrate_enable: in_atomic:1 p->migrate_disable_atomic:0 p->migrate_disable:1
>
> Hm. Seems this is home grown a non-preemptive wakeup in the making.

Does the below cure things? It breaks !rt builds, but we can cure that if it works..

---
include/linux/sem.h | 2 ++
ipc/sem.c | 20 ++++----------------
2 files changed, 6 insertions(+), 16 deletions(-)

Index: linux-rt/include/linux/sem.h
===================================================================
--- linux-rt.orig/include/linux/sem.h
+++ linux-rt/include/linux/sem.h
@@ -80,6 +80,7 @@ struct seminfo {
#include <asm/atomic.h>
#include <linux/rcupdate.h>
#include <linux/cache.h>
+#include <linux/wait.h>

struct task_struct;

@@ -114,6 +115,7 @@ struct sem_queue {
struct sembuf *sops; /* array of pending operations */
int nsops; /* number of operations */
int alter; /* does the operation alter the array? */
+ wait_queue_head_t wait;
};

/* Each task has a list of undo requests. They are executed automatically
Index: linux-rt/ipc/sem.c
===================================================================
--- linux-rt.orig/ipc/sem.c
+++ linux-rt/ipc/sem.c
@@ -415,13 +415,6 @@ static int try_atomic_semop (struct sem_
static void wake_up_sem_queue_prepare(struct list_head *pt,
struct sem_queue *q, int error)
{
- if (list_empty(pt)) {
- /*
- * Hold preempt off so that we don't get preempted and have the
- * wakee busy-wait until we're scheduled back on.
- */
- preempt_disable();
- }
q->status = IN_WAKEUP;
q->pid = error;

@@ -450,7 +443,7 @@ static void wake_up_sem_queue_do(struct
q->status = q->pid;
}
if (did_something)
- preempt_enable();
+ wake_up_all(&q->wait);
}

static void unlink_queue(struct sem_array *sma, struct sem_queue *q)
@@ -1275,15 +1268,9 @@ static struct sem_undo *find_alloc_undo(
*/
static int get_queue_result(struct sem_queue *q)
{
- int error;
-
- error = q->status;
- while (unlikely(error == IN_WAKEUP)) {
- cpu_relax();
- error = q->status;
- }
+ wait_event(q->wait, ACCESS_ONCE(q->status) != IN_WAKEUP);

- return error;
+ return q->status;
}

@@ -1432,6 +1419,7 @@ SYSCALL_DEFINE4(semtimedop, int, semid,

queue.status = -EINTR;
queue.sleeper = current;
+ init_waitqueue_head(&queue.wait);
current->state = TASK_INTERRUPTIBLE;
sem_unlock(sma);