Re: [patch] preempt-trace.patch (mono preempt-trace)

Ingo Molnar
2005-08-05T20:08:46Z

* Dominik Karall <dominik.karall@gmx.net> wrote:

> With FRAME_POINTERS enabled:
>
> BUG: mono[3193] exited with nonzero preempt_count 1!
> ---------------------------
> | preempt count: 00000001 ]
> | 1 level deep critical section nesting:
> ----------------------------------------
> .. [<ffffffff80400a46>] .... _spin_lock+0x16/0x80
> .....[<ffffffff801ed30c>] .. ( <= sys_semtimedop+0x28c/0x7c0)

thanks. It seems semundo->lock somehow leaked. One possibility would be
of semundo->refcount going from 2 to 1 while another thread has it
locked. I dont see what prevents this scenario from happening. To test
this theory, could you apply the patch below, which will do semundo
locking not conditional on the refcount - does it fix the bug?

Ingo

ipc/sem.c | 10 +++-------
1 files changed, 3 insertions(+), 7 deletions(-)

Index: linux-preempt-trace/ipc/sem.c
===================================================================
--- linux-preempt-trace.orig/ipc/sem.c
+++ linux-preempt-trace/ipc/sem.c
@@ -895,7 +895,7 @@ static inline void lock_semundo(void)
struct sem_undo_list *undo_list;

undo_list = current->sysvsem.undo_list;
- if ((undo_list != NULL) && (atomic_read(&undo_list->refcnt) != 1))
+ if (undo_list)
spin_lock(&undo_list->lock);
}

@@ -915,7 +915,7 @@ static inline void unlock_semundo(void)
struct sem_undo_list *undo_list;

undo_list = current->sysvsem.undo_list;
- if ((undo_list != NULL) && (atomic_read(&undo_list->refcnt) != 1))
+ if (undo_list)
spin_unlock(&undo_list->lock);
}

@@ -943,9 +943,7 @@ static inline int get_undo_list(struct s
if (undo_list == NULL)
return -ENOMEM;
memset(undo_list, 0, size);
- /* don't initialize unodhd->lock here. It's done
- * in copy_semundo() instead.
- */
+ spin_lock_init(&undo_list->lock);
atomic_set(&undo_list->refcnt, 1);
current->sysvsem.undo_list = undo_list;
}
@@ -1231,8 +1229,6 @@ int copy_semundo(unsigned long clone_fla
error = get_undo_list(&undo_list);
if (error)
return error;
- if (atomic_read(&undo_list->refcnt) == 1)
- spin_lock_init(&undo_list->lock);
atomic_inc(&undo_list->refcnt);
tsk->sysvsem.undo_list = undo_list;
} else