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SubjectRe: oops in __delayacct_blkio_ticks with 2.6.18-rc4
Olaf Hering wrote:
> https://bugzilla.novell.com/show_bug.cgi?id=200526
>

Thanks for detecting this.

I suspect the oops is caused by a reading of /proc/<tgid>/stat for some task
that is late in exit. Currently tsk->delays is being freed up too early (before
the tsk is removed from the tasklist).

Could you try the patch below ? It was unclear from the bug what userspace
actions were being done to reproduce the oops - I suspect some kind of
reading of /proc/.../stat for all processes ?

>
> tsk->delays became 0 after or during the call to spinlock. _spin_lock cant be
> called with 0.
>

<snip>

> Call Trace:
> [C0000000BB3078B0] [C00000000009D604] .__delayacct_blkio_ticks+0x24/0x6c (unreliable)
> [C0000000BB307940] [C000000000128320] .do_task_stat+0x4b0/0x6fc
> [C0000000BB307C40] [C000000000124F4C] .proc_info_read+0x9c/0x144
> [C0000000BB307CF0] [C0000000000D394C] .vfs_read+0x118/0x200
> [C0000000BB307D90] [C0000000000D3E30] .sys_read+0x4c/0x8c
> [C0000000BB307E30] [C00000000000871C] syscall_exit+0x0/0x40




Cleanup allocation and freeing of tsk->delays used by delay accounting.

Currently tsk->delays is getting freed too early in task exit
which can cause a NULL tsk->delays to get accessed via reading
of /proc/<tgid>/stats. The patch fixes this problem by freeing
tsk->delays closer to when task_struct itself is freed up. As a result,
it also eliminates the use of tsk->delays_lock which was only being
used (inadequately) to safeguard access to tsk->delays
while a task was exiting.

The patch also cleans up tsk->delays allocations after a bad fork which
was missing earlier and might lead to leaks.

Signed-Off-By: Shailabh Nagar <nagar@watson.ibm.com>

include/linux/delayacct.h | 10 +++++++---
include/linux/sched.h | 1 -
kernel/delayacct.c | 16 ----------------
kernel/exit.c | 1 -
kernel/fork.c | 6 ++++--
5 files changed, 11 insertions(+), 23 deletions(-)
Index: linux-2.6.18-rc4/kernel/fork.c
===================================================================
--- linux-2.6.18-rc4.orig/kernel/fork.c 2006-08-22 14:42:08.000000000 -0400
+++ linux-2.6.18-rc4/kernel/fork.c 2006-08-22 14:52:52.000000000 -0400
@@ -117,6 +117,7 @@ void __put_task_struct(struct task_struc
security_task_free(tsk);
free_uid(tsk->user);
put_group_info(tsk->group_info);
+ delayacct_tsk_free(tsk);

if (!profile_handoff_task(tsk))
free_task(tsk);
@@ -1011,7 +1012,7 @@ static struct task_struct *copy_process(
retval = -EFAULT;
if (clone_flags & CLONE_PARENT_SETTID)
if (put_user(p->pid, parent_tidptr))
- goto bad_fork_cleanup;
+ goto bad_fork_cleanup_delays_binfmt;

INIT_LIST_HEAD(&p->children);
INIT_LIST_HEAD(&p->sibling);
@@ -1277,7 +1278,8 @@ bad_fork_cleanup_policy:
bad_fork_cleanup_cpuset:
#endif
cpuset_exit(p);
-bad_fork_cleanup:
+bad_fork_cleanup_delays_binfmt:
+ delayacct_tsk_free(p);
if (p->binfmt)
module_put(p->binfmt->module);
bad_fork_cleanup_put_domain:
Index: linux-2.6.18-rc4/include/linux/delayacct.h
===================================================================
--- linux-2.6.18-rc4.orig/include/linux/delayacct.h 2006-08-22 14:42:03.000000000 -0400
+++ linux-2.6.18-rc4/include/linux/delayacct.h 2006-08-22 14:52:52.000000000 -0400
@@ -59,10 +59,14 @@ static inline void delayacct_tsk_init(st
__delayacct_tsk_init(tsk);
}

-static inline void delayacct_tsk_exit(struct task_struct *tsk)
+/* Free tsk->delays. Called from bad fork and __put_task_struct
+ * where there's no risk of tsk->delays being accessed elsewhere
+ */
+static inline void delayacct_tsk_free(struct task_struct *tsk)
{
if (tsk->delays)
- __delayacct_tsk_exit(tsk);
+ kmem_cache_free(delayacct_cache, tsk->delays);
+ tsk->delays = NULL;
}

static inline void delayacct_blkio_start(void)
@@ -101,7 +105,7 @@ static inline void delayacct_init(void)
{}
static inline void delayacct_tsk_init(struct task_struct *tsk)
{}
-static inline void delayacct_tsk_exit(struct task_struct *tsk)
+static inline void delayacct_tsk_free(struct task_struct *tsk)
{}
static inline void delayacct_blkio_start(void)
{}
Index: linux-2.6.18-rc4/include/linux/sched.h
===================================================================
--- linux-2.6.18-rc4.orig/include/linux/sched.h 2006-08-22 14:42:03.000000000 -0400
+++ linux-2.6.18-rc4/include/linux/sched.h 2006-08-22 14:52:52.000000000 -0400
@@ -994,7 +994,6 @@ struct task_struct {
*/
struct pipe_inode_info *splice_pipe;
#ifdef CONFIG_TASK_DELAY_ACCT
- spinlock_t delays_lock;
struct task_delay_info *delays;
#endif
};
Index: linux-2.6.18-rc4/kernel/exit.c
===================================================================
--- linux-2.6.18-rc4.orig/kernel/exit.c 2006-08-22 14:42:03.000000000 -0400
+++ linux-2.6.18-rc4/kernel/exit.c 2006-08-22 14:52:52.000000000 -0400
@@ -908,7 +908,6 @@ fastcall NORET_TYPE void do_exit(long co
audit_free(tsk);
taskstats_exit_send(tsk, tidstats, group_dead, mycpu);
taskstats_exit_free(tidstats);
- delayacct_tsk_exit(tsk);

exit_mm(tsk);

Index: linux-2.6.18-rc4/kernel/delayacct.c
===================================================================
--- linux-2.6.18-rc4.orig/kernel/delayacct.c 2006-08-22 14:42:03.000000000 -0400
+++ linux-2.6.18-rc4/kernel/delayacct.c 2006-08-22 14:52:52.000000000 -0400
@@ -41,24 +41,11 @@ void delayacct_init(void)

void __delayacct_tsk_init(struct task_struct *tsk)
{
- spin_lock_init(&tsk->delays_lock);
- /* No need to acquire tsk->delays_lock for allocation here unless
- __delayacct_tsk_init called after tsk is attached to tasklist
- */
tsk->delays = kmem_cache_zalloc(delayacct_cache, SLAB_KERNEL);
if (tsk->delays)
spin_lock_init(&tsk->delays->lock);
}

-void __delayacct_tsk_exit(struct task_struct *tsk)
-{
- struct task_delay_info *delays = tsk->delays;
- spin_lock(&tsk->delays_lock);
- tsk->delays = NULL;
- spin_unlock(&tsk->delays_lock);
- kmem_cache_free(delayacct_cache, delays);
-}
-
/*
* Start accounting for a delay statistic using
* its starting timestamp (@start)
@@ -118,8 +105,6 @@ int __delayacct_add_tsk(struct taskstats
struct timespec ts;
unsigned long t1,t2,t3;

- spin_lock(&tsk->delays_lock);
-
/* Though tsk->delays accessed later, early exit avoids
* unnecessary returning of other data
*/
@@ -161,7 +146,6 @@ int __delayacct_add_tsk(struct taskstats
spin_unlock(&tsk->delays->lock);

done:
- spin_unlock(&tsk->delays_lock);
return 0;
}

-
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