Skip to contentSkip to conversation

Fix initialization of runqueues

PATCH In review Samuel Thibault 3 participants 5 messages Aug 2, 2006 – Aug 3, 2006

lore
Share & tools
lore

Samuel Thibault

author
#0 lore raw

PATCH 1/1 Fix initialization of runqueues

Hi,

There's an odd thing about the nr_active field in arrays of runqueue_t:
it is actually never initialized to 0!... This doesn't yet trigger a bug probably because the way runqueues are allocated make it so that it is already initialized to 0, but that's not a safe way. Here is a patch:

Initialize to zero the nr_active field of arrays of runqueues. (fixes future potential dynamic allocation of runqueues).

Signed-off-by Samuel Thibault <[email protected]>
kernel/sched.c +1 -0
@@ -6132,6 +6132,7 @@
 
 		for (j = 0; j < 2; j++) {
 			array = rq->arrays + j;
+			array->nr_active = 0;
 			for (k = 0; k < MAX_PRIO; k++) {
 				INIT_LIST_HEAD(array->queue + k);
 				__clear_bit(k, array->bitmap);
#1 lore raw
Quoted 7 lines · Samuel Thibault

Hi,

There's an odd thing about the nr_active field in arrays of
runqueue_t: it is actually never initialized to 0!... This doesn't yet trigger a bug probably because the way runqueues are allocated make it so that it is already initialized to 0, but that's not a safe way. Here is a patch:

we do rely on zero initialization of bss (and percpu) data in a number of places.

Ingo

author
#2 lore raw
Quoted 5 lines · Ingo Molnar, le Wed 02 Aug 2006 17:24:19 +0200, a écrit :
Quoted 2 lines · Samuel Thibault

Hi,

Quoted 8 lines
Quoted 5 lines

There's an odd thing about the nr_active field in arrays of
runqueue_t: it is actually never initialized to 0!... This doesn't yet trigger a bug probably because the way runqueues are allocated make it so that it is already initialized to 0, but that's not a safe way. Here is a patch:

we do rely on zero initialization of bss (and percpu) data in a number of places.

The rest of runqueue initialization doesn't rely on that, and as a result people might think that it is safe to allocate runqueues dynamically.

Samuel

#3 lore raw
Quoted 12 lines · Samuel Thibault
Quoted 11 lines · Ingo Molnar, le Wed 02 Aug 2006 17:24:19 +0200, a écrit :
Quoted 7 lines · Samuel Thibault

Hi,

There's an odd thing about the nr_active field in arrays of
runqueue_t: it is actually never initialized to 0!... This doesn't yet trigger a bug probably because the way runqueues are allocated make it so that it is already initialized to 0, but that's not a safe way. Here is a patch:

Quoted 6 lines
Quoted 2 lines

we do rely on zero initialization of bss (and percpu) data in a number of places.

The rest of runqueue initialization doesn't rely on that, and as a result people might think that it is safe to allocate runqueues dynamically.

I don't buy the "safe to allocate runqueues dynamically" bit since they are local to sched.c and if you do do that (I did for a customer once) you better know what you're doing.

That said, ...

Hmm, Ingo I guess he's right on the first part:

<sched_init snipit>

		rq->nr_running = 0;

[...]

#ifdef CONFIG_SMP
		rq->sd = NULL;
		for (j = 1; j < 3; j++)
			rq->cpu_load[j] = 0;
		rq->active_balance = 0;
		rq->push_cpu = 0;
		rq->migration_thread = NULL;

</sched_init snipit>

So I guess we should add his zero initializer, or we should remove all the other zero initializers. Either way, we should be consistent.

-- Steve

author
#4 lore raw
Quoted 16 lines · Steven Rostedt, le Thu 03 Aug 2006 11:07:37 -0400, a écrit :
Quoted 15 lines · Samuel Thibault
Quoted 13 lines · Ingo Molnar, le Wed 02 Aug 2006 17:24:19 +0200, a écrit :
Quoted 7 lines · Samuel Thibault

Hi,

There's an odd thing about the nr_active field in arrays of
runqueue_t: it is actually never initialized to 0!... This doesn't yet trigger a bug probably because the way runqueues are allocated make it so that it is already initialized to 0, but that's not a safe way. Here is a patch:

we do rely on zero initialization of bss (and percpu) data in a number of places.

Quoted 7 lines
Quoted 3 lines

The rest of runqueue initialization doesn't rely on that, and as a result people might think that it is safe to allocate runqueues dynamically.

I don't buy the "safe to allocate runqueues dynamically" bit since they are local to sched.c and if you do do that (I did for a customer once) you better know what you're doing.

Yes, but as you agreed, initializing some members to 0 and not others doesn't help to know what you're doing :)

Samuel