kmem_cache_destroy() badness with SLUB

kmem_cache_destroy() badness with SLUB

[Benjamin Herrenschmidt]

Hi folks !

Internally, I'm hitting a little "nit"...

sysfs_slab_add() has this check:

	if (slab_state < SYSFS)
		/* Defer until later */
		return 0;

But sysfs_slab_remove() doesn't.

So if the slab is created -and- destroyed at, for example, arch_initcall
time, then we hit a WARN in the kobject code, trying to dispose of a
non-existing kobject.

Now, at first sight, just adding the same test to sysfs_slab_remove()
would do the job... but it all seems very racy to me.

I don't understand in fact how this slab_state deals with races at all. 

What prevents us from hitting slab_sysfs_init() at the same time as
another CPU deos sysfs_slab_add() ? How do that deal with collisions
trying to register the same kobject twice ? Similar race with remove...

Shouldn't we have a mutex around those guys ?

Cheers,
Ben.


--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: kmem_cache_destroy() badness with SLUB

[David Rientjes]

On Mon, 28 Jun 2010, Benjamin Herrenschmidt wrote:

> Hi folks !
> 
> Internally, I'm hitting a little "nit"...
> 
> sysfs_slab_add() has this check:
> 
> 	if (slab_state < SYSFS)
> 		/* Defer until later */
> 		return 0;
> 
> But sysfs_slab_remove() doesn't.
> 
> So if the slab is created -and- destroyed at, for example, arch_initcall
> time, then we hit a WARN in the kobject code, trying to dispose of a
> non-existing kobject.
> 

Indeed, but shouldn't we be appropriately handling the return value of 
sysfs_slab_add() so that it fails cache creation?  We wouldn't be calling 
sysfs_slab_remove() on a cache that was never created.

> Now, at first sight, just adding the same test to sysfs_slab_remove()
> would do the job... but it all seems very racy to me.
> 
> I don't understand in fact how this slab_state deals with races at all. 
> 

All modifiers of slab_state are intended to be run only on the boot cpu so 
the only concern is the ordering.  We need slab_state to indicate how far 
slab has been initialized since we can't otherwise enforce how code uses 
slab in between things like kmem_cache_init(), kmem_cache_init_late(), and 
initcalls on the boot cpu.

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: kmem_cache_destroy() badness with SLUB

[Benjamin Herrenschmidt]

On Mon, 2010-06-28 at 02:03 -0700, David Rientjes wrote:
> On Mon, 28 Jun 2010, Benjamin Herrenschmidt wrote:
> 
> > Hi folks !
> > 
> > Internally, I'm hitting a little "nit"...
> > 
> > sysfs_slab_add() has this check:
> > 
> > 	if (slab_state < SYSFS)
> > 		/* Defer until later */
> > 		return 0;
> > 
> > But sysfs_slab_remove() doesn't.
> > 
> > So if the slab is created -and- destroyed at, for example, arch_initcall
> > time, then we hit a WARN in the kobject code, trying to dispose of a
> > non-existing kobject.
> > 
> Indeed, but shouldn't we be appropriately handling the return value of 
> sysfs_slab_add() so that it fails cache creation?  We wouldn't be calling 
> sysfs_slab_remove() on a cache that was never created.

It's eventually created, but yes, we should probably store a state,
unless we have a clean way to know the kobject in there is uninitialized
and test for that.

> > Now, at first sight, just adding the same test to sysfs_slab_remove()
> > would do the job... but it all seems very racy to me.
> > 
> > I don't understand in fact how this slab_state deals with races at all. 
> > 
> All modifiers of slab_state are intended to be run only on the boot cpu so 
> the only concern is the ordering.  We need slab_state to indicate how far 
> slab has been initialized since we can't otherwise enforce how code uses 
> slab in between things like kmem_cache_init(), kmem_cache_init_late(), and 
> initcalls on the boot cpu.

But initcalls aren't pinned to the boot CPU... IE. I don't see how the
sysfs creation avoids racing with SLAB creation, or am I missing
something ?

Cheers,
Ben.

> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>


--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: kmem_cache_destroy() badness with SLUB

[Christoph Lameter]

On Mon, 28 Jun 2010, Benjamin Herrenschmidt wrote:

> So if the slab is created -and- destroyed at, for example, arch_initcall
> time, then we hit a WARN in the kobject code, trying to dispose of a
> non-existing kobject.

Yes dont do that.

> Now, at first sight, just adding the same test to sysfs_slab_remove()
> would do the job... but it all seems very racy to me.

Yes lets leave as is. Dont remove slabs during boot.

> Shouldn't we have a mutex around those guys ?

At boot time?

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: kmem_cache_destroy() badness with SLUB

[Roland Dreier]

Hi folks !
Internally, I'm hitting a little "nit"...

sysfs_slab_add() has this check:

	if (slab_state < SYSFS)
		/* Defer until later */
		return 0;

But sysfs_slab_remove() doesn't.

So if the slab is created -and- destroyed at, for example, arch_initcall
time, then we hit a WARN in the kobject code, trying to dispose of a
non-existing kobject.

Now, at first sight, just adding the same test to sysfs_slab_remove()
would do the job... but it all seems very racy to me.

I don't understand in fact how this slab_state deals with races at all. 

What prevents us from hitting slab_sysfs_init() at the same time as
another CPU deos sysfs_slab_add() ? How do that deal with collisions
trying to register the same kobject twice ? Similar race with remove...

Shouldn't we have a mutex around those guys ?

Cheers,
Ben.


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/


-- 
Roland Dreier <rolandd@cisco.com> || For corporate legal information go to:
http://www.cisco.com/web/about/doing_business/legal/cri/index.html

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>