kmem_cache_destroy() badness with SLUB
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From: Benjamin Herrenschmidt on 27 Jun 2010 21:50 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(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: David Rientjes on 28 Jun 2010 05:10 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 from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Benjamin Herrenschmidt on 28 Jun 2010 17:50 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(a)kvack.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont(a)kvack.org"> email(a)kvack.org </a> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Christoph Lameter on 29 Jun 2010 11:50 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 from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Roland Dreier on 6 Jul 2010 00:00
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(a)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(a)cisco.com> || For corporate legal information go to: http://www.cisco.com/web/about/doing_business/legal/cri/index.html -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |