Re: Hung task when calling clone() due to netfilter/slab
From: Eric W. Biederman <hidden>
Date: 2012-01-19 21:41:28
Also in:
linux-mm, lkml, netfilter-devel
Christoph Lameter [off-list ref] writes:
Another version that drops the slub lock for both invocations of sysfs functions from kmem_cache_create. The invocation from slab_sysfs_init is not a problem since user space is not active at that point. Subject: slub: Do not take the slub lock while calling into sysfs This patch avoids holding the slub_lock during kmem_cache_create() when calling sysfs. It is possible because kmem_cache_create() allocates the kmem_cache object and therefore is the only one context that can access the newly created object. It is therefore possible to drop the slub_lock early. We defer the adding of the new kmem_cache to the end of processing because the new kmem_cache structure would be reachable otherwise via scans over slabs. This allows sysfs_slab_add() to run without holding any locks. The case is different if we are creating an alias instead of a new kmem_cache structure. In that case we can also drop the slub lock early because we have taken a refcount on the kmem_cache structure. It therefore cannot vanish from under us. But if the sysfs_slab_alias() call fails we can no longer simply decrement the refcount since the other references may have gone away in the meantime. Call kmem_cache_destroy() to cause the refcount to be decremented and the kmem_cache structure to be freed if all references are gone. Signed-off-by: Christoph Lameter <redacted>
I am dense. Is the deadlock here that you are fixing slub calling sysfs with the slub_lock held but sysfs then calling kmem_cache_zalloc? I don't see what sysfs is doing in the creation path that would cause a deadlock except for using slab.
quoted hunk
--- mm/slub.c | 25 +++++++++++-------------- 1 file changed, 11 insertions(+), 14 deletions(-) Index: linux-2.6/mm/slub.c ===================================================================--- linux-2.6.orig/mm/slub.c 2012-01-17 09:53:26.599505365 -0600 +++ linux-2.6/mm/slub.c 2012-01-17 09:59:57.131497273 -0600@@ -3912,13 +3912,14 @@ struct kmem_cache *kmem_cache_create(con s->objsize = max(s->objsize, (int)size); s->inuse = max_t(int, s->inuse, ALIGN(size, sizeof(void *))); + up_write(&slub_lock); if (sysfs_slab_alias(s, name)) { - s->refcount--; + kmem_cache_destroy(s); goto err; } - up_write(&slub_lock); return s; } + up_write(&slub_lock); n = kstrdup(name, GFP_KERNEL); if (!n)@@ -3928,27 +3929,23 @@ struct kmem_cache *kmem_cache_create(con if (s) { if (kmem_cache_open(s, n, size, align, flags, ctor)) { - list_add(&s->list, &slab_caches); - if (sysfs_slab_add(s)) { - list_del(&s->list); - kfree(n); - kfree(s); - goto err; + + if (sysfs_slab_add(s) == 0) { + down_write(&slub_lock); + list_add(&s->list, &slab_caches); + up_write(&slub_lock); + return s; } - up_write(&slub_lock); - return s; } kfree(n); kfree(s); } err: - up_write(&slub_lock); if (flags & SLAB_PANIC) panic("Cannot create slabcache %s\n", name); - else - s = NULL; - return s; + + return NULL; } EXPORT_SYMBOL(kmem_cache_create); --To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html