Re: Hung task when calling clone() due to netfilter/slab
From: Eric W. Biederman <hidden>
Date: 2012-01-19 22:12:39
ebiederm@xmission.com (Eric W. Biederman) writes:
Christoph Lameter [off-list ref] writes:quoted
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.
Oh. I see. The problem is calling kobject_uevent (which happens to live in slabs sysfs_slab_add) with a lock held. And kobject_uevent makes a blocking call to userspace. No locks held seems to be a good policy on that one. Eric -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>