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

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