Christoph Lameter <cl@xxxxxxxxx> 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 <cl@xxxxxxxxx>
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.
> ---
> 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);
>
> --
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