Messages in this thread | | | Subject | Re: [PATCH] lib/idr.c: Zero memory properly in idr_remove_all | From | Kristian Høgsberg <> | Date | Mon, 12 Jan 2009 10:20:20 -0500 |
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On Sat, 2009-01-10 at 11:05 +0100, Stefan Richter wrote: > Andrew Morton wrote: ... > > Are we sure that all the other callers of free_layer() are freeing > > zeroed objects? > > > > It would be cleaner, safer and quite possibly faster to remove the > > constructor altogether and use kmem_cache_zalloc() to allocate new > > objects. > > Yes, it sounds at least safer if the allocation path should be fixed up. > > The zeroing was done in idr_remove_all() though since Kristian added it > in 2.6.23, until 2.6.26 inclusive. > > Kristian, was there a deeper reason to do it at deallocation instead of > allocation, and does the reason still apply today?
Oh wow, that's a long time ago... The idr implementation caches unused, zeroed out idr_layers in a free list in the idr struct for later use. If no layers are in the cache, the idr_pre_get() function will allocate one from the kmem_cache, which will zero out the layer in the ctor, and then add it to the idr free list. There are two other ways a layer can go back to the free list: 1) when we free up a layer by freeing the entries one by one, in which case the layer is already zeroed out, or in idr_remove_all(), where we just zero it out brute force. The problem isn't about returning un-zeroed-out objects to the kmem cache, the problem is returning them to the idr free list.
As far as I know the kmem_cache allocator is plenty fast and we could just drop the entire free list and allocate out of the kmem cache everytime in idr_pre_alloc(). If we're doing that, we should really, please, just drop the stupid idr_pre_get() then idr_get_new() and retry if fail scheme. Every idr use I've seen could just do the whole thing under a mutex and not worry about the awkward retry idea. We don't have to break API, we can just add a new function idr_alloc_new(idr, ptr, id, gfp) that just does idr_pre_alloc() and then idr_get_new() under the assumption that the client has taken the required mutex. If the client protects access to the idr using a mutex or spinlock, we can do idr_pre_get() and idr_get_new() in succession without anybody else grabbing that new layer from under us in the meantime. Same thing for idr_get_new_above(), of course. Anyways... :)
cheers, Kristian
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