Re: [PATCH 4/4] rhashtable: fix vmalloc() deadlock in atomic rehash

[Herbert Xu]

On Wed, Oct 28, 2026 at 11:42:18AM -0400, Liam R. Howlett (Google) wrote:
>
> Below, one CPU adds an FDB entry for an RTM_NEWNEIGH netlink request,
> while another CPU is inside vmalloc() or vfree():
> 
>     RTM_NEWNEIGH                      vmalloc() or vfree()
>     ============                      ====================
>     spin_lock_bh(&br->hash_lock)
>                                       spin_lock(&vn->busy.lock)
>     rhashtable_insert_rehash()
>       kvmalloc(GFP_ATOMIC)
>         vmalloc()
>           spin_lock(&vn->busy.lock)
>             waits for the other CPU
>                                       softirq: a frame for the bridge
>                                         spin_lock(&br->hash_lock)
>                                           waits for RTM_NEWNEIGH

I think this is a fundamental bug in the kvmalloc non-blocking
support patch.  How can it be used at all if it is susceptible
to such dead-locks?

And if you can't use it safely why don't we just get rid of the
broken kvmalloc non-block path and have it always go to kmalloc
like it did before?
 
> Fixes: c6307674ed82 ("mm: kvmalloc: add non-blocking support for vmalloc")

BTW, rhashtable patches should be sent to linux-crypto.

Thanks,
-- 
Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt