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

[Uladzislau Rezki <urezki@gmail.com>]

On Thu, Oct 29, 2026 at 10:16:36AM +0800, Herbert Xu wrote:
> 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?
>  
If we have a look at this as we did not have vmalloc() here:


     RTM_NEWNEIGH                      foo() or ...
     ============                      ====================
     lock_bh(&br->hash_lock)
                                       foo()
                                        lock(B)
     rhashtable_insert_rehash()
         foo()
           lock(B)
             waits for the other CPU
                                       softirq: a frame for the bridge
                                         lock(&br->hash_lock)
                                           waits for RTM_NEWNEIGH

Same issue?

I have a question:

GFP_ATOMIC/GFP_NOWAIT guarantee they do not sleep. Do they guarantee
not hitting ABBA dead-locks or they are not interrupted?

--
Uladzislau Rezki