Re: [PATCH lib] lib: alloc_tag_module_unload must wait for pending kfree_rcu calls

[Uladzislau Rezki <urezki@gmail.com>]

On Mon, Oct 07, 2024 at 06:49:32PM -0700, Suren Baghdasaryan wrote:
> On Mon, Oct 7, 2024 at 6:15 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > On Mon,  7 Oct 2024 22:52:24 +0200 Florian Westphal <fw@strlen.de> wrote:
> >
> > > Ben Greear reports following splat:
> > >  ------------[ cut here ]------------
> > >  net/netfilter/nf_nat_core.c:1114 module nf_nat func:nf_nat_register_fn has 256 allocated at module unload
> > >  WARNING: CPU: 1 PID: 10421 at lib/alloc_tag.c:168 alloc_tag_module_unload+0x22b/0x3f0
> > >  Modules linked in: nf_nat(-) btrfs ufs qnx4 hfsplus hfs minix vfat msdos fat
> > > ...
> > >  Hardware name: Default string Default string/SKYBAY, BIOS 5.12 08/04/2020
> > >  RIP: 0010:alloc_tag_module_unload+0x22b/0x3f0
> > >   codetag_unload_module+0x19b/0x2a0
> > >   ? codetag_load_module+0x80/0x80
> > >
> > > nf_nat module exit calls kfree_rcu on those addresses, but the free
> > > operation is likely still pending by the time alloc_tag checks for leaks.
> > >
> > > Wait for outstanding kfree_rcu operations to complete before checking
> > > resolves this warning.
> > >
> > > Reproducer:
> > > unshare -n iptables-nft -t nat -A PREROUTING -p tcp
> > > grep nf_nat /proc/allocinfo # will list 4 allocations
> > > rmmod nft_chain_nat
> > > rmmod nf_nat                # will WARN.
> > >
> > > ...
> > >
> > > --- a/lib/codetag.c
> > > +++ b/lib/codetag.c
> > > @@ -228,6 +228,8 @@ bool codetag_unload_module(struct module *mod)
> > >       if (!mod)
> > >               return true;
> > >
> > > +     kvfree_rcu_barrier();
> > > +
> > >       mutex_lock(&codetag_lock);
> > >       list_for_each_entry(cttype, &codetag_types, link) {
> > >               struct codetag_module *found = NULL;
> >
> > It's always hard to determine why a thing like this is present, so a
> > comment is helpful:
> >
> > --- a/lib/codetag.c~lib-alloc_tag_module_unload-must-wait-for-pending-kfree_rcu-calls-fix
> > +++ a/lib/codetag.c
> > @@ -228,6 +228,7 @@ bool codetag_unload_module(struct module
> >         if (!mod)
> >                 return true;
> >
> > +       /* await any module's kfree_rcu() operations to complete */
> >         kvfree_rcu_barrier();
> >
> >         mutex_lock(&codetag_lock);
> > _
> >
> > But I do wonder whether this is in the correct place.
> >
> > Waiting for a module's ->exit() function's kfree_rcu()s to complete
> > should properly be done by the core module handling code?
> 
> I don't think core module code cares about kfree_rcu()s being complete
> before the module is unloaded.
> Allocation tagging OTOH cares because it is about to destroy tags
> which will be accessed when kfree() actually happens, therefore a
> strict ordering is important.
> 
> >
> > free_module() does a full-on synchronize_rcu() prior to freeing the
> > module memory itself and I think codetag_unload_module() could be
> > called after that?
> 
> I think we could move codetag_unload_module() after synchronize_rcu()
> inside free_module() but according to the reply in
> https://lore.kernel.org/all/20241007112904.GA27104@breakpoint.cc/
> synchronize_rcu() does not help. I'm not quite sure why...
>
It is because, synchronize_rcu() is used for a bit different things,
i.e. it is about a GP completion. Offloading objects can span several
GPs.

> Note that once I'm done upstreaming
> https://lore.kernel.org/all/20240902044128.664075-3-surenb@google.com/,
> this change will not be needed and I'm planning to remove this call,
> however this change is useful for backporting. It should be sent to
> stable@vger.kernel.org # v6.10+
>
The kvfree_rcu_barrier() has been added into v6.12:

<snip>
urezki@pc638:~/data/raid0/coding/linux.git$ git tag --contains 2b55d6a42d14c8675e38d6d9adca3014fdf01951
next-20240912
next-20240919
next-20240920
next-20241002
v6.12-rc1
urezki@pc638:~/data/raid0/coding/linux.git$
<snip>

For 6.10+, it implies that the mentioned commit should be backported also.

--
Uladzislau Rezki