Re: [zswap?] BUG: sleeping function called from invalid context at kernel/cgroup/rstat.c:421

[Shakeel Butt <shakeel.butt@linux.dev>]

On Wed, Jun 10, 2026 at 11:38:29AM -0700, Nhat Pham wrote:
> On Wed, Jun 10, 2026 at 10:31 AM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> >
> > +Roman, Qi
> >
> > On Wed, Jun 10, 2026 at 09:38:03AM -0700, Nhat Pham wrote:
> > > On Wed, Jun 10, 2026 at 9:05 AM Zenghui Yu <zenghui.yu@linux.dev> wrote:
> > >
> > > Thanks for reporting, Zenghui.
> > >
> > >
> > > >
> > > > Hi all,
> > > >
> > > > The following splat was triggered on the mainline kernel:
> > > >
> > > >  BUG: sleeping function called from invalid context at kernel/cgroup/rstat.c:421
> > > >  in_atomic(): 0, irqs_disabled(): 0, non_block: 0, pid: 1126, name: cat
> > > >  preempt_count: 0, expected: 0
> > > >  RCU nest depth: 1, expected: 0
> > > >  CPU: 7 UID: 0 PID: 1126 Comm: cat Kdump: loaded Not tainted 7.1.0-rc7-00056-gacb7500801e9-dirty #304 PREEMPT
> > > >  Hardware name: QEMU QEMU Virtual Machine, BIOS edk2-stable202408-prebuilt.qemu.org 08/13/2024
> > > >  Call trace:
> > > >   show_stack+0x18/0x24 (C)
> > > >   dump_stack_lvl+0x78/0x90
> > > >   dump_stack+0x18/0x24
> > > >   __might_resched+0x114/0x170
> > > >   __might_sleep+0x48/0x98
> > > >   css_rstat_flush+0x54/0x564
> > > >   mem_cgroup_flush_stats+0x9c/0xb0
> > > >   zswap_shrinker_count+0xe4/0x1e4
> > > >   shrinker_debugfs_count_show+0xd8/0x268
> > >
> > > Ah, this seems a bit tricky.
> > >
> > > Seems like shrinker_debugfs_count_show() is invoking
> > > zswap_shrinker_count() in rcu_read_section(). zswap_shrinker_count()
> > > triggers a stats flushing, which might sleep. Not ideal.
> > >
> > > Is the rcu_read_section() here to protect memcg or shrinker? For
> > > memcg, i dont think it's necessary, no? mem_cgroup_iter() pins the
> > > memcg before returning.
> > >
> > > (memcg maintainers please fact check me).
> >
> > mem_cgroup_iter() handles the lifetime of memcg, so there is no need for rcu
> > read section for memcg.
> >
> > >
> > > If this is for the shrinker think this needs to follow shrink_slab()'s pattern.:
> > >
> > > rcu_read_lock();
> > > list_for_each_entry_rcu(shrinker, &shrinker_list, list)
> > > {
> > >     if (!shrinker_try_get(shrinker))
> > >         continue;
> > >     rcu_read_unlock();
> > > }
> > >
> > > But OTOH, doesn't seem like rcu_read_section() is what keeping it safe:
> >
> > Shouldn't the caller already holds the reference to the shrinker which it is
> > giving to this function? Does debugfs file entry holds a reference to the
> > shrinker which it is giving.
> >
> > After looking at shrinker_free(), it has call_rcu(&shrinker->rcu,
> > shrinker_free_rcu_cb), so this rcu read section is against that.
> >
> > I think we can simply use shrinker_try_get() here as Nhat said.
> 
> Hmm, so is this unsafe even with the current rcu shennanigans? What's
> stopping shrinker to be freed by that callback before we enter
> rcu_read_section()?
> 
> Seems like this is just implicitly correct - shrinker_debugfs_detach()
> and shrinker_debugfs_remove() happens before call_rcu(&shrinker->rcu,
> shrinker_free_rcu_cb);, so if you're reading this file, then it's
> before shrinker_free_rcu_cb() is even registered?
> 
> Do we still need rcu or shrinker_try_get() here?

I think you are right that we don't need rcu or shrinker_try_get() but it is
more about an active debugfs file reader. Suppose we are sleeping within rstat
flush from shrinker_debugfs_count_show() and there is a parallel
shrinker_debugfs_remove() call.

shrinker_debugfs_remove calls debugfs_remove_recursive and deep in the stack
there is a call wait_for_completion(&fsd->active_users_drained) which will wait
for active users, one of which is sleeping within rstat flush.

So, let's simply remove rcu read here.

>