Re: [PATCH 00/14] replace call_rcu by kfree_rcu for simple kmem_cache_free callback

Paul E. McKenney
2024-06-13T03:38:02Z

On Thu, Jun 13, 2024 at 02:31:53AM +0200, Jason A. Donenfeld wrote:
> On Thu, Jun 13, 2024 at 01:31:57AM +0200, Jason A. Donenfeld wrote:
> > On Wed, Jun 12, 2024 at 03:37:55PM -0700, Paul E. McKenney wrote:
> > > On Wed, Jun 12, 2024 at 02:33:05PM -0700, Jakub Kicinski wrote:
> > > > On Sun, 9 Jun 2024 10:27:12 +0200 Julia Lawall wrote:
> > > > > Since SLOB was removed, it is not necessary to use call_rcu
> > > > > when the callback only performs kmem_cache_free. Use
> > > > > kfree_rcu() directly.
> > > > >
> > > > > The changes were done using the following Coccinelle semantic patch.
> > > > > This semantic patch is designed to ignore cases where the callback
> > > > > function is used in another way.
> > > >
> > > > How does the discussion on:
> > > > [PATCH] Revert "batman-adv: prefer kfree_rcu() over call_rcu() with free-only callbacks"
> > > > https://lore.kernel.org/all/20240612133357.2596-1-linus.luessing@c0d3.blue/
> > > > reflect on this series? IIUC we should hold off..
> > >
> > > We do need to hold off for the ones in kernel modules (such as 07/14)
> > > where the kmem_cache is destroyed during module unload.
> > >
> > > OK, I might as well go through them...
> > >
> > > [PATCH 01/14] wireguard: allowedips: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
> > > Needs to wait, see wg_allowedips_slab_uninit().
> >
> > Right, this has exactly the same pattern as the batman-adv issue:
> >
> > void wg_allowedips_slab_uninit(void)
> > {
> > rcu_barrier();
> > kmem_cache_destroy(node_cache);
> > }
> >
> > I'll hold off on sending that up until this matter is resolved.
>
> BTW, I think this whole thing might be caused by:
>
> a35d16905efc ("rcu: Add basic support for kfree_rcu() batching")
>
> The commit message there mentions:
>
> There is an implication with rcu_barrier() with this patch. Since the
> kfree_rcu() calls can be batched, and may not be handed yet to the RCU
> machinery in fact, the monitor may not have even run yet to do the
> queue_rcu_work(), there seems no easy way of implementing rcu_barrier()
> to wait for those kfree_rcu()s that are already made. So this means a
> kfree_rcu() followed by an rcu_barrier() does not imply that memory will
> be freed once rcu_barrier() returns.
>
> Before that, a kfree_rcu() used to just add a normal call_rcu() into the
> list, but with the function offset < 4096 as a special marker. So the
> kfree_rcu() calls would be treated alongside the other call_rcu() ones
> and thus affected by rcu_barrier(). Looks like that behavior is no more
> since this commit.

You might well be right, and thank you for digging into this!

> Rather than getting rid of the batching, which seems good for
> efficiency, I wonder if the right fix to this would be adding a
> `should_destroy` boolean to kmem_cache, which kmem_cache_destroy() sets
> to true. And then right after it checks `if (number_of_allocations == 0)
> actually_destroy()`, and likewise on each kmem_cache_free(), it could
> check `if (should_destroy && number_of_allocations == 0)
> actually_destroy()`. This way, the work is delayed until it's safe to do
> so. This might also mitigate other lurking bugs of bad code that calls
> kmem_cache_destroy() before kmem_cache_free().

Here are the current options being considered, including those that
are completely brain-dead:

o Document current state. (Must use call_rcu() if module
destroys slab of RCU-protected objects.)

Need to review Julia's and Uladzislau's series of patches
that change call_rcu() of slab objects to kfree_rcu().

o Make rcu_barrier() wait for kfree_rcu() objects. (This is
surprisingly complex and will wait unnecessarily in some cases.
However, it does preserve current code.)

o Make a kfree_rcu_barrier() that waits for kfree_rcu() objects.
(This avoids the unnecessary waits, but adds complexity to
kfree_rcu(). This is harder than it looks, but could be done,
for example by maintaining pairs of per-CPU counters and handling
them in an SRCU-like fashion. Need some way of communicating the
index, though.)

(There might be use cases where both rcu_barrier() and
kfree_rcu_barrier() would need to be invoked.)

A simpler way to implement this is to scan all of the in-flight
objects, and queue each (either separately or in bulk) using
call_rcu(). This still has problems with kfree_rcu_mightsleep()
under low-memory conditions, in which case there are a bunch
of synchronize_rcu() instances waiting. These instances could
use SRCU-like per-CPU arrays of counters. Or just protect the
calls to synchronize_rcu() and the later frees with an SRCU
reader, then have the other end call synchronize_srcu().

o Make the current kmem_cache_destroy() asynchronously wait for
all memory to be returned, then complete the destruction.
(This gets rid of a valuable debugging technique because
in normal use, it is a bug to attempt to destroy a kmem_cache
that has objects still allocated.)

o Make a kmem_cache_destroy_rcu() that asynchronously waits for
all memory to be returned, then completes the destruction.
(This raises the question of what to is it takes a "long time"
for the objects to be freed.)

o Make a kmem_cache_free_barrier() that blocks until all
objects in the specified kmem_cache have been freed.

o Make a kmem_cache_destroy_wait() that waits for all memory to
be returned, then does the destruction. This is equivalent to:

kmem_cache_free_barrier(&mycache);
kmem_cache_destroy(&mycache);

Uladzislau has started discussions on the last few of these:
https://lore.kernel.org/all/ZmnL4jkhJLIW924W@pc636/

I have also added this information to a Google Document for
easier tracking:
https://docs.google.com/document/d/1v0rcZLvvjVGejT3523W0rDy_sLFu2LWc_NR3fQItZaA/edit?usp=sharing

Other thoughts?

Thanx, Paul