The rcutree.do_rcu_barrier test hook currently waits for ordinary RCU
callbacks, but it can return while kfree_rcu() still retains an object in a
partial per-CPU sheaf. This defeats the hook's purpose of preventing deferred
frees from one test spilling into the next.
The patch drains kfree_rcu sheaves and batches before retaining the hook's
explicit ordinary rcu_barrier(). Four counterbalanced fresh-VM pairs with the
full validation fixture reported 60 -> 60 active objects on the unpatched
kernel and 60 -> 59 on the patched kernel. An ordinary-callback regression
test passed on both kernels.
The primary reproducer below removes that separate regression machinery. One
fresh control/treatment pair with this exact 41-line source reproduced the
same 60 -> 60 versus 60 -> 59 split; both cells reached TEST SUCCESS with no
problem-class kernel records.
Save the source as rcu_barrier_sheaf_repro.c and create a Makefile containing:
obj-m := rcu_barrier_sheaf_repro.o
Build it with:
make -C /lib/modules/$(uname -r)/build M="$PWD" modules
Then, as root on a disposable test kernel:
insmod rcu_barrier_sheaf_repro.ko
awk '$1 == "rcu_barrier_sheaf_repro" { print $2 }' /proc/slabinfo
cat /sys/kernel/slab/rcu_barrier_sheaf_repro/sheaf_capacity
echo 1 > /sys/module/rcutree/parameters/do_rcu_barrier
awk '$1 == "rcu_barrier_sheaf_repro" { print $2 }' /proc/slabinfo
rmmod rcu_barrier_sheaf_repro
The first and second slabinfo readings are 60 and 60 without the patch, and
60 and 59 with it. kmem_cache_destroy() performs per-cache deferred-free
cleanup when the module is removed, after the measurement.
// SPDX-License-Identifier: GPL-2.0
#include <linux/init.h>
#include <linux/module.h>
#include <linux/rcupdate.h>
#include <linux/slab.h>
struct repro_object {
struct rcu_head rcu;
unsigned long payload;
};
static struct kmem_cache *repro_cache;
static int __init rcu_barrier_sheaf_repro_init(void)
{
struct repro_object *object;
repro_cache = kmem_cache_create("rcu_barrier_sheaf_repro",
sizeof(*object), 0, SLAB_NO_MERGE,
NULL);
if (!repro_cache)
return -ENOMEM;
object = kmem_cache_alloc(repro_cache, GFP_KERNEL);
if (!object) {
kmem_cache_destroy(repro_cache);
return -ENOMEM;
}
kfree_rcu(object, rcu);
return 0;
}
static void __exit rcu_barrier_sheaf_repro_exit(void)
{
kmem_cache_destroy(repro_cache);
}
module_init(rcu_barrier_sheaf_repro_init);
module_exit(rcu_barrier_sheaf_repro_exit);
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Reproduce incomplete rcutree.do_rcu_barrier drains");
Matthias Goergens (1):
rcu: drain kfree_rcu sheaves from the userspace barrier hook
.../admin-guide/kernel-parameters.txt | 7 ++---
kernel/rcu/tree.c | 27 ++++++++++++-------
2 files changed, 21 insertions(+), 13 deletions(-)
base-commit: 50d05c7c76c96b90462f24debacca971d2e86713
--
2.55.0
