In the Linux kernel, the following vulnerability has been resolved:

rcu/nocb: Fix WARN_ON_ONCE() in the rcu_nocb_bypass_lock()

For the kernels built with CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y and
CONFIG_RCU_LAZY=y, the following scenarios will trigger WARN_ON_ONCE()
in the rcu_nocb_bypass_lock() and rcu_nocb_wait_contended() functions:

        CPU2                                               CPU11
kthread
rcu_nocb_cb_kthread                                       ksys_write
rcu_do_batch                                              vfs_write
rcu_torture_timer_cb                                      proc_sys_write
__kmem_cache_free                                         proc_sys_call_handler
kmemleak_free                                             drop_caches_sysctl_handler
delete_object_full                                        drop_slab
__delete_object                                           shrink_slab
put_object                                                lazy_rcu_shrink_scan
call_rcu                                                  rcu_nocb_flush_bypass
__call_rcu_commn                                            rcu_nocb_bypass_lock
                                                            raw_spin_trylock(&rdp->nocb_bypass_lock) fail
                                                            atomic_inc(&rdp->nocb_lock_contended);
rcu_nocb_wait_contended                                     WARN_ON_ONCE(smp_processor_id() != rdp->cpu);
 WARN_ON_ONCE(atomic_read(&rdp->nocb_lock_contended))                                          |
                            |_ _ _ _ _ _ _ _ _ _same rdp and rdp->cpu != 11_ _ _ _ _ _ _ _ _ __|

Reproduce this bug with "echo 3 > /proc/sys/vm/drop_caches".

This commit therefore uses rcu_nocb_try_flush_bypass() instead of
rcu_nocb_flush_bypass() in lazy_rcu_shrink_scan().  If the nocb_bypass
queue is being flushed, then rcu_nocb_try_flush_bypass will return
directly.

References:
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2024-35929
https://www.cve.org/CVERecord?id=CVE-2024-35929
https://git.kernel.org/stable/c/4d58c9fb45c70e62c19e8be3f3605889c47601bc
https://git.kernel.org/stable/c/927d1f4f77e4784ab3944a9df86ab14d1cd3185a
https://git.kernel.org/stable/c/dda98810b552fc6bf650f4270edeebdc2f28bd3f
https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2024/CVE-2024-35929.mbox
https://bugzilla.redhat.com/show_bug.cgi?id=2281514