As part of testing the 5.16 rc series we noticed a new BUG message originating from check_preemption_disabled(). We submitted a patch to move a call to smp_processor_id() into an rcu critical section within the same function. See https://lore.kernel.org/linux-rdma/20211129191958.101968.87329.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxx/T/#u. Much to my surprise, additional testing still sees the BUG! Additional testing has shown that an explicit preempt_disable()/preempt_enable() silences the warning when placed around the RCU critical section. The RCU config is: # # RCU Subsystem # CONFIG_TREE_RCU=y CONFIG_PREEMPT_RCU=y # CONFIG_RCU_EXPERT is not set CONFIG_SRCU=y CONFIG_TREE_SRCU=y CONFIG_TASKS_RCU_GENERIC=y CONFIG_TASKS_RCU=y CONFIG_TASKS_RUDE_RCU=y CONFIG_TASKS_TRACE_RCU=y CONFIG_RCU_STALL_COMMON=y CONFIG_RCU_NEED_SEGCBLIST=y CONFIG_RCU_NOCB_CPU=y # end of RCU Subsystem It looks like there is a difference between the checking in check_preemption_disabled() and the implicit preemption disabling in __rcu_read_lock(). The implicit disable looks like: static void rcu_preempt_read_enter(void) { WRITE_ONCE(current->rcu_read_lock_nesting, READ_ONCE(current->rcu_read_lock_nesting) + 1); } The checking code uses the x86 define preempt_count(): static __always_inline void __preempt_count_add(int val) { raw_cpu_add_4(__preempt_count, val); } An explicit disable uses this x86 code: static __always_inline void __preempt_count_add(int val) { raw_cpu_add_4(__preempt_count, val); } The difference seems to be the use of __preempt_count vs. rcu_read_lock_nesting. This can't be good... Mike
