> From: Paul E. McKenney <paul...@kernel.org> > Sent: Thursday, November 26, 2020 7:47 AM > ... > The rcu_segcblist_n_cbs() function returns non-zero because something > invoked call_rcu() some time previously. The ftrace facility (or just > a printk) should help you work out where that call_rcu() is located.
call_rcu() is indeed called multiple times, but as you said, this should be normal. > My best guess is that the underlying bug is that you are invoking > rcu_barrier() before the RCU grace-period kthread has been created. > This means that RCU grace periods cannot complete, which in turn means > that if there has been even one invocation of call_rcu() since boot, > rcu_barrier() cannot complete, which is what you are in fact seeing. > Please note that it is perfectly legal to invoke call_rcu() very early in > the boot process, as in even before the call to rcu_init(). Therefore, > if this is the case, the bug is the early call to rcu_barrier(), not > the early calls to call_rcu(). > > To check this, at the beginning of rcu_barrier(), check the value of > rcu_state.gp_kthread. If my guess is correct, it will be NULL. Unluckily, it's not NULL here. :-) > > Another possibility is that rcu_state.gp_kthread is non-NULL, but that > something else is preventing RCU grace periods from completing, but in It looks like somehow the scheduling is not working here: in rcu_barrier() , if I replace the wait_for_completion() with wait_for_completion_timeout(&rcu_state.barrier_completion, 30*HZ), the issue persists. > that case you should see RCU CPU stall warnings. Unless of course they > have been disabled. > Thanx, Paul I guess I didn't disable the wanrings (I don't even know how to do that :) grep RCU .config # RCU Subsystem CONFIG_TREE_RCU=y # CONFIG_RCU_EXPERT is not set CONFIG_SRCU=y CONFIG_TREE_SRCU=y CONFIG_TASKS_RCU_GENERIC=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 CONFIG_MMU_GATHER_RCU_TABLE_FREE=y # RCU Debugging # CONFIG_RCU_SCALE_TEST is not set # CONFIG_RCU_TORTURE_TEST is not set # CONFIG_RCU_REF_SCALE_TEST is not set CONFIG_RCU_CPU_STALL_TIMEOUT=30 CONFIG_RCU_TRACE=y CONFIG_RCU_EQS_DEBUG=y # end of RCU Debugging Thanks, -- Dexuan
