Hello! I am seeing the following splat in rcutorture testing of v4.11-rc1:
[ 30.694013] ============================= [ 30.694013] WARNING: suspicious RCU usage [ 30.694013] 4.11.0-rc1+ #1 Not tainted [ 30.694013] ----------------------------- [ 30.694013] /home/git/linux-2.6-tip/kernel/workqueue.c:712 sched RCU or wq_pool_mutex should be held! [ 30.694013] [ 30.694013] other info that might help us debug this: [ 30.694013] [ 30.694013] [ 30.694013] RCU used illegally from offline CPU! [ 30.694013] rcu_scheduler_active = 2, debug_locks = 0 [ 30.694013] no locks held by swapper/1/0. [ 30.694013] [ 30.694013] stack backtrace: [ 30.694013] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.11.0-rc1+ #1 [ 30.694013] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011 [ 30.694013] Call Trace: [ 30.694013] dump_stack+0x67/0x99 [ 30.694013] lockdep_rcu_suspicious+0xe7/0x120 [ 30.694013] get_work_pool+0x82/0x90 [ 30.694013] __queue_work+0x70/0x5f0 [ 30.694013] queue_work_on+0x33/0x70 [ 30.694013] clear_sched_clock_stable+0x33/0x40 [ 30.694013] early_init_intel+0xe7/0x2f0 [ 30.694013] init_intel+0x11/0x350 [ 30.694013] identify_cpu+0x344/0x5a0 [ 30.694013] identify_secondary_cpu+0x18/0x80 [ 30.694013] smp_store_cpu_info+0x39/0x40 [ 30.694013] start_secondary+0x4e/0x100 [ 30.694013] start_cpu+0x14/0x14 Here is the relevant code from x86's smp_callin(): /* * Save our processor parameters. Note: this information * is needed for clock calibration. */ smp_store_cpu_info(cpuid); /* * Get our bogomips. * Update loops_per_jiffy in cpu_data. Previous call to * smp_store_cpu_info() stored a value that is close but not as * accurate as the value just calculated. */ calibrate_delay(); cpu_data(cpuid).loops_per_jiffy = loops_per_jiffy; pr_debug("Stack at about %p\n", &cpuid); /* * This must be done before setting cpu_online_mask * or calling notify_cpu_starting. */ set_cpu_sibling_map(raw_smp_processor_id()); wmb(); notify_cpu_starting(cpuid); The problem is that smp_store_cpu_info() indirectly invokes schedule_work(), which wants to use RCU. But RCU isn't informed of the incoming CPU until the call to notify_cpu_starting(), which causes lockdep to complain bitterly about the use of RCU by the premature call to schedule_work(). I considered just moving the notify_cpu_starting() earlier in the sequence, but the comments make it seem like this would not be a wise choice. Any suggestions? Thanx, Paul