Re: [PATCH tip/core/rcu 0/2] rcu: fixes for RCU lockdep

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sat, Feb 27, 2010 at 01:59:53PM +0100, Ingo Molnar wrote:
> 
> here's another one:
> 
> [    0.000000] Memory: 914996k/1047744k available (15146k kernel code, 452k absent, 131584k reserved, 12516k data, 2552k init)
> [    0.000000] SLUB: Genslabs=13, HWalign=64, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
> [    0.000000] 
> [    0.000000] ===================================================
> [    0.000000] [ INFO: suspicious rcu_dereference_check() usage. ]
> [    0.000000] ---------------------------------------------------
> [    0.000000] include/linux/cgroup.h:492 invoked rcu_dereference_check() without protection!
> [    0.000000] 
> [    0.000000] other info that might help us debug this:
> [    0.000000] 
> [    0.000000] 1 lock held by swapper/0:
> [    0.000000]  #0:  (&rq->lock){......}, at: [<ffffffff81ec0bad>] init_idle+0x31/0x1ee
> [    0.000000] 
> [    0.000000] stack backtrace:
> [    0.000000] Pid: 0, comm: swapper Not tainted 2.6.33-tip+ #10563
> [    0.000000] Call Trace:
> [    0.000000]  [<ffffffff810ad1c1>] lockdep_rcu_dereference+0xa1/0xb0
> [    0.000000]  [<ffffffff81ec0cbd>] init_idle+0x141/0x1ee
> [    0.000000]  [<ffffffff82ceff40>] sched_init+0x43a/0x4b6
> [    0.000000]  [<ffffffff82cdcceb>] start_kernel+0x1b3/0x49e
> [    0.000000]  [<ffffffff82cdc319>] x86_64_start_reservations+0x120/0x124
> [    0.000000]  [<ffffffff82cdc46b>] x86_64_start_kernel+0x14e/0x15d
> [    0.000000] Hierarchical RCU implementation.
> [    0.000000] RCU-based detection of stalled CPUs is enabled.
> [    0.000000] NR_IRQS:4352
> 
> on:
> 
> 1bc9767: Merge branch 'tracing/core'

Thank you very much, found it trivially this time.  ;-)

> Config attached. This too seems to a sched-init artifact. Could we use 
> inc_preempt_count() perhaps to make sure we have the preempt count disabled 
> even on !PREEMPT? Instead of hacking more 'the scheduler is not initialized 
> yet' checks into various debug checks?

This is based on plain RCU rather than RCU-sched, so inc_preempt_count()
won't help in this case (though I expect that it might work very well
in other cases).  So I hack another 'the scheduler is not initialized
yet' check in a separate set of patches. :-(

							Thanx, Paul