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SubjectRe: linux-next-20110923: warning kernel/rcutree.c:1833
On Sun, Sep 25, 2011 at 03:06:25PM +0200, Frederic Weisbecker wrote:
> On Sun, Sep 25, 2011 at 02:26:37PM +0300, Kirill A. Shutemov wrote:
> > On Sat, Sep 24, 2011 at 10:08:26PM -0700, Paul E. McKenney wrote:
> > > On Sun, Sep 25, 2011 at 03:24:09AM +0300, Kirill A. Shutemov wrote:
> > > > [ 29.974288] ------------[ cut here ]------------
> > > > [ 29.974308] WARNING: at /home/kas/git/public/linux-next/kernel/rcutree.c:1833 rcu_needs_cpu+0xff
> > > > [ 29.974316] Hardware name: HP EliteBook 8440p
> > > > [ 29.974321] Modules linked in: ip6table_filter ip6_tables ebtable_nat ebtables ipt_MASQUERADE iple_mangle xt_tcpudp iptable_filter ip_tables x_tables bridge stp llc rfcomm bnep acpi_cpufreq mperfckd fscache auth_rpcgss nfs_acl sunrpc ext2 loop kvm_intel kvm snd_hda_codec_hdmi snd_hda_codec_idtideodev media v4l2_compat_ioctl32 snd_seq bluetooth drm_kms_helper snd_timer tpm_infineon snd_seq_drt tpm_tis hp_accel intel_ips soundcore lis3lv02d tpm rfkill i2c_algo_bit snd_page_alloc i2c_core c16 sha256_generic aesni_intel cryptd aes_x86_64 aes_generic cbc dm_crypt dm_mod sg sr_mod sd_mod cd thermal_sys [last unloaded: scsi_wait_scan]
> > > > [ 29.974517] Pid: 0, comm: kworker/0:1 Not tainted 3.1.0-rc7-next-20110923 #2
> > > > [ 29.974521] Call Trace:
> > > > [ 29.974525] <IRQ> [<ffffffff8104d72a>] warn_slowpath_common+0x7a/0xb0
> > > > [ 29.974540] [<ffffffff8104d775>] warn_slowpath_null+0x15/0x20
> > > > [ 29.974546] [<ffffffff810bffdf>] rcu_needs_cpu+0xff/0x110
> > > > [ 29.974555] [<ffffffff8108396f>] tick_nohz_stop_sched_tick+0x13f/0x3d0
> > > > [ 29.974563] [<ffffffff814329c0>] ? notifier_call_chain+0x70/0x70
> > > > [ 29.974571] [<ffffffff81055622>] irq_exit+0xa2/0xd0
> > > > [ 29.974578] [<ffffffff8101ee75>] smp_apic_timer_interrupt+0x85/0x1c0
> > > > [ 29.974585] [<ffffffff814329c0>] ? notifier_call_chain+0x70/0x70
> > > > [ 29.974592] [<ffffffff81436e1e>] apic_timer_interrupt+0x6e/0x80
> > > > [ 29.974596] <EOI> [<ffffffff81297abd>] ? acpi_hw_read+0x4a/0x51
> > > > [ 29.974609] [<ffffffff81087a07>] ? lock_acquire+0xa7/0x160
> > > > [ 29.974615] [<ffffffff814329c0>] ? notifier_call_chain+0x70/0x70
> > > > [ 29.974622] [<ffffffff81432a16>] __atomic_notifier_call_chain+0x56/0xb0
> > > > [ 29.974631] [<ffffffff814329c0>] ? notifier_call_chain+0x70/0x70
> > > > [ 29.974642] [<ffffffff8130ebb6>] ? cpuidle_idle_call+0x106/0x350
> > > > [ 29.974651] [<ffffffff81432a81>] atomic_notifier_call_chain+0x11/0x20
> > > > [ 29.974661] [<ffffffff81001233>] cpu_idle+0xe3/0x120
> > > > [ 29.974672] [<ffffffff8141e34b>] start_secondary+0x1fd/0x204
> > > > [ 29.974681] ---[ end trace 6c1d44095a3bb7c5 ]---
> > >
> > > Do the following help?
> > >
> > > https://lkml.org/lkml/2011/9/17/47
> > > https://lkml.org/lkml/2011/9/17/45
> > > https://lkml.org/lkml/2011/9/17/43
> >
> > Yes. Thanks.
>
> I believe that doesn't really fix the issue. But the warning is not
> easy to trigger. You simply haven't hit it by chance after applying
> the patches.
>
> This happens when the idle notifier callchain is called in idle
> and is interrupted in the middle. So we have called rcu_read_lock()
> but haven't yet released with rcu_read_unlock(), and in the end
> of the interrupt we call tick_nohz_stop_sched_tick() -> rcu_needs_cpu()
> which is illegal while in an rcu read side critical section.
>
> No idea how to solve that. Any use of RCU after the tick gets stopped
> is concerned here. If it is really required that rcu_needs_cpu() can't
> be called in an rcu read side critical sectionn then it's not going
> to be easy to fix.
>
> But I don't really understand that requirement. rcu_needs_cpu() simply
> checks if we don't have callbacks to handle. So I don't understand how
> read side is concerned. It's rather the write side.
> The rule I can imagine instead is: don't call __call_rcu() once the tick is
> stopped.
>
> But I'm certainly missing something.
>
> Paul?

This is required for RCU_FAST_NO_HZ, which checks to see whether the
current CPU can accelerate the current grace period so as to enter
dyntick-idle mode sooner than it would otherwise. This takes effect
in the situation where rcu_needs_cpu() sees that there are callbacks.
It then notes a quiescent state (which is illegal in an RCU read-side
critical section), calls force_quiescent_state(), and so on. For this
to work, the current CPU must be in an RCU read-side critical section.

If this cannot be made to work, another option is to call a new RCU
function in the case where rcu_needs_cpu() returned false, but after
the RCU read-side critical section has exited. This new RCU function
could then attempt to rearrange RCU so as to allow the CPU to enter
dyntick-idle mode more quickly. It is more important for this to
happen when the CPU is going idle than when it is executing a user
process.

So, is this doable?

Thanx, Paul


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