Messages in this thread | | | Date | Mon, 23 Jun 2008 17:19:41 +0530 | From | Gautham R Shenoy <> | Subject | Re: [PATCH] fix rcu vs hotplug race |
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On Mon, Jun 23, 2008 at 12:58:44PM +0200, Ingo Molnar wrote: > > * Dhaval Giani <dhaval@linux.vnet.ibm.com> wrote: > > > On running kernel compiles in parallel with cpu hotplug, > > > > ------------[ cut here ]------------ > > WARNING: at arch/x86/kernel/smp.c:118 > > native_smp_send_reschedule+0x21/0x36() > > Modules linked in: > > Pid: 27483, comm: cc1 Not tainted 2.6.26-rc7 #1 > > [<c01217d9>] warn_on_slowpath+0x41/0x5d > > [<c01515b7>] ? generic_file_aio_read+0x10f/0x137 > > [<c0151340>] ? file_read_actor+0x0/0xf7 > > [<c013ae4c>] ? validate_chain+0xaa/0x29c > > [<c013c854>] ? __lock_acquire+0x612/0x666 > > [<c013c854>] ? __lock_acquire+0x612/0x666 > > [<c013ae4c>] ? validate_chain+0xaa/0x29c > > [<c01715d3>] ? file_kill+0x2d/0x30 > > [<c013cbd7>] ? __lock_release+0x4b/0x51 > > [<c01715d3>] ? file_kill+0x2d/0x30 > > [<c0110355>] native_smp_send_reschedule+0x21/0x36 > > [<c014fe8f>] force_quiescent_state+0x47/0x57 > > [<c014fef0>] call_rcu+0x51/0x6d > > [<c01713b3>] __fput+0x130/0x158 > > [<c0171231>] fput+0x17/0x19 > > [<c016fd99>] filp_close+0x4d/0x57 > > [<c016fdff>] sys_close+0x5c/0x97 > > [<c0103861>] sysenter_past_esp+0x6a/0xb1 > > ======================= > > ---[ end trace aa35f3913ddf2d06 ]--- > > > > This is because a reschedule is sent to a CPU which is offline. > > Just ensure that the CPU we send the smp_send_reschedule is actually > > online. > > > > Signed-off-by: Dhaval Giani <dhaval@linux.vnet.ibm.com> > > --- > > kernel/rcuclassic.c | 3 ++- > > 1 files changed, 2 insertions(+), 1 deletion(-) > > > > Index: linux-2.6.26-rc7/kernel/rcuclassic.c > > =================================================================== > > --- linux-2.6.26-rc7.orig/kernel/rcuclassic.c > > +++ linux-2.6.26-rc7/kernel/rcuclassic.c > > @@ -93,7 +93,8 @@ static void force_quiescent_state(struct > > cpumask = rcp->cpumask; > > cpu_clear(rdp->cpu, cpumask); > > for_each_cpu_mask(cpu, cpumask) > > - smp_send_reschedule(cpu); > > + if (cpu_online(cpu)) > > + smp_send_reschedule(cpu); > > } >
Hi Ingo,
> hm, not sure - we might just be fighting the symptom and we might now > create a silent resource leak instead. Isnt a full RCU quiescent state > forced (on all CPUs) before a CPU is cleared out of cpu_online_map? That > way the to-be-offlined CPU should never actually show up in > rcp->cpumask.
No, this does not happen currently. The rcp->cpumask is always initialized to cpu_online_map&~nohz_cpu_mask when we start a new batch. Hence, before the batch ends, if a cpu goes offline we _can_ have a stale rcp->cpumask, till the RCU subsystem has handled it's CPU_DEAD notification.
Thus for a tiny interval, the rcp->cpumask would contain the offlined CPU. One of the alternatives is probably to handle this using CPU_DYING notifier instead of CPU_DEAD where we can call __rcu_offline_cpu().
The warn_on that dhaval was hitting was because of some cpu-offline that was called just before we did a local_irq_save inside call_rcu(). But at that time, the rcp->cpumask was still stale, and hence we ended up sending a smp_reschedule() to an offlined cpu. So the check may not create any resource leak.
But probably there's a better way to fix this. > > Ingo
-- Thanks and Regards gautham
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