lkml.org 
[lkml]   [2008]   [Jun]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] fix rcu vs hotplug race
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


\
 
 \ /
  Last update: 2008-06-23 13:51    [W:0.101 / U:0.360 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site