Re: [PATCH] fix rcu vs hotplug race

From: Ingo Molnar
Date: Mon Jun 23 2008 - 07:04:48 EST



* Dhaval Giani <dhaval@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
> ---
> 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);
> }

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.

Ingo
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