Re: [RFC PATCH] x86, espfix: postpone the initialization of espfix stack for AP

[Gu Zheng <guz.fnst@cn.fujitsu.com>]

ping...

On 05/22/2015 06:13 PM, Gu Zheng wrote:

> The following lockdep warning occurs when running with 4.1.0-rc3:
> [    3.178000] ------------[ cut here ]------------
> [    3.183000] WARNING: CPU: 128 PID: 0 at kernel/locking/lockdep.c:2755 lockdep_trace_alloc+0xdd/0xe0()
> [    3.193000] DEBUG_LOCKS_WARN_ON(irqs_disabled_flags(flags))
> [    3.199000] Modules linked in:
> 
> [    3.203000] CPU: 128 PID: 0 Comm: swapper/128 Not tainted 4.1.0-rc3 #70
> [    3.221000]  0000000000000000 2d6601fb3e6d4e4c ffff88086fd5fc38 ffffffff81773f0a
> [    3.230000]  0000000000000000 ffff88086fd5fc90 ffff88086fd5fc78 ffffffff8108c85a
> [    3.238000]  ffff88086fd60000 0000000000000092 ffff88086fd60000 00000000000000d0
> [    3.246000] Call Trace:
> [    3.249000]  [<ffffffff81773f0a>] dump_stack+0x4c/0x65
> [    3.255000]  [<ffffffff8108c85a>] warn_slowpath_common+0x8a/0xc0
> [    3.261000]  [<ffffffff8108c8e5>] warn_slowpath_fmt+0x55/0x70
> [    3.268000]  [<ffffffff810ee24d>] lockdep_trace_alloc+0xdd/0xe0
> [    3.274000]  [<ffffffff811cda0d>] __alloc_pages_nodemask+0xad/0xca0
> [    3.281000]  [<ffffffff810ec7ad>] ? __lock_acquire+0xf6d/0x1560
> [    3.288000]  [<ffffffff81219c8a>] alloc_page_interleave+0x3a/0x90
> [    3.295000]  [<ffffffff8121b32d>] alloc_pages_current+0x17d/0x1a0
> [    3.301000]  [<ffffffff811c869e>] ? __get_free_pages+0xe/0x50
> [    3.308000]  [<ffffffff811c869e>] __get_free_pages+0xe/0x50
> [    3.314000]  [<ffffffff8102640b>] init_espfix_ap+0x17b/0x320
> [    3.320000]  [<ffffffff8105c691>] start_secondary+0xf1/0x1f0
> [    3.327000] ---[ end trace 1b3327d9d6a1d62c ]---
> 
> This seems a mis-warning by lockdep, as we alloc pages with GFP_KERNEL in
> init_espfix_ap() which is called before enabled local irq, and the lockdep
> sub-system considers this behaviour as allocating memory with GFP_FS with
> local irq disabled, then trigger the warning as mentioned about.
> 
> Though we could allocate them on the boot CPU side and hand them over to
> the secondary CPU, but it seems a waste if some of cpus are still offline.
> As there is no need to these pages(espfix stack) until we try to run user
> code, so we can postpone the initialization of espfix stack after cpu
> booted to avoid the noise.
> 
> Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
> ---
>  arch/x86/kernel/smpboot.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 50e547e..3ce05de 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -240,13 +240,6 @@ static void notrace start_secondary(void *unused)
>  	check_tsc_sync_target();
>  
>  	/*
> -	 * Enable the espfix hack for this CPU
> -	 */
> -#ifdef CONFIG_X86_ESPFIX64
> -	init_espfix_ap();
> -#endif
> -
> -	/*
>  	 * We need to hold vector_lock so there the set of online cpus
>  	 * does not change while we are assigning vectors to cpus.  Holding
>  	 * this lock ensures we don't half assign or remove an irq from a cpu.
> @@ -901,6 +894,13 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle)
>  		}
>  	}
>  
> +	/*
> +	 * Enable the espfix hack for this CPU
> +	 */
> +#ifdef CONFIG_X86_ESPFIX64
> +	init_espfix_ap();
> +#endif
> +
>  	/* mark "stuck" area as not stuck */
>  	*trampoline_status = 0;
>