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

[Andy Lutomirski]

On Wed, May 27, 2015 at 6:20 PM, Gu Zheng <guz.fnst@xxxxxxxxxxxxxx> wrote:
> 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.

Does this pass the sigreturn_32 test on both 32-bit and 64-bit kernels
and sigreturn_64 test on 64-bit kernels?  (The test is in
tools/testing/selftests/x86.)  If so, looks good to me.

--Andy

>>
>> Signed-off-by: Gu Zheng <guz.fnst@xxxxxxxxxxxxxx>
>> ---
>>  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;
>>
>
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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