[PATCH] ARM: Stop irqsoff tracing if return to user with interrupts enabled

[Todd Poynor]

On Sat, Nov 13, 2010 at 12:48 AM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Fri, Nov 12, 2010 at 07:40:48PM -0800, Todd Poynor wrote:
>> Signed-off-by: Todd Poynor <toddpoynor at google.com>
>
> NAK.  Firstly, you're not explaining why.  Secondly, we purposely leave
> IRQs off when returning to userspace to avoid race conditions.  Thirdly,
> we purposely leave lockdep believing that IRQs are off while userspace
> is running, even though IRQs are not.

Apologies for the lack of explanation.  The change makes the irqsoff
tracer account for whether the return to userspace will or will not
enable interrupts; it will not override the kernel's decision on
interrupts being enabled or not, and I don't believe it will affect
lockdep's beliefs.

The reason why is to properly terminate an IRQs-disabled interval if
IRQs will in fact be enabled on return, so that the tracer does not
continue to measure the time spent in userspace as a continuation of
the IRQs-off sequence started by the call to disable_irq at
ret_slow_syscall or ret_fast_syscall.  The x86 architecture implements
macro TRACE_IRQS_IRET for the same purpose.

I'll continue to search the lists in case there's anything I've missed.

>
> I've explained why this is in the past - please search the mailing list.
>
>> ---
>>  arch/arm/kernel/entry-common.S |   14 ++++++++++++++
>>  1 files changed, 14 insertions(+), 0 deletions(-)
>>
>> diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
>> index 8bfa987..b81d270 100644
>> --- a/arch/arm/kernel/entry-common.S
>> +++ b/arch/arm/kernel/entry-common.S
>> @@ -29,6 +29,13 @@ ret_fast_syscall:
>>       ldr     r1, [tsk, #TI_FLAGS]
>>       tst     r1, #_TIF_WORK_MASK
>>       bne     fast_work_pending
>> +#if defined(CONFIG_TRACE_IRQFLAGS)
>> +     ldr     r1, [sp, #S_OFF + S_PSR]        @ get calling cpsr
>> +     tst     r1, #PSR_I_BIT
>> +     bne     1f
>> +     asm_trace_hardirqs_on
>> +1:
>> +#endif
>>
>>       /* perform architecture specific actions before user return */
>>       arch_ret_to_user r1, lr
>> @@ -65,6 +72,13 @@ ret_slow_syscall:
>>       tst     r1, #_TIF_WORK_MASK
>>       bne     work_pending
>>  no_work_pending:
>> +#if defined(CONFIG_TRACE_IRQFLAGS)
>> +     ldr     r1, [sp, #S_PSR]                @ get calling cpsr
>> +     tst     r1, #PSR_I_BIT
>> +     bne     2f
>> +     asm_trace_hardirqs_on
>> +2:
>> +#endif
>>       /* perform architecture specific actions before user return */
>>       arch_ret_to_user r1, lr
>>
>> --
>> 1.7.3.1
>>
>>
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>