Re: [Xenomai-core] [RFC][PATCH] x86: Fix root domain state restoring on exception return

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

Gilles Chanteperdrix wrote:
> Jan Kiszka wrote:
>> If we enter __ipipe_handle_exception over a non-root domain and leave it
>> due to migration in the event handler over root, we must not restore the
>> root domain state so far saved on entry. This caused subtle pipeline
>> state corruptions. Actually, we only need to save the state if we enter
>> over the root domain and have to align its state to the hardware
>> interrupt mask.
>>
>> Moreover, the x86-32 regs.eflags fix-up must happen based on the current
>> root domain state to avoid more spurious corruptions.
>>
>> Signed-off-by: Jan Kiszka <jan.kiszka@domain.hid>
>> ---
>>
>> This patch is so far running fine on the x86-64 boxes of our colleagues
>> @Healthcare. It currently makes most sense to me, also for (untested)
>> x86-32, but maybe I'm still missing a problematic scenario.
>>
>>  arch/x86/kernel/ipipe.c |   64 ++++++++++++++++++++++++++--------------------
>>  1 files changed, 36 insertions(+), 28 deletions(-)
>>
>> diff --git a/arch/x86/kernel/ipipe.c b/arch/x86/kernel/ipipe.c
>> index 4442d96..8253993 100644
>> --- a/arch/x86/kernel/ipipe.c
>> +++ b/arch/x86/kernel/ipipe.c
>> @@ -702,19 +702,17 @@ static int __ipipe_xlate_signo[] = {
>>  
>>  int __ipipe_handle_exception(struct pt_regs *regs, long error_code, int vector)
>>  {
>> -	unsigned long flags;
>> -
>> -	/* Pick up the root domain state of the interrupted context. */
>> -	local_save_flags(flags);
>> +	bool restore_flags = false;
>> +	unsigned long flags = 0;
>>  
>> -	if (ipipe_root_domain_p) {
>> +	if (ipipe_root_domain_p && irqs_disabled_hw()) {
> 
> I really do not understand this hunk. It differs a lot from the current
> situation. In the current situation __fixup_if really does something,
> even if irqs were not masked on entry.
> 

Ok, but on x86_64, you have:
+	bool restore_flags = false;
+	if (ipipe_root_domain_p && irqs_disabled_hw()) {
+		restore_flags = true;
 	}
+	if (restore_flags)
+		local_irq_restore_nosync(flags);

Which I do not understand. local_irq_restore_nosync still has an effect
even if irqs were not disabled on entry.

-- 
Gilles Chanteperdrix, Free Electrons
Kernel, drivers, real-time and embedded Linux
development, consulting, training and support.
http://free-electrons.com