Re: [Devel] Re: [PATCH 08/10] Introduce functions to restart a process

[Oren Laadan <orenl@cs.columbia.edu>]

Andrey Mirkin wrote:
> On Wednesday 22 October 2008 13:25 Louis Rilling wrote:
>> On Wed, Oct 22, 2008 at 12:49:54PM +0400, Andrey Mirkin wrote:
>>> On Monday 20 October 2008 13:23 Cedric Le Goater wrote:
>>>> Hello Andrey !
>>>>
>>>>> diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
>>>>> index 109792b..a4848a3 100644
>>>>> --- a/arch/x86/kernel/entry_32.S
>>>>> +++ b/arch/x86/kernel/entry_32.S
>>>>> @@ -225,6 +225,7 @@ ENTRY(ret_from_fork)
>>>>>  	GET_THREAD_INFO(%ebp)
>>>>>  	popl %eax
>>>>>  	CFI_ADJUST_CFA_OFFSET -4
>>>>> +ret_from_fork_tail:
>>>>>  	pushl $0x0202			# Reset kernel eflags
>>>>>  	CFI_ADJUST_CFA_OFFSET 4
>>>>>  	popfl
>>>>> @@ -233,6 +234,26 @@ ENTRY(ret_from_fork)
>>>>>  	CFI_ENDPROC
>>>>>  END(ret_from_fork)
>>>>>
>>>>> +ENTRY(i386_ret_from_resume)
>>>>> +	CFI_STARTPROC
>>>>> +	pushl %eax
>>>>> +	CFI_ADJUST_CFA_OFFSET 4
>>>>> +	call schedule_tail
>>>>> +	GET_THREAD_INFO(%ebp)
>>>>> +	popl %eax
>>>>> +	CFI_ADJUST_CFA_OFFSET -4
>>>>> +	movl (%esp), %eax
>>>>> +	testl %eax, %eax
>>>>> +	jz    1f
>>>>> +	pushl %esp
>>>>> +	call  *%eax
>>>>> +	addl  $4, %esp
>>>>> +1:
>>>>> +	addl  $256, %esp
>>>>> +	jmp   ret_from_fork_tail
>>>>> +	CFI_ENDPROC
>>>>> +END(i386_ret_from_resume)
>>>> Could you explain why you need to do this
>>>>
>>>> 	call  *%eax
>>>>
>>>> is it related to the freezer code ?
>>> It is not related to the freezer code actually.
>>> That is needed to restart syscalls. Right now I don't have a code in my
>>> patchset which restarts a syscall, but later I plan to add it.
>>> In OpenVZ checkpointing we restart syscalls if process was caught in
>>> syscall during checkpointing.
>> Do you checkpoint uninterruptible syscalls as well? If only interruptible
>> syscalls are checkpointed, I'd say that either this syscall uses
>> ERESTARTSYS or ERESTART_RESTARTBLOCK, and then signal handling code already
>> does the trick, or this syscall does not restart itself when interrupted,
>> and well, this is life, userspace just sees -EINTR, which is allowed by the
>> syscall spec.
>> Actually this is how we checkpoint/migrate tasks in interruptible syscalls
>> in Kerrighed and this works.
> 
> We checkpoint only interruptible syscalls. Some syscalls do not restart 
> themself, that is why after restarting a process we restart syscall to 
> complete it.

Can you please elaborate on this ?  I don't recall having had issues
with that.

Thanks,

Oren.

> 
> Andrey
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