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

[Andrey Mirkin <amirkin@parallels.com>]

On Wednesday 22 October 2008 19:25 Oren Laadan wrote:
> 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.

Right now in 2.6.18 kernel we restarts in such a way pause, rt_sigtimedwait 
and futex syscalls. Recently futex syscall was reworked and we will not need 
such hooks for it.

Andrey