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

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Date:	Wed, 22 Oct 2008 14:12:12 +0400
From:	Andrey Mirkin <major@...nvz.org>
To:	Louis.Rilling@...labs.com
Cc:	devel@...nvz.org, Pavel Emelyanov <xemul@...nvz.org>,
	Cedric Le Goater <clg@...ibm.com>,
	containers@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [Devel] Re: [PATCH 08/10] Introduce functions to restart a process

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.

Andrey
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