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

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Message-ID: <20081022092502.GW15171@hawkmoon.kerlabs.com>
Date:	Wed, 22 Oct 2008 11:25:02 +0200
From:	Louis Rilling <Louis.Rilling@...labs.com>
To:	Andrey Mirkin <major@...nvz.org>
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 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.

Louis

-- 
Dr Louis Rilling			Kerlabs
Skype: louis.rilling			Batiment Germanium
Phone: (+33|0) 6 80 89 08 23		80 avenue des Buttes de Coesmes
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