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

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Date:	Thu, 23 Oct 2008 13:54:45 +0400
From:	Andrey Mirkin <major@...nvz.org>
To:	Cedric Le Goater <clg@...ibm.com>
Cc:	devel@...nvz.org, Pavel Emelyanov <xemul@...nvz.org>,
	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 16:47 Cedric Le Goater wrote:
> >>> +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.
>
> ok. I get it now. why 256 bytes of extra stack ? I'm sure it's not random.
We are putting special structure on stack, which is used at the very end of 
the whole restart procedure to restore complex states (ptrace is one of such 
cases). Right now I don't need to use this structure as we have a deal with 
simple cases, but reservation of 256 bytes on stack is needed for future.

Andrey
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