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SubjectRe: [Devel] Re: [PATCH 08/10] Introduce functions to restart a process
On Wed, Oct 22, 2008 at 02:12:12PM +0400, 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.

I guess you do that to avoid breaking application that are badly written and do
not handle -EINTR correctly with interruptible syscalls. Right?

Louis

--
Dr Louis Rilling Kerlabs
Skype: louis.rilling Batiment Germanium
Phone: (+33|0) 6 80 89 08 23 80 avenue des Buttes de Coesmes
http://www.kerlabs.com/ 35700 Rennes
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