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


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      Last update: 2008-10-23 11:05    [W:0.027 / U:31.104 seconds]
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