[PATCH RFC] s390: let tasks know to restart syscalls after sys_restart()

[Oren Laadan]

Serge E. Hallyn wrote:
> (This is a patch against the checkpoint/restart kernel tree at
> http://git.ncl.cs.columbia.edu/?p=linux-cr.git;a=shortlog;h=refs/heads/ckpt-v19-rc2.9)
> 
> On x86, do_signal() leaves -516 in eax while it freezes, which
> sys_restart() can use to detect that it should restart the
> syscall which was interrupted by a signal (or the freezer).
> 
> On s390, gprs[2] gets tweaked to -EINTR (-4) instead, leaving
> us no reliable way to tell whether should be restarted.  If the
> task is checkpointed here and then restarted, then the last part
> of do_signal() won't be done, and the interrupted syscall won't
> be restarted.
> 
> This patch defines TIF_RESTARTBLOCK as a thread flag showing that
> the thread expects to be frozen while kicked out of a restartable
> syscall by a signal.
> 
> The TIF_RESTARTBLOCK flag is only set for the duration of the
> get get_signal_to_deliver() which is where the task may get
> frozen.  We also set it in sys_restart() if the checkpointed task
> had had TIF_RESTARTBLOCK set.  It will get cleared if upon exiting
> sys_restart() we jump to sysc_sigpending.  If instead we jump back
> to do_signal(), then TIF_RESTARTBLOCK will stay set again until
> after get_signal_to_deliver() so that if it immediately freezes and
> is re-checkpointed, the resulting second checkpoint image again
> will have TIF_RESTARTBLOCK set.

Two comments:

1) This note will be lost once we fold this patch into a clean
patchset. Can you please make it part of the code ?

2) Maybe I'm missing something, but I'm not convinced. Can you
elaborate on why this is correct in different cases ?  Also, in
particular with respect to the post-signal-sent snippet in the
signal handling code:

        if (retval == -ERESTART_RESTARTBLOCK
            && regs->psw.addr == continue_addr) {

                regs->gprs[2] = __NR_restart_syscall;

                set_thread_flag(TIF_RESTART_SVC);

        }


Would it do what you expect after a restart ?  (restart modifies
the psw.addr)

Oren.