try_to_freeze() called with IRQs disabled on ARM

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Fri, Sep 02, 2011 at 04:47:35PM +0200, Ulrich Weigand wrote:
> Assume the scenario you initally describe, where a first signal is
> ignored and leads to system call restart.  With your latest patch,
> you call into syscall_restart which sets everything up to restart
> the call (with interrupts disabled).

I don't think SIG_IGN signals even set the TIF work flag, so they
never even cause a call into do_signal().  Therefore, as far as
syscalls go, attempting to send a process (eg) a SIGINT which its
handler is set to SIG_IGN results in the process not even being
notified about the attempt - we won't even wake up while the
syscall is sleeping.

> To really fix this case would probably require some way for the
> debugger to save and restore the restore_block saved state.  This
> is not quite trivial, since it would expose that state to user space,
> effectively creating a new ABI (and probably requiring sanity checks
> to ensure a valid state is restored).  This probably cannot be fixed
> by one architecture for itself, but would need support from common
> kernel code.

Such state would have to be crytographically signed or kept entirely
within the kernel, as it would otherwise mean that you could redirect
the kernel PC to anywhere...