Re: [RFC] Fix SMP brokenness for PF_FREEZE and make freezing usable for other purposes

[Pavel Machek <pavel@ucw.cz>]

Hi!

> > It includes whitespace changes and most of patch is nice cleanup that
> > should probably go in separately. (Hint hint :-). 
> 
> > Previous code had important property: try_to_freeze was optimized away
> > in !CONFIG_PM case. Please keep that.
> > 
> > Best way is to introduce macros and cleanup the code to use the
> > macros, without actually changing any object code. That can go in very
> > fast. Then we can switch to atomic_t ... yeah I think that's
> > neccessary, but I'd like cleanups first.
> 
> Here is such a patch:
> 
> ---
> Cleanup patch for freezing against 2.6.12-git5.
> 
> 1. Establish a simple API for process freezing defined in linux/include/sched.h:
> 
> frozen(process)		Check for frozen process
> freezing(process)	Check if a process is being frozen
> freeze(process)		Tell a process to freeze (go to refrigerator)
> thaw_process(process)	Restart process
> frozen_process(process)		Process is frozen now
> 
> 2. Remove all references to PF_FREEZE and PF_FROZEN from all
>    kernel sources except sched.h
> 
> 3. Fix numerous locations where try_to_freeze is manually done by a driver
> 
> 4. Remove the argument that is no longer necessary from two function
> calls.

Can you just keep the argument? Rename it to int unused or whatever,
but if you do it, it stays backwards-compatible (and smaller patch,
too).

> 5. Some whitespace cleanup
> 
> 6. Clear potential race in refrigerator (provides an open window of PF_FREEZE
>    cleared before setting PF_FROZEN, recalc_sigpending does not check 
>    PF_FROZEN).
> 
> This patch does not address the problem of freeze_processes() violating the rule
> that a task may only modify its own flags by setting PF_FREEZE. This is not clean
> in an SMP environment. freeze(process) is therefore not SMP safe!
> 
> Signed-off-by: Christoph Lameter <christoph@lameter.com>

Looks mostly good but:

> Index: linux-2.6.12/arch/i386/kernel/signal.c
> ===================================================================
> --- linux-2.6.12.orig/arch/i386/kernel/signal.c	2005-06-25 05:01:26.000000000 +0000
> +++ linux-2.6.12/arch/i386/kernel/signal.c	2005-06-25 05:01:28.000000000 +0000
> @@ -608,10 +608,8 @@ int fastcall do_signal(struct pt_regs *r
>  	if (!user_mode(regs))
>  		return 1;
>  
> -	if (current->flags & PF_FREEZE) {
> -		refrigerator(0);
> +	if (try_to_freeze)
>  		goto no_signal;
> -	}
>  

This is not good. Missing ().

							Pavel
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