[RFC] removal of sync in panic

[Christian Borntraeger <linux-kernel@borntraeger.net>]

I have a question regarding the sys_sync() call within the panic function. 
--------snip---------------
        printk(KERN_EMERG "Kernel panic: %s\n",buf);
        if (in_interrupt())
                printk(KERN_EMERG "In interrupt handler - not syncing\n");
        else if (!current->pid)
                printk(KERN_EMERG "In idle task - not syncing\n");
        else
                sys_sync(); <--------------------
        bust_spinlocks(0);

#ifdef CONFIG_SMP
        smp_send_stop();
#endif
--------------------------


I have seen panic failing two times lately on an SMP system. The box 
panic'ed but was running happily on the other cpus. The culprit of this 
failure is the fact, that these panics have been caused by a block device 
or a filesystem (e.g. using errors=panic). In these cases the  likelihood 
of a failure/hang of  sys_sync() is high. This is exactly what happened in 
both cases I have seen. Meanwhile the other cpus are happily continuing  
destroying data as the kernel has a severe problem but its not aware of 
that as smp_send_stop happens after sys_sync.

I can imagine several changes but I am not sure if this is a problem which 
must be fixed and which fix is the best.
Here are my alternatives:

1. remove sys_sync completely: syslogd and klogd use fsync. No need to help 
them. Furthermore we have a severe problem which is worth a panic, so we 
better dont do any I/O.
2. move smp_send_stop before sys_sync. This at least prevents other cpus of 
doing harm if sys_sync hangs. Here I am not sure if this is really working.
3. Add an 
        if (doing_io())
                printk(KERN_EMERG "In I/O routine - not syncing\n");
check like in_interrupt check. Unfortunately I have no clue how this can be 
achieved and it looks quite ugly.

Thanks for any ideas and clarifications
cheers

Christian