Potentially racy del_timer(&timer); ... ; add_timer(&timer) sequence

Potentially racy del_timer(&timer); ... ; add_timer(&timer) sequence

[Hareesh Nagarajan]

I was going through the kernel sources looking for places where the 
following sequence occurs:
	
	del_timer(&timer);
	... modify timer ...
	add_timer(&timer)

To my surprise, I found numerous place where such code appears. I 
figured this kind of code will always be (potentially) racy on SMP machines.

Examples are:
1. net/lapb/lapb_timer.c

void lapb_start_t1timer(struct lapb_cb *lapb)
{
         del_timer(&lapb->t1timer);

         lapb->t1timer.data     = (unsigned long)lapb;
         lapb->t1timer.function = &lapb_t1timer_expiry;
         lapb->t1timer.expires  = jiffies + lapb->t1;

         add_timer(&lapb->t1timer);
}

mod_timer could have been used.

2. arch/i386/mach-voyager/voyager_thread.c
3. kernel/acct.c

         del_timer(&acct_globals.timer);
         acct_globals.needcheck = 0;
         acct_globals.timer.expires = jiffies + ACCT_TIMEOUT*HZ;
         add_timer(&acct_globals.timer);

Would anyone on LKML interested in getting this cleaned up? I could do 
it, if it would be useful. Or perhaps someone on Kernel Janitors is 
already working on it.

Thanks,

Hareesh Nagarajan
www.cs.uic.edu/~hnagaraj