Re: [bug report] powerpc/mm/radix: Avoid flushing the PWC on every flush_tlb_range

On Fri, Aug 11, 2017 at 03:16:08PM -0700, Tyrel Datwyler wrote:

On 08/11/2017 01:15 PM, Dan Carpenter wrote:

quoted

Hello Benjamin Herrenschmidt,

This is a semi-automatic email about new static checker warnings.

The patch 424de9c6e3f8: "powerpc/mm/radix: Avoid flushing the PWC on 
every flush_tlb_range" from Jul 19, 2017, leads to the following 
Smatch complaint:

arch/powerpc/mm/tlb-radix.c:368 radix__flush_tlb_collapsed_pmd()
	 error: we previously assumed 'mm' could be null (see line 362)

arch/powerpc/mm/tlb-radix.c
   361	
   362		pid = mm ? mm->context.id : 0;
                      ^^
Check for NULL.

   363		if (unlikely(pid == MMU_NO_CONTEXT))
   364			goto no_context;
   365	
   366		/* 4k page size, just blow the world */
   367		if (PAGE_SIZE == 0x1000) {
   368			radix__flush_all_mm(mm);
                                            ^^
Unchecked dereference.

Appears to be a false positive. MMU_NO_CONTEXT I believe is defined as "0". So, maybe it
would be clearer that we take the goto branch if this line read:

362		pid = mm ? mm->context.id : MMU_NO_CONTEXT;

Ah...  This is because I'm compiling code for other arches in a "best
effort" way.  It doesn't pull in the right headers so it doesn't know
the value of MMU_NO_CONTEXT.  Otherwise it would read the code correctly
and not complain.

Sorry for that.

regards,
dan carpenter