Re: cpusets: BUG: cpuset_excl_nodes_overlap() may sleep under tasklist_lock

[Kirill Korotaev]

> > FYI, there is an obvious bug in cpusets in 2.6.15-rcX:
> > cpuset_excl_nodes_overlap() may sleep (as it takes semaphore), but is 
> > called from atomic context - select_bad_process() under tasklist_lock.
> > BUG. Found by Denis Lunev.
>
>
> Sorry for not responding sooner - I was off the air for a week.
>
> Thanks for finding and reporting this.
>
> Apparently, from KUROSAWA Takahiro's report, this bug was also in
> 2.6.14.  My initial reading of the code in 2.6.14 and 2.6.15-* agrees,
> and finds that this bug was present since the cpuset_excl_nodes_overlap
> call was added, Sept 8, 2005 (in Linus's tree.)
>
>
>
> > the same actually applies to cpuset_zone_allowed() which is called e.g. 
> > from __alloc_pages()->get_page_from_freelist() and doesn't check for 
> > GPF_NOATOMIC anyhow...
>
>
> I don't think so.  Please read the comments in kernel/cpuset.c above
> the routine cpuset_zone_allowed().  Either that routine is called with
> the __GFP_HARDWALL flag set, so returns before it gets to the semaphore
> call, or it is not called at all, due to the check for ATOMIC (!wait)
> in mm/page_alloc.c.
>
> I don't see any bugs like this, in the cpuset_zone_allowed code path.
this piece of code in __alloc_pages():

        if (((p->flags & PF_MEMALLOC) || 
unlikely(test_thread_flag(TIF_MEMDIE)))
                        && !in_interrupt()) {
                if (!(gfp_mask & __GFP_NOMEMALLOC)) {
nofail_alloc:
                        /* go through the zonelist yet again, ignoring 
mins */
                        page = get_page_from_freelist(gfp_mask, order,
                                zonelist, 
ALLOC_NO_WATERMARKS|ALLOC_CPUSET);

ALLOC_CPUSET is specified, gfp_mask can be GFP_ATOMIC still and no 
__GFP_HARDWALL. Am I wrong?

Kirill


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