On Mon, 2009-03-16 at 21:00 +1100, Nick Piggin wrote: > On Monday 16 March 2009 01:56:00 Matt Mackall wrote: > > On Sun, 2009-03-15 at 20:06 +1100, Nick Piggin wrote: > > > On Sunday 15 March 2009 17:48:18 Ingo Molnar wrote: > > > > > Cc: Nick Piggin <npiggin@xxxxxxx> > > > > > Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> > > > > > LKML-Reference: <20090128135457.350751756@xxxxxxxxx> > > > > > Signed-off-by: Ingo Molnar <mingo@xxxxxxx> > > > > > > > > and with this fixed, and with SLOB now being tested in -tip, the > > > > new lockdep assert attached below (followed by a real lockup) > > > > pops up. > > > > > > > > Seems like a genuine SLOB bug, probably present upstream as > > > > well. > > > > > > Hmmf. debugobjects calls back into the slab allocator from the page > > > allocator. The following patch would improve SLOB, but I think it > > > would be a good idea to avoid a dependency in that direction. Can > > > debugobjects defer this freeing? > > > > Yeah. I don't think any of the allocators are designed with recursion in > > mind. That the others aren't (visibly) failing here is blind luck. > > > > Nick, not really sure what your patch is accomplishing. It narrows the > > lock window, but it doesn't eliminate it. But I think we can take the > > page allocator case out from under the lock entirely, no? > > Oh, it was trying to accomplish exactly this, but wasn't tested (just > for illustration). > > I think Thomas's deferred freeing work should be a good way to fix this > problem, but of course reducing locking in SLOB doesn't hurt in the > slightest either ;) > > > > diff -r 8e0f1cee0a71 mm/slob.c > > --- a/mm/slob.c Sat Jan 24 15:41:13 2009 -0600 > > +++ b/mm/slob.c Sun Mar 15 09:50:42 2009 -0500 > > @@ -387,8 +387,6 @@ > > sp = (struct slob_page *)virt_to_page(block); > > units = SLOB_UNITS(size); > > > > - spin_lock_irqsave(&slob_lock, flags); > > - > > if (sp->units + units == SLOB_UNITS(PAGE_SIZE)) { > > /* Go directly to page allocator. Do not pass slob allocator */ > > if (slob_page_free(sp)) > > This doesn't work because you have to hold the lock over the test > otherwise another thread can concurrently meddle with sp->units. Ahh, yes, I was glossing over that code because of the misleading comment. I was assuming this was the case where the object itself was a page, rather than object is the only allocation on the page. > For that matter my previous patch was buggy, aside from the obvious > that Ingo pointed out, because I unlocked before removing the page > from the freelist too. > > This should be pretty close to correct ;) Yes. Now the only question that remains is if we want to change a nearly negligible performance improvement for a nearly negligible size increase. > -- > > Don't hold SLOB lock when freeing the page. Reduces lock hold width. > > Signed-off-by: Nick Piggin <npiggin@xxxxxxx> Acked-by: Matt Mackall <mpm@xxxxxxxxxxx> > --- > mm/slob.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > Index: linux-2.6/mm/slob.c > =================================================================== > --- linux-2.6.orig/mm/slob.c > +++ linux-2.6/mm/slob.c > @@ -393,10 +393,11 @@ static void slob_free(void *block, int s > /* Go directly to page allocator. Do not pass slob allocator */ > if (slob_page_free(sp)) > clear_slob_page_free(sp); > + spin_unlock_irqrestore(&slob_lock, flags); > clear_slob_page(sp); > free_slob_page(sp); > free_page((unsigned long)b); > - goto out; > + return; > } > > if (!slob_page_free(sp)) { -- http://selenic.com : development and support for Mercurial and Linux -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html