On Wed, Nov 23, 2011 at 10:52:39AM -0800, Hugh Dickins wrote: > On Wed, 23 Nov 2011, Johannes Weiner wrote: > > > From: Johannes Weiner <jweiner@xxxxxxxxxx> > > > > This bit is protected by zone->lru_lock, there is no need for locked > > operations when setting and clearing it. > > > > Signed-off-by: Johannes Weiner <jweiner@xxxxxxxxxx> > > Unless there are special considerations which you have not mentioned at > all in the description above, this 7/8 and the similar 8/8 are mistaken. > > The atomic operation is not for guaranteeing the setting and clearing > of the bit in question: it's for guaranteeing that you don't accidentally > set or clear any of the other bits in the same word when you're doing so, > if another task is updating them at the same time as you're doing this. > > There are circumstances when non-atomic shortcuts can be taken, when > you're sure the field cannot yet be visible to other tasks (we do that > when setting PageLocked on a freshly allocated page, for example - but > even then have to rely on others using get_page_unless_zero properly). > But I don't think that's the case here. I have no idea how I could oversee this. You are, of course, right. That said, I *think* that it is safe for PageCgroupCache because nobody else should be modifying any pc->flags concurrently: PCG_LOCK: by definition exclusive and held during setting and clearing PCG_CACHE PCG_CACHE: serialized by PCG_LOCK PCG_USED: serialized by PCG_LOCK PCG_MIGRATION: serialized by PCG_LOCK PCG_MOVE_LOCK: 1. update_page_stat() is only called against file pages, and the page lock serializes charging against mapping. the page is also charged before establishing a pte mapping, so an unmap can not race with a charge. 2. split_huge_fixup runs against already charged pages. 3. move_account is serialized both by LRU-isolation during charging and PCG_LOCK PCG_FILE_MAPPED: same as PCG_MOVE_LOCK's 1. PCG_ACCT_LRU: pages are isolated from the LRU during charging But all this is obviously anything but robust, and so I retract the broken 7/8 and the might-actually-work 8/8. -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
