Hi Gu, On Fri, Mar 14, 2014 at 06:25:16PM +0800, Gu Zheng wrote: > Hi Ben, > On 03/13/2014 06:17 AM, Benjamin LaHaise wrote: > > > Hello Tang, > > > > On Wed, Mar 12, 2014 at 01:25:26PM +0800, Tang Chen wrote: > > ... <snip> ... > > > >>> Another spot is in > >>> aio_read_events_ring() where head and tail are fetched from the ring > >>> without > >>> any locking. I also fear we'll be introducing new performance issues with > >>> all the additonal spinlock bouncing, despite the fact that is only ever > >>> needed for migration. I'm going to continue looking into this today and > >>> will try to send out a followup to this email later. > >> > >> In the beginning of aio_read_events_ring(), it reads head and tail, not > >> write. > >> So even if ring pages are migrated, the contents of the pages will not > >> be changed. > >> So reading it is OK, from old page or from the new page, I think. > > > > Your assumption that reading it is okay is incorrect. Since we do not have > > a reference on the page at that point, it is possible that the read of the > > page takes place after the page has been freed and allocated to another part > > of the kernel. This would result in the read returning invalid information. > > What about the following patch? It adds additional reference to protect the page > avoid being freed when we reading it. > ps.It is applied on linux-next(3-13). I think that's even worse than the spinlock approach since we'll end up bouncing around the struct page's cacheline in addition to spinlock we're going to end up taking anyways. -ben -- "Thought is the essence of where you are now." -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
