On Fri, Oct 9, 2015 at 5:18 PM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote: > > On Fri, Oct 09, 2015 at 06:29:44AM +0000, Kosuke Tatsukawa wrote: > > Neil Brown wrote: > > > Kosuke Tatsukawa <tatsu@xxxxxxxxxxxxx> writes: > > > > > >> There are several places in net/sunrpc/svcsock.c which calls > > >> waitqueue_active() without calling a memory barrier. Add a memory > > >> barrier just as in wq_has_sleeper(). > > >> > > >> I found this issue when I was looking through the linux source code > > >> for places calling waitqueue_active() before wake_up*(), but without > > >> preceding memory barriers, after sending a patch to fix a similar > > >> issue in drivers/tty/n_tty.c (Details about the original issue can be > > >> found here: https://lkml.org/lkml/2015/9/28/849). > > > > > > hi, > > > this feels like the wrong approach to the problem. It requires extra > > > 'smb_mb's to be spread around which are hard to understand as easy to > > > forget. > > > > > > A quick look seems to suggest that (nearly) every waitqueue_active() > > > will need an smb_mb. Could we just put the smb_mb() inside > > > waitqueue_active()?? > > <snip> > > > > There are around 200 occurrences of waitqueue_active() in the kernel > > source, and most of the places which use it before wake_up are either > > protected by some spin lock, or already has a memory barrier or some > > kind of atomic operation before it. > > > > Simply adding smp_mb() to waitqueue_active() would incur extra cost in > > many cases and won't be a good idea. > > > > Another way to solve this problem is to remove the waitqueue_active(), > > making the code look like this; > > if (wq) > > wake_up_interruptible(wq); > > This also fixes the problem because the spinlock in the wake_up*() acts > > as a memory barrier and prevents the code from being reordered by the > > CPU (and it also makes the resulting code is much simpler). > > I might not care which we did, except I don't have the means to test > this quickly, and I guess this is some of our most frequently called > code. > > I suppose your patch is the most conservative approach, as the > alternative is a spinlock/unlock in wake_up_interruptible, which I > assume is necessarily more expensive than an smp_mb(). > > As far as I can tell it's been this way since forever. (Well, since a > 2002 patch "NFSD: TCP: rationalise locking in RPC server routines" which > removed some spinlocks from the data_ready routines.) > > I don't understand what the actual race is yet (which code exactly is > missing the wakeup in this case? nfsd threads seem to instead get > woken up by the wake_up_process() in svc_xprt_do_enqueue().) > Those threads still use blocking calls for sendpage() and sendmsg(), so presumably they may be affected. -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
