On Thu, Jun 08, 2017 at 02:28:14PM -0700, Krister Johansen wrote: > On Thu, Jun 08, 2017 at 01:55:00PM -0700, Paul E. McKenney wrote: > > On Thu, Jun 08, 2017 at 01:11:48PM -0700, Krister Johansen wrote: > > > May I impose upon you to CC this patch to stable, and tag it as fixing > > > abedf8e241? I ran into this on a production 4.9 branch. When I > > > debugged it, I discovered that it went all the way back to 4.6. The > > > tl;dr is that at least for some environments, the missed wakeup > > > manifests itself as a series of hung-task warnings to console and if I'm > > > unlucky it can also generate a hang that can block interactive logins > > > via ssh. > > > > Interesting! This is the first that I have heard that this was anything > > other than a theoretical bug. To the comment in your second URL, it is > > wise to recall that a seismologist was in fact arrested for failing to > > predict an earthquake. Later acquitted/pardoned/whatever, but arrested > > nonetheless. ;-) > > Point taken. I do realize that we all make mistakes, and certainly I do > too. Indeed! Let's just say that the author of that email will have no trouble returning the favor, and sooner rather than later. ;-) > Perhaps I should have said that my survey of current callers of > swake_up() was enough to convince me that I didn't have an immediate > problem elsewhere, but that I'm not familiar enough with the code base > to make that statement with a lot of authority. The concern being that if > the patch came from RT-linux where the barrier was present in > swake_up(), are there other places where swake_up() callers still assume > this is being handled on their behalf? > > As part of this, I also pondered whether I should add a comment around > swake_up(), similar to what's already there for waitqueue_active. > I wasn't sure how subtle this is for other consumers, though. In my case, I assume I need barriers for swake_up(), which is why I found this bug by inspection. Still, I wouldn't mind a comment. Others might have other opinions. > > Silliness aside, does my patch actually fix your problem in practice as > > well as in theory? If so, may I have your Tested-by? > > Yes, it absolutely does. Consider it given: > > Tested-by: Krister Johansen <kjlx@xxxxxxxxxxxxxxxxxx> Thank you!!! Thanx, Paul > > Impressive investigative effort, by the way! > > Thanks! > > -K >