McKenney: Stupid RCU Tricks: rcutorture Catches an RCU Bug

Posted Dec 11, 2014 3:09 UTC (Thu) by Baylink (guest, #755)
Parent article: McKenney: Stupid RCU Tricks: rcutorture Catches an RCU Bug

"This causes call_rcu() to take evasive action" may be the most enjoyable sentence fragment I have ever seen in a debugging saga, and I have read "email only goes 500 miles" and "user can only log in while sitting down, not while standing up".

McKenney: Stupid RCU Tricks: rcutorture Catches an RCU Bug

Posted Dec 11, 2014 17:00 UTC (Thu) by nix (subscriber, #2304) [Link] (3 responses)

I'm very curious about the cause of the second bug. Something to do with the user's body absorbing a crucial amount of wifi signal? (I saw something similar with my home cinema kit, though it could better be described as "box can only play video if it can also set my sitting room on fire": it had to cool by standing on metal, not the room's wooden floor, but doing so absorbed enough signal that it couldn't pick up the local wifi net any more).

The best comment I can remember seeing recently must be in nethack, from src/pray.c, explaining why gods can zap you with fire out of heaven when you're underground but only if you're not wearing suitably high-grade gear (thanks to David Damerell for pointing it out):

/* "I am sometimes shocked by...  the nuns who never take a bath without
 * wearing a bathrobe all the time.  When asked why, since no man can see them,
 * they reply 'Oh, but you forget the good God'.  Apparently they conceive of
 * the Deity as a Peeping Tom, whose omnipotence enables Him to see through
 * bathroom walls, but who is foiled by bathrobes." --Bertrand Russell, 1943
 * Divine wrath, dungeon walls, and armor follow the same principle.
 */

McKenney: Stupid RCU Tricks: rcutorture Catches an RCU Bug

Posted Dec 11, 2014 19:43 UTC (Thu) by cesarb (subscriber, #6266) [Link] (2 responses)

I recall reading that one somewhere too. IIRC, the cause was a keyboard layout mismatch; while sitting down, the user didn't look at the keyboard (typing according to the key positions), and while standing up, the user looked at the keyboard (typing according to the key caps).

McKenney: Stupid RCU Tricks: rcutorture Catches an RCU Bug

Posted Dec 11, 2014 19:45 UTC (Thu) by mathstuf (subscriber, #69389) [Link] (1 responses)

Yeah, the 'c' and 'v' keys were switched, IIRC. Which is why it didn't affect some people, but it did for others. Personally, I just cause problems[1] for anyone at my computer who can't touch type :D .

[1]https://cdn.shopify.com/s/files/1/0152/0433/products/Top_...

McKenney: Stupid RCU Tricks: rcutorture Catches an RCU Bug

Posted Dec 26, 2014 15:13 UTC (Fri) by Baylink (guest, #755) [Link]

Specifically, that story is told as being first-person from an IBM field engineer*; the issue was that *keys in the user's password* had had their keytops switched while the terminal was being cleaned, and when the user was sitting, he touchtyped the password.

(* That is, when I read it, the writer said "when I was an IBM FE...")