On Tue, 2011-05-17 at 23:27 +0200, Ingo Molnar wrote: > * John Stultz <john.stultz@xxxxxxxxxx> wrote: > > > The implicit rules for current->comm access being safe without locking are no > > longer true. Accessing current->comm without holding the task lock may result > > in null or incomplete strings (however, access won't run off the end of the > > string). > > This is rather unfortunate - task->comm is used in a number of performance > critical codepaths such as tracing. > > Why does this matter so much? A NULL string is not a big deal. I'll defer to KOSAKI Motohiro and David on this bit. :) > Note, since task->comm is 16 bytes there's the CMPXCHG16B instruction on x86 > which could be used to update it atomically, should atomicity really be > desired. Could we use this where cmpxchg16b is available and fall back to locking if not? Or does that put too much of a penalty on arches that don't have cmpxchg16b support? Alternatively, we can have locked accessors that are safe in the majority of slow-path warning printks, and provide unlocked accessors for cases where the performance is critical and the code can properly handle possibly incomplete comms. thanks -john -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>