Messages in this thread |  | | Subject | Re: [PATCH 1/3] comm: Introduce comm_lock spinlock to protect task->comm access | | From | John Stultz <> | | Date | Tue, 17 May 2011 15:27:05 -0700 |
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On Tue, 2011-05-17 at 23:27 +0200, Ingo Molnar wrote: > * John Stultz <john.stultz@linaro.org> 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
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