Re: [PATCH 1/3] comm: Introduce comm_lock spinlock to protecttask->comm access

[Peter Zijlstra]

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.
> 
> 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.

The changelog also fails to mention _WHY_ this is no longer true. Nor
does it treat why making it true again isn't an option.

Who is changing another task's comm? That's just silly.
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