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

[David Rientjes]

On Tue, 17 May 2011, Peter Zijlstra wrote:

> 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.
> 

It's been true since:

	4614a696bd1c3a9af3a08f0e5874830a85b889d4
	Author: john stultz <johnstul@xxxxxxxxxx>
	Date:   Mon Dec 14 18:00:05 2009 -0800

	    procfs: allow threads to rename siblings via /proc/pid/tasks/tid/comm

Although at the time it appears that nobody was concerned about races so 
proper syncronization was never implemented.  We always had the 
prctl(PR_SET_NAME) so the majority of comm reads, those to current, 
required no locking, but this commit changed that.  The remainder of comm 
dereferences always required task_lock() and the helper get_task_comm() to 
read the string into a (usually stack-allocated) buffer.

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

I agree, and I suggested taking write privileges away from /proc/pid/comm, 
but others find that it is useful to be able to differentiate between 
threads in the same thread group without using the prctl() for debugging?
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