Re: [PATCH 1/3] comm: Introduce comm_lock spinlock to protect task->commaccess

[KOSAKI Motohiro]

(2011/05/18 7:27), John Stultz wrote:
> 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.

Right.


> > 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. :)

Heh, I did ask you current locking rule of task->comm after you introduced
writable /proc/<pid>/comm.


> > 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.

Probably, this is safer choice.

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