Messages in this thread | | | Date | Wed, 18 May 2011 10:02:44 +0900 | From | KOSAKI Motohiro <> | Subject | Re: [PATCH 1/3] comm: Introduce comm_lock spinlock to protect task->comm access |
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(2011/05/18 7:27), John Stultz wrote: > 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.
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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