Re: [PATCH] CPU time clock support in clock_* syscalls

[Nick Piggin <nickpiggin@yahoo.com.au>]

Roland McGrath wrote:
>>It seemed like a syscall could read the values from a task currently
>>running on another CPU. If not, great.
> 
> 
> Indeed it can.  And yes, there is no locking against updates for this.  For
> sched_time on 32-bit platforms, there is the possibility it could be read
> during an update and give a bogus value if the high half is updated before
> the low half.  Since there are no guarantees about accuracy, period, I
> decided not to worry about such an anomaly.  Perhaps it would be better to
> do something about this, but AFAIK nothing perfect can be done without
> adding more words to task_struct (e.g. seqcount).  I don't know if the
> nature of SMP cache behavior makes something like:
> 
> 	do {
> 		sample = p->sched_time;
> 	} while (p->sched_time != sample);
> 
> sufficient.  That would certainly be easy to do.
> 

I don't think that will be quite sufficient.

A seqcount would probably be the way to go.