[PATCH] sched_clock: Prevent callers from seeing half-updated data

[sboyd@codeaurora.org (Stephen Boyd)]

On 02/07, Stephen Boyd wrote:
> If two sched_clock sources are registered we may end up in a
> situation where a call to sched_clock() may be accessing the
> epoch cycle count for the old counter and the cycle count for the
> new counter. This can lead to confusing results where
> sched_clock() values jump and then are reset to 0 (due to the way
> the registration function forces the epoch_ns to be 0). Fix this
> by reorganizing the registration function to hold the seqlock for
> as short a time as possible while we update the clock_data
> structure for a new counter and stop resetting the epoch_ns count
> to 0.

Hmm.. This won't properly accumulate time. We need to put
whatever time has elapsed into epoch_ns when we register the new
counter for this to work. I don't have a board with this
configuration but I'll send a v2 that should fix this. Hopefully
Will can test it.

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