Messages in this thread | | | Subject | Re: [PATCH] Time precision, adjtime(x) vs. gettimeofday | From | Benjamin Herrenschmidt <> | Date | Wed, 08 Oct 2003 20:43:02 +0200 |
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> The following will prevent adjtime from causing time regression. > It delays starting the adjtime mechanism for one tick, and > keeps gettimeofday inside the window. > > Only fixes i386, but changes to other arch would be similar. > > Running a simple clock test program and playing with adjtime demonstrates > that this fixes the problem (and 2.6.0-test6 is broken). > But given the fragile nature of the timer code, it should go through some > more testing before inclusion. Andrew could you put this in the next > -mm tree?
I like that solution. There is still a possible small issue in 2.4 but I don't think we need to care about it (see below)
Note about the 2.4 SMP race I talked about, x86 is indeed safe, as it also uses (jiffies - wall_jiffies) to adjust the offset, I missed it at first as it's not done from the do_gettimeoffset() function where I was looking for it.
However, that that means we may apply more than one jiffie to xtime at once, thus the above workaround would still have a small hole. But since that happens only with insane interrupt latencies that I don't expect to see in real life, it's probably a non-issue.
2.6 should always have jiffies and wall_jiffies in perfect sync as they are manipulated within the same write_lock block.
Ben.
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