From: Linus Torvalds Date: Fri, 31 Jul 2009 19:45:41 +0000 (-0700) Subject: x86: Fix serialization in pit_expect_msb() X-Git-Tag: v2.6.31-rc6~21^2 X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=b6e61eef4f9f94714ac3ee4a5c96862d9bcd1836;p=pandora-kernel.git x86: Fix serialization in pit_expect_msb() Wei Chong Tan reported a fast-PIT-calibration corner-case: | pit_expect_msb() is vulnerable to SMI disturbance corner case | in some platforms which causes /proc/cpuinfo to show wrong | CPU MHz value when quick_pit_calibrate() jumps to success | section. I think that the real issue isn't even an SMI - but the fact that in the very last iteration of the loop, there's no serializing instruction _after_ the last 'rdtsc'. So even in the absense of SMI's, we do have a situation where the cycle counter was read without proper serialization. The last check should be done outside the outer loop, since _inside_ the outer loop, we'll be testing that the PIT has the right MSB value has the right value in the next iteration. So only the _last_ iteration is special, because that's the one that will not check the PIT MSB value any more, and because the final 'get_cycles()' isn't serialized. In other words: - I'd like to move the PIT MSB check to after the last iteration, rather than in every iteration - I think we should comment on the fact that it's also a serializing instruction and so 'fences in' the TSC read. Here's a suggested replacement. Signed-off-by: Linus Torvalds Reported-by: "Tan, Wei Chong" Tested-by: "Tan, Wei Chong" LKML-Reference: Signed-off-by: Ingo Molnar --- Reading git-diff-tree failed