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trace: reorder defaultIDGenerator fields for 8byte alignment #866
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Fixes census-instrumentation#865 This bug manifested as a consequence of https://golang.org/pkg/sync/atomic/#pkg-note-BUG and was exposed by PR census-instrumentation#851 which switched to atomically incrementing defaultIDGenerator.nextSpanID The organization of the fields was misaligned on 32-bit machines because the field `traceIDRand *rand.Rand`, a pointer was included as the second field of the struct. This is because the size of a pointer on 32-bit machines is 4 bytes, hence after the second field, we'll have offset from 12 bytes and for atomic access of *int64 fields, which are accessed in 4 byte increments by atomic operations, on 32-bit machines, their addresses are on non-8-byte divisible alignments i.e. * nextSpanID -- [28, 36] * spanIDInc -- [36, 44] but on 64-bit machines, sizeof(pointer) = 8 bytes hence their addresses are on 8-byte divisible alignments i.e. * nextSpanID -- [32, 40] * spanIDInc -- [40, 48] Thus the required reorganization but making the pointer the last field fixes the problem for both 32-bit and 64-bit. This fix can be verified by prefixing `GOARCH=386` before running code or tests.
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Aug 16, 2018
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thanks @odeke-em! |
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Thanks for the review and merge @Ramonza! |
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Fixes #865
This bug manifested as a consequence of
https://golang.org/pkg/sync/atomic/#pkg-note-BUG
and was exposed by PR #851 which switched to atomically
incrementing defaultIDGenerator.nextSpanID
The organization of the fields was misaligned on 32-bit
machines because the field
traceIDRand *rand.Rand, apointer was included as the second field of the struct.
This is because the size of a pointer on 32-bit machines
is 4 bytes, hence after the second field, we'll have
offset from 12 bytes and for atomic access of *int64
fields, which are accessed in 4 byte increments by
atomic operations, on 32-bit machines, their addresses
are on non-8-byte divisible alignments i.e.
nextSpanID -- [28, 36]
spanIDInc -- [36, 44]
but on 64-bit machines, sizeof(pointer) = 8 bytes hence
their addresses are on 8-byte divisible alignments i.e.
nextSpanID -- [32, 40]
spanIDInc -- [40, 48]
Thus the required reorganization but making the pointer
the last field fixes the problem for both 32-bit and 64-bit.
This fix can be verified by prefixing
GOARCH=386beforerunning code or tests.