Commits on Aug 16, 2018

1. trace: reorder defaultIDGenerator fields for 8byte alignment …

   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.

   

   odeke-em committed Aug 16, 2018
   Configuration menu

   • View commit details
   • Copy full SHA for 0856a85
   • Browse repository at this point

   Copy the full SHA

   0856a85 View commit details

   Browse the repository at this point in the history