[Bug 490371] Re: Atomic operations not safe for ARMv7, Thumb-2 and multicore

[Michael K. Edwards]

I believe that upstream is mistaken.  It is not safe to allow other
memory accesses to be speculated into the ldrex/strex region, and (at
least on Cortex A9/A15) the only way to prevent that is to issue full
memory barriers before and after.  Arguably the compiler should be
allowed to move memory accesses across the whole block, but that seems
like a small optimization (compared to the memory barrier overhead), and
I have seen no benchmark data to support it.  I don't know whether GCC's
later optimization passes can be relied upon not to move opcodes into
the critical region if the "memory" attribute on the assembly block were
omitted.

See also http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48126, which
suggests a fix to GCC's compare-and-swap intrinsics to prevent them from
skipping the trailing memory barrier.

** Bug watch added: GCC Bugzilla #48126
   http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48126

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https://bugs.launchpad.net/bugs/490371

Title:
  Atomic operations not safe for ARMv7,Thumb-2 and multicore