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

Michael K. Edwards m.k.edwards at gmail.com
Fri Jun 24 19:28:22 UTC 2011


It seems that the ordered primitives do need fixing, as dmart suggests;
for confirmation see http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48126 .
I would also think that the ref()/deref() methods should have ordered
semantics, since they function like locks (a caller which sees deref()
return 0 is entitled to tear down the associated instance).

My personal preference is to use the GCC intrinsics, since they're more
likely to get full review, and I'm skeptical of the performance benefits
of the "relaxed" primitives in practice (on any system that actually has
atomics and doesn't need kernel help).  Particularly in the ordered
cases, I see no reason whatsoever to embed inline assembly.  But I
suppose it is not wrong for Qt's "relaxed" primitives to contain
equivalent assembly, without the memory barriers -- as long as the
compiler won't insert memory operations in between the ldrex/strex pair.
(The ARM folks' reading of the ARM ARM is that hardware speculation of
memory accesses into the critical region should not break the local
monitor.)

Another advantage of using the GCC intrinsics is that they will work for
64-bit operands once this support is in GCC, without embedding yet more
complex assembly into Qt.  David Gilbert has implemented 64-bit atomics
for ARMv7, and I'm helping test; I intend to implement them also for
32-bit x86 (using cmpxchg8b).

Would you entertain a patch that makes the use of inline assembly for
the non-ordered primitives optional at compile time?  (Via a template
specialization, so it still works for double-word operands if the
compiler can handle them.)

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

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