Re: [tsan] Instrument atomics

[Jakub Jelinek <jakub at redhat dot com>]

On Tue, Nov 27, 2012 at 12:13:42PM +0400, Dmitry Vyukov wrote:
> I've added 128-bit atomic ops:
> http://llvm.org/viewvc/llvm-project?view=rev&revision=168683

Thanks.
> 
> Refactored atomic ops so that the atomic operation itself is done
> under the mutex:
> http://llvm.org/viewvc/llvm-project?view=rev&revision=168682

This is IMHO very wrong.  It is fine if you hold some mutex over the atomic
operation, but you do need to perform the operation atomicly, e.g. you can't
rely that all atomic accesses to that memory location are performed from
-fsanitize=thread compiled code (it could be from non-instrumented code
from e.g. other libraries, could be from user written assembly, etc.).
By not doing the operation atomicly, the other code might observe
inconsistent state, or the tsan non-atomic code might be doing the wrong
thing.  One thing is that the tsan analysis will be right, but another
thing is that the program itself might misbehave.

> And added atomic nand operation:
> http://llvm.org/viewvc/llvm-project?view=rev&revision=168584

Thanks.  Can you please also add the memory range read/write functions?
That could be used both for builtins (e.g. if user writes
  __builtin_memcpy (&a, &b, sizeof (a));
in his code, no -fno-builtin or similar actually disables that), compiler
generated builtins - e.g. structure copies
struct large a, b;
  ...
  a = b;
and also for accesses of sizes that aren't supported by the library
(consider 32-byte or 64-byte vectors etc.).

	Jakub