Adding DECLSPEC_ALIGN(4) to structs for atomic DWORD access

Joerg-Cyril.Hoehle at Joerg-Cyril.Hoehle at
Mon Jan 21 11:52:23 CST 2013


there are some places in Wine where I believe atomic access to 32bit entities
is expected by the programmer, yet I see little or no measures to ensure this.

DWORD-Alignment is essential when several threads can access
the same DWORD variable outside of a mutex (or within the implementation of the mutex).

DECLSPEC_ALIGN(4) is the declaration that could be used to enforce
it within a struct, assuming the struct itself is somehow aligned.

E.g. shouldn't CRITICAL_SECTION objects be aligned when used within other (non-public) structs?
(Presumably public MS structs cannot be augmented with alignment declarations)

DECLSPEC_ALIGN is hardly used in Wine

Other examples where I'm considering adding DWORD-alignment
to read DWORD-sized objects atomically:
 - MCI position values (for MCI_STATUS_POSITION)
 - MIDI end_thread control variable
 - mmdevapi GetCurrentPadding (as part of lock-less patch)

Would such patches be welcome?  Are they superfluous, assuming
that DWORD-alignment is granted and using DECLSPEC_ALIGN() only
for larger alignments, e.g. 8 or 16 (that seems to be the current use)?

One benefit I see is to highlight those variables shared among several threads.

	Jörg Höhle

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