Re: [PATCH 1/1] win32u: Fix data race in NtUserGetProcessDpiAwarenessContext.

[Jin-oh Kang]

On Wed, Oct 12, 2022 at 3:08 AM Zebediah Figura  wrote:
> On 10/3/22 07:45, Jinoh Kang wrote:
> > @@ -5326,14 +5326,17 @@ BOOL WINAPI NtUserSetProcessDpiAwarenessContext( ULONG awareness, ULONG unknown
> >    */
> >   ULONG WINAPI NtUserGetProcessDpiAwarenessContext( HANDLE process )
> >   {
> > - DPI_AWARENESS val;
> > -
> > if (process && process != GetCurrentProcess())
> > {
> >     WARN( "not supported on other process %p\n", process );
> >     return NTUSER_DPI_UNAWARE;
> > }
> >
> > - if (!dpi_awareness) return NTUSER_DPI_UNAWARE;
> > - return dpi_awareness;
> > - val = ReadAcquire( &dpi_awareness );
> > - if (!val) return NTUSER_DPI_UNAWARE;
> > - return val;
>
> Why does this need acquire semantics?

It was intended to ensure that loads by two successive
NtUserGetProcessDpiAwarenessContext() calls don't get reordered with
each other.

If you think this won't be a problem (i.e. such reordering cannot
actually happen in C++ memory model, or the user is responsible for
serializing access), I'll downgrade it to ReadNoFence.