Messages in this thread | | | From | Arvind Sankar <> | Date | Wed, 2 Sep 2020 14:24:49 -0400 | Subject | Re: [PATCH v2] x86/asm: Replace __force_order with memory clobber |
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On Wed, Sep 02, 2020 at 08:19:25PM +0200, Miguel Ojeda wrote: > On Wed, Sep 2, 2020 at 5:33 PM Arvind Sankar <nivedita@alum.mit.edu> wrote: > > > > + * The compiler should not reorder volatile asm, however older versions of GCC > > + * had a bug (which was fixed in 8.1, 7.3 and 6.5) where they could sometimes > > + * reorder volatile asm. The write functions are not a problem since they have > > + * memory clobbers preventing reordering. To prevent reads from being reordered > > + * with respect to writes, use a dummy memory operand. > > I see you added the information to the commit message, but I'd still > reword this to something like:
Ah sorry, I forgot to change the comment in the source as well.
> > "The compiler should not reorder volatile asm, however GCC 4.9.x and > 5.x have a bug where they could sometimes reorder volatile asm. The > bug was fixed in 8.1, 7.3 and 6.5. The write functions are not a > problem since they have memory clobbers preventing reordering. To > prevent reads from being reordered with respect to writes, use a dummy > memory operand." > > The important point is that 4.9.x and 5.x *have* the bug and that is > the reason for having the hack. In the old wording it seems like the > bug is no more. Then one wonders why the hack is still there (i.e. > perhaps because we don't trust it, perhaps to support the rest of the > minor versions which are newer, perhaps to avoid regressions, perhaps > only the comment was updated, etc.). > > Cheers, > Miguel
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