On Sun, 24 May 2020, Maciej W. Rozycki wrote:
> Hi Mikulas,
>
> > This patch makes barriers confiorm to the specification.
> >
> > 1. We add mb() before readX_relaxed and writeX_relaxed -
> > memory-barriers.txt claims that these functions must be ordered w.r.t.
> > each other. Alpha doesn't order them, so we need an explicit barrier.
> > 2. We add mb() before reads from the I/O space - so that if there's a
> > write followed by a read, there should be a barrier between them.
> >
> > Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> > Fixes: cd0e00c10672 ("alpha: io: reorder barriers to guarantee writeX() and iowriteX() ordering")
> > Fixes: 92d7223a7423 ("alpha: io: reorder barriers to guarantee writeX() and iowriteX() ordering #2")
> > Cc: stable@xxxxxxxxxxxxxxx # v4.17+
> > Acked-by: Ivan Kokshaysky <ink@xxxxxxxxxxxxxxxxxxxx>
>
> Thank you for your effort to address this regression. I have looked
> through your code and the context it is to be applied to. Overall it
> looks good to me, however I still have one concern as detailed below
> (please accept my apologies if you find it tedious to address all the
> points raised in the course of this review).
>
> > Index: linux-stable/arch/alpha/include/asm/io.h
> > ===================================================================
> > --- linux-stable.orig/arch/alpha/include/asm/io.h 2020-05-23 10:01:22.000000000 +0200
> > +++ linux-stable/arch/alpha/include/asm/io.h 2020-05-23 17:29:22.000000000 +0200
> [...]
> > @@ -487,16 +501,59 @@ extern inline void writeq(u64 b, volatil
> > #define outb_p outb
> > #define outw_p outw
> > #define outl_p outl
> > -#define readb_relaxed(addr) __raw_readb(addr)
> > -#define readw_relaxed(addr) __raw_readw(addr)
> > -#define readl_relaxed(addr) __raw_readl(addr)
> > -#define readq_relaxed(addr) __raw_readq(addr)
> > -#define writeb_relaxed(b, addr) __raw_writeb(b, addr)
> > -#define writew_relaxed(b, addr) __raw_writew(b, addr)
> > -#define writel_relaxed(b, addr) __raw_writel(b, addr)
> > -#define writeq_relaxed(b, addr) __raw_writeq(b, addr)
> >
> > /*
> > + * The _relaxed functions must be ordered w.r.t. each other, but they don't
> > + * have to be ordered w.r.t. other memory accesses.
> > + */
> > +static inline u8 readb_relaxed(const volatile void __iomem *addr)
> > +{
> > + mb();
> > + return __raw_readb(addr);
> > +}
> [etc.]
>
> Please observe that changing the `*_relaxed' entry points from merely
> aliasing the corresponding `__raw_*' handlers to more elaborate code
> sequences (though indeed slightly only, but still) makes the situation
> analogous to one we have with the ordinary MMIO accessor entry points.
> Those regular entry points have been made `extern inline' and wrapped
> into:
>
> #if IO_CONCAT(__IO_PREFIX,trivial_rw_bw) == 1
>
> and:
>
> #if IO_CONCAT(__IO_PREFIX,trivial_rw_lq) == 1
>
> respectively, with corresponding out-of-line entry points available, so
> that there is no extra inline code produced where the call to the relevant
> MMIO accessor is going to end up with an actual function call, as this
> would not help performance in any way and would expand code unnecessarily
> at all call sites.
>
> Therefore I suggest that your new `static inline' functions follow the
> pattern, perhaps by grouping them with the corresponding ordinary accessor
> functions in arch/alpha/include/asm/io.h within the relevant existing
> #ifdef, and then by making them `extern inline' and providing out-of-line
> implementations in arch/alpha/kernel/io.c, with the individual symbols
> exported. Within arch/alpha/kernel/io.c the compiler will still inline
> code as it sees fit as it already does, e.g. `__raw_readq' might get
> inlined in `readq' if it turns out cheaper than arranging for an actual
> call, including all the stack frame preparation for `ra' preservation;
> it's less likely with say `writeq' which probably always ends with a tail
> call to `__raw_writeq' as no stack frame is required in that case.
>
> That for the read accessors.
I think that making the read*_relaxed functions extern inline just causes
source code bloat with no practical gain - if we make them extern inline,
we would need two implementations (one in the include file, the other in
the C file) - and it is not good practice to duplicate code.
The functions __raw_read* are already extern inline, so the compiler will
inline/noinline them depending on the macros trivial_io_bw and
trivial_io_lq - so we can just call them from read*_relaxed without
repeating the extern inline pattern.
> > +static inline void writeb_relaxed(u8 b, volatile void __iomem *addr)
> > +{
> > + mb();
> > + __raw_writeb(b, addr);
> > +}
> [etc.]
>
> Conversely for the write accessors, keeping in mind what I have noted
> above, I suggest that you just redirect the existing aliases to the
> ordinary accessors, as there will be no difference anymore between the
> respective ordinary and relaxed accessors. That is:
>
> #define writeb_relaxed(b, addr) writeb(b, addr)
Yes - that's a good point.
> etc.
>
> Let me know if you have any further questions or comments.
>
> Maciej
Mikulas
