Re: [PATCH 1/5] clump_bits: Introduce the for_each_set_clump macro

[William Breathitt Gray <vilhelm.gray@gmail.com>]

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On Sun, Dec 27, 2020 at 11:03:06PM +0100, Arnd Bergmann wrote:
> On Sat, Dec 26, 2020 at 7:42 AM Syed Nayyar Waris <syednwaris@gmail.com> wrote:
> >
> > This macro iterates for each group of bits (clump) with set bits,
> > within a bitmap memory region. For each iteration, "start" is set to
> > the bit offset of the found clump, while the respective clump value is
> > stored to the location pointed by "clump". Additionally, the
> > bitmap_get_value() and bitmap_set_value() functions are introduced to
> > respectively get and set a value of n-bits in a bitmap memory region.
> > The n-bits can have any size from 1 to BITS_PER_LONG. size less
> > than 1 or more than BITS_PER_LONG causes undefined behaviour.
> > Moreover, during setting value of n-bit in bitmap, if a situation arise
> > that the width of next n-bit is exceeding the word boundary, then it
> > will divide itself such that some portion of it is stored in that word,
> > while the remaining portion is stored in the next higher word. Similar
> > situation occurs while retrieving the value from bitmap.
> >
> > GCC gives warning in bitmap_set_value(): https://godbolt.org/z/rjx34r
> > Add explicit check to see if the value being written into the bitmap
> > does not fall outside the bitmap.
> > The situation that it is falling outside would never be possible in the
> > code because the boundaries are required to be correct before the
> > function is called. The responsibility is on the caller for ensuring the
> > boundaries are correct.
> > The code change is simply to silence the GCC warning messages
> > because GCC is not aware that the boundaries have already been checked.
> > As such, we're better off using __builtin_unreachable() here because we
> > can avoid the latency of the conditional check entirely.
> 
> Didn't the __builtin_unreachable() end up leading to an objtool
> warning about incorrect stack frames for the code path that leads
> into the undefined behavior? I thought I saw a message from the 0day
> build bot about that and didn't expect to see it again after that.
> 
> Can you actually measure any performance difference compared
> to BUG_ON() that avoids the undefined behavior? Practically
> all CPUs from the past 20 years have branch predictors that should
> completely hide measurable overhead from this.
> 
>       Arnd

When I initially recommended using __builtin_unreachable(), I was
anticipating the use of bitmap_set_value() in kernel at large -- so the
possible performance hit from a conditional check was a concern for me.
However, now that we're restricting the scope of bitmap_set_value() to
only the GPIO subsystem, such optimization is no longer a major concern
I feel: gpio-xilinx is the only driver utilizing bitmap_set_value() --
and we know it won't be called in a loop -- so whatever hypothetical
performance hit there might be is inconsequential in the end.

Instead, we should focus on code clarity now. I believe it makes sense
given the new scope of this function to revert back to the earlier
suggestion of passing in and checking the boundary explicitly, and to
remove the __builtin_unreachable() call for now. If bitmap_set_value()
becomes available to the rest of the kernel in the future, we can
reconsider whether or not to use __builtin_unreachable().

William Breathitt Gray

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