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

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 [off-list ref] wrote:

quoted

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