On 16/01/2019 21:53, Martijn Dekker wrote:
Op 16-01-19 om 22:39 schreef Martijn Dekker:
Op 16-01-19 om 14:32 schreef Herbert Xu:
Thanks for the patch. I took a deeper look into the history of
the bug and it turned out that I added REALLY_CLOSED as an
optimisation in order to avoid an unnecessary close(2) syscall.
Does this actually save cycles? I'm probably missing something, but that
code looks to me like it probably uses more cycles than close(2) going
'descriptor not open, return'.
Never mind, stupid question that I should have googled before asking it.
The answer is that a switch to kernel mode and back is expensive.
Still, if that's slow enough and happens commonly enough that it's worth
avoiding, it seems like it would still be simpler, shorter and probably
faster to just write
if (fd != -1)
close(fd);
in a few places, since we know that the only invalid fd that ever gets
passed to close() is -1. That avoids the other cases where dash already
happily calls close(-1) as well.
Cheers,
Harald van Dijk
