linux-kernel - Re: [PATCH v2 5/6] signal: sys_sigprocmask() needs retarget_shared

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Date:	26 Apr 2011 21:35:41 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	torvalds@...ux-foundation.org
Cc:	linux@...izon.com, linux-kernel@...r.kernel.org, oleg@...hat.com
Subject: Re: [PATCH v2 5/6] signal: sys_sigprocmask() needs retarget_shared_pending()

> and notice how you now can do that helper function *WITHOUT* any
> conditionals, and just make it do
> 
>     sigprocmask(&clear, &set, NULL);
> 
> which handles all cases correctly (just "andn clear" + "or set") with
> no if's or switch'es.

Gripe: Arrgh, apostrophe disease!  Linus, youre picking up nasty habits.

Suggestion: Actually, the usual, more flexible implementation, is
"andn mask" + "xor set".  That gives all 4 bit operations (nop, set,
clear, and toggle) unique bit combinations.

Not that it's of any use in many cases, but it has better hack value.

That would convert to:
  switch (how) {
  case SIG_BLOCK:
      mask_bits = new_set;
      set_bits = new_set;
      break;
  case SIG_UNBLOCK:
      mask_bits = new_set;
      set_bits = 0;
      break
  case SIG_SET:
     mask_bits = low_bits | new_set;
     set_bits = new_set;
     break;
   default:
     return -EINVAL;
  }

If you prefer separate set & clear fields, with both set meaning "toggle",
which admittedly is a more elegant representation, then it's
"andn (set|clear)" + "xor set".
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