linux-kernel - Re: tasks getting stuck on mmap

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Date:	Tue, 17 Aug 2010 14:28:49 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Tony Luck <tony.luck@...el.com>
Cc:	David Miller <davem@...emloft.net>,
	Michel Lespinasse <walken@...gle.com>,
	David Howells <dhowells@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: tasks getting stuck on mmap_sem?

Oh, and it strikes me when looking at the sparc rwsem-const.h that it
has some unsigned constants in it. I think

  #define RWSEM_WAITING_BIAS              0xffff0000

is actually unsigned, because it's a hex constant that doesn't fit in
a signed int. So it's of type 'unsigned int' if I recall the C type
rules right.

(That's one of the really _subtle_ C type rules: hex constants
actually act differently from decimal constants).

So now RWSEM_ACTIVE_WRITE_BIAS is unsigned too.

The x86 constants are all explicitly signed.

Btw, Michel sent out an email saying:

  FYI, I just verified that RWSEM_WAITING_BIAS is defined as signed on
  all architectures except for ia64. So, this would be consistent with
  the issue being observed only on ia64.

but I think he missed that subtlety of the C type system. Other architectures do

  #define RWSEM_WAITING_BIAS             (-0x00010000)

which is indeed signed, but Sparc is unsigned despite the lack of a "u".

Of course, maybe I only _think_ I know all the C typing rules. I might
be full of sh*t. It happens..I didn't actually look them up.

             Linus

                  Linus
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