linux-kernel - Re: [GIT PULL] ext4 changes for 3.15

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Message-ID: <alpine.LFD.2.11.1406242251230.23403@eddie.linux-mips.org>
Date:	Tue, 24 Jun 2014 23:03:32 +0100 (BST)
From:	"Maciej W. Rozycki" <macro@...ux-mips.org>
To:	T Makphaibulchoke <tmac@...com>
cc:	Theodore Ts'o <tytso@....edu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [GIT PULL] ext4 changes for 3.15

On Thu, 3 Apr 2014, Theodore Ts'o wrote:

>       fs/mbcache.c: doucple the locking of local from global data

 This change causes breakage:

fs/built-in.o: In function `__mb_cache_entry_release':
mbcache.c:(.text+0x56b3c): undefined reference to `log2'
mbcache.c:(.text+0x56b3c): relocation truncated to fit: R_MIPS_26 against `log2'
mbcache.c:(.text+0x56b74): undefined reference to `__fixunsdfsi'
mbcache.c:(.text+0x56b74): relocation truncated to fit: R_MIPS_26 against `__fixunsdfsi'
mbcache.c:(.text+0x56be4): undefined reference to `log2'
mbcache.c:(.text+0x56be4): relocation truncated to fit: R_MIPS_26 against `log2'
mbcache.c:(.text+0x56bf0): undefined reference to `__fixunsdfsi'
mbcache.c:(.text+0x56bf0): relocation truncated to fit: R_MIPS_26 against `__fixunsdfsi'

and so on, and so on.  It uses __builtin_log2() that is a floating-point 
function, its corresponding <math.h> prototype is:

 -- Function: double log2 (double X)

(GCC builtins are of course implicitly prototyped).  Please note that 
floating-point calculations are not allowed in the kernel and relying on 
the compiler to optimise them away is risky to say the least.

 Can you please rewrite the fragment using __builtin_log2() so as to avoid 
the floating-point calculation, using ffs() or suchlike perhaps?

  Maciej
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