linux-kernel - xchg in fastpath for mutex unlock [mutex-xchg.h]

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Message-ID: <CAPDArR+9Kd+Rzgxj9FgQR0NCayZskXkq4bg3U-GRpCKFP5uC+Q@mail.gmail.com>
Date:	Fri, 28 Sep 2012 16:18:41 +0530
From:	Shantanu Gupta <shans95g@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: xchg in fastpath for mutex unlock [mutex-xchg.h]

Hello,

I am currently studying mutex(es) and was referring to linux sources
for it's implementation. However I noticed one shortcoming, in
"/include/asm-generic/mutex-xchg.h" of the master branch inside
__mutex_fastpath_trylock, Why doesn't the kernel use a cmpxchg instead
of dirtying the value with xchg ?

(The only thing that came to my mind was that x86_x64 implementation
mentioned that all x86_x64 cpu(s) have cmpxchg leading me to
assumption that certain archs can have xchg but not cmpxchg, so is it
this way just to work with architectures without a cmpxchg but with
xchg ?)

Thanks,
Rick
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