arch, locking: Ciao arch_mutex_cpu_relax()

arch, locking: Ciao arch_mutex_cpu_relax()

The arch_mutex_cpu_relax() function, introduced by 34b133f, is
hacky and ugly. It was added a few years ago to address the fact
that common cpu_relax() calls include yielding on s390, and thus
impact the optimistic spinning functionality of mutexes. Nowadays
we use this function well beyond mutexes: rwsem, qrwlock, mcs and
lockref. Since the macro that defines the call is in the mutex header,
any users must include mutex.h and the naming is misleading as well.

This patch (i) renames the call to cpu_relax_lowlatency  ("relax, but
only if you can do it with very low latency") and (ii) defines it in
each arch's asm/processor.h local header, just like for regular cpu_relax
functions. On all archs, except s390, cpu_relax_lowlatency is simply cpu_relax,
and thus we can take it out of mutex.h. While this can seem redundant,
I believe it is a good choice as it allows us to move out arch specific
logic from generic locking primitives and enables future(?) archs to
transparently define it, similarly to System Z.

Signed-off-by: Davidlohr Bueso <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Anton Blanchard <[email protected]>
Cc: Aurelien Jacquiot <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Bharat Bhushan <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Chen Liqin <[email protected]>
Cc: Chris Metcalf <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Chris Zankel <[email protected]>
Cc: David Howells <[email protected]>
Cc: David S. Miller <[email protected]>
Cc: Deepthi Dharwar <[email protected]>
Cc: Dominik Dingel <[email protected]>
Cc: Fenghua Yu <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Cc: Guan Xuetao <[email protected]>
Cc: Haavard Skinnemoen <[email protected]>
Cc: Hans-Christian Egtvedt <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Helge Deller <[email protected]>
Cc: Hirokazu Takata <[email protected]>
Cc: Ivan Kokshaysky <[email protected]>
Cc: James E.J. Bottomley <[email protected]>
Cc: James Hogan <[email protected]>
Cc: Jason Wang <[email protected]>
Cc: Jesper Nilsson <[email protected]>
Cc: Joe Perches <[email protected]>
Cc: Jonas Bonn <[email protected]>
Cc: Joseph Myers <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Koichi Yasutake <[email protected]>
Cc: Lennox Wu <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Mark Salter <[email protected]>
Cc: Martin Schwidefsky <[email protected]>
Cc: Matt Turner <[email protected]>
Cc: Max Filippov <[email protected]>
Cc: Michael Neuling <[email protected]>
Cc: Michal Simek <[email protected]>
Cc: Mikael Starvik <[email protected]>
Cc: Nicolas Pitre <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Paul Burton <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Paul Gortmaker <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Qais Yousef <[email protected]>
Cc: Qiaowei Ren <[email protected]>
Cc: Rafael Wysocki <[email protected]>
Cc: Ralf Baechle <[email protected]>
Cc: Richard Henderson <[email protected]>
Cc: Richard Kuo <[email protected]>
Cc: Russell King <[email protected]>
Cc: Steven Miao <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Stratos Karafotis <[email protected]>
Cc: Tim Chen <[email protected]>
Cc: Tony Luck <[email protected]>
Cc: Vasily Kulikov <[email protected]>
Cc: Vineet Gupta <[email protected]>
Cc: Vineet Gupta <[email protected]>
Cc: Waiman Long <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Wolfram Sang <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>