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2014-01-12locking/doc: Rename LOCK/UNLOCK to ACQUIRE/RELEASEPeter Zijlstra
The LOCK and UNLOCK barriers as described in our barrier document are generally known as ACQUIRE and RELEASE barriers in other literature. Since we plan to introduce the acquire and release nomenclature in generic kernel primitives we should amend the document to avoid confusion as to what an acquire/release means. Reviewed-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> Signed-off-by: Peter Zijlstra <peterz@infradead.org> Acked-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: Frederic Weisbecker <fweisbec@gmail.com> Cc: Michael Ellerman <michael@ellerman.id.au> Cc: Michael Neuling <mikey@neuling.org> Cc: Russell King <linux@arm.linux.org.uk> Cc: Geert Uytterhoeven <geert@linux-m68k.org> Cc: Heiko Carstens <heiko.carstens@de.ibm.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com> Cc: Victor Kaplansky <VICTORK@il.ibm.com> Cc: Tony Luck <tony.luck@intel.com> Cc: Oleg Nesterov <oleg@redhat.com> Link: http://lkml.kernel.org/r/20131217092435.GC21999@twins.programming.kicks-ass.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2013-12-16Documentation/memory-barriers.txt: Downgrade UNLOCK+BLOCKPaul E. McKenney
Historically, an UNLOCK+LOCK pair executed by one CPU, by one task, or on a given lock variable has implied a full memory barrier. In a recent LKML thread, the wisdom of this historical approach was called into question: http://www.spinics.net/lists/linux-mm/msg65653.html, in part due to the memory-order complexities of low-handoff-overhead queued locks on x86 systems. This patch therefore removes this guarantee from the documentation, and further documents how to restore it via a new smp_mb__after_unlock_lock() primitive. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: <linux-arch@vger.kernel.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Link: http://lkml.kernel.org/r/1386799151-2219-6-git-send-email-paulmck@linux.vnet.ibm.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
2013-12-16Documentation/memory-barriers.txt: Document ACCESS_ONCE()Paul E. McKenney
The situations in which ACCESS_ONCE() is required are not well documented, so this commit adds some verbiage to memory-barriers.txt. Reported-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org> Reviewed-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: <linux-arch@vger.kernel.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Link: http://lkml.kernel.org/r/1386799151-2219-4-git-send-email-paulmck@linux.vnet.ibm.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
2013-12-16Documentation/memory-barriers.txt: Prohibit speculative writesPeter Zijlstra
No SMP architecture currently supporting Linux allows speculative writes, so this commit updates Documentation/memory-barriers.txt to prohibit them in Linux core code. It also records restrictions on their use. Signed-off-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org> Reviewed-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: <linux-arch@vger.kernel.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Link: http://lkml.kernel.org/r/1386799151-2219-3-git-send-email-paulmck@linux.vnet.ibm.com [ Paul modified the original patch from Peter. ] Signed-off-by: Ingo Molnar <mingo@kernel.org>
2013-12-16Documentation/memory-barriers.txt: Add long atomic examples to ↵Paul E. McKenney
memory-barriers.txt Although the atomic_long_t functions are quite useful, they are a bit obscure. This commit therefore adds the common ones alongside their atomic_t counterparts in Documentation/memory-barriers.txt. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org> Reviewed-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: <linux-arch@vger.kernel.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Link: http://lkml.kernel.org/r/1386799151-2219-2-git-send-email-paulmck@linux.vnet.ibm.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
2013-12-16Documentation/memory-barriers.txt: Add needed ACCESS_ONCE() calls to ↵Paul E. McKenney
memory-barriers.txt The Documentation/memory-barriers.txt file was written before the need for ACCESS_ONCE() was fully appreciated. It therefore contains no ACCESS_ONCE() calls, which can be a problem when people lift examples from it. This commit therefore adds ACCESS_ONCE() calls. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org> Reviewed-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: <linux-arch@vger.kernel.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Link: http://lkml.kernel.org/r/1386799151-2219-1-git-send-email-paulmck@linux.vnet.ibm.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
2013-11-22Documentation/memory-barriers.txt: Fix a typo in the data dependency descriptionIngo Molnar
This typo has been there forever, it is 7.5 years old, looks like this section of our memory ordering documentation is a place where most eyes are glazed over already ;-) [ Also fix some stray spaces and stray tabs while at it, shrinking the file by 49 bytes. Visual output unchanged. ] Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Link: http://lkml.kernel.org/n/tip-gncea9cb8igosblizfqMXrie@git.kernel.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
2013-08-19doc: Fix memory-barrier control-dependency examplePaul E. McKenney
Each control-dependency example needs its barriers between the "if" condition and the body of the "if" because a control dependency is a dependency induced by a branch. This commit makes the needed adjustment. Reported-by: Yongming Shen <symingz@gmail.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2013-01-08Documentation: Memory barrier semantics of atomic_xchg()Richard Braun
Add atomic_xchg() to documentation for atomic operations and memory barriers. Signed-off-by: Richard Braun <rbraun@sceen.net> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2012-10-23Documentation: Fix memory-barriers.txt examplePaul E. McKenney
This commit fixes a broken example of overlapping stores in the Documentation/memory-barriers.txt file. Reported-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2011-09-27doc: fix broken referencesPaul Bolle
There are numerous broken references to Documentation files (in other Documentation files, in comments, etc.). These broken references are caused by typo's in the references, and by renames or removals of the Documentation files. Some broken references are simply odd. Fix these broken references, sometimes by dropping the irrelevant text they were part of. Signed-off-by: Paul Bolle <pebolle@tiscali.nl> Signed-off-by: Jiri Kosina <jkosina@suse.cz>
2011-03-04smp: Document transitivity for memory barriers.Paul E. McKenney
Transitivity is guaranteed only for full memory barriers (smp_mb()). Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2010-03-24Document Linux's circular buffering capabilitiesDavid Howells
Document the circular buffering capabilities available in Linux. Signed-off-by: David Howells <dhowells@redhat.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Randy Dunlap <rdunlap@xenotime.net> Reviewed-by: Stefan Richter <stefanr@s5r6.in-berlin.de> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2009-04-29sched: Document memory barriers implied by sleep/wake-up primitivesDavid Howells
Add a section to the memory barriers document to note the implied memory barriers of sleep primitives (set_current_state() and wrappers) and wake-up primitives (wake_up() and co.). Also extend the in-code comments on the wake_up() functions to note these implied barriers. [ Impact: add documentation ] Signed-off-by: David Howells <dhowells@redhat.com> Cc: Oleg Nesterov <oleg@redhat.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> LKML-Reference: <20090428140138.1192.94723.stgit@warthog.procyon.org.uk> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-05-14read_barrier_depends arch fixletsNick Piggin
read_barrie_depends has always been a noop (not a compiler barrier) on all architectures except SMP alpha. This brings UP alpha and frv into line with all other architectures, and fixes incorrect documentation. Signed-off-by: Nick Piggin <npiggin@suse.de> Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2008-04-20PCI: doc/pci: create Documentation/PCI/ and move files into itRandy Dunlap
Create Documentation/PCI/ and move PCI-related files to it. Fix a few instances of trailing whitespace. Update references to the new file locations. Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com> Cc: Jesse Barnes <jbarnes@virtuousgeek.org> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
2008-02-23documentation: atomic_add_unless() doesn't imply mb() on failureOleg Nesterov
(sorry for being offtpoic, but while experts are here...) A "typical" implementation of atomic_add_unless() can return 0 immediately after the first atomic_read() (before doing cmpxchg). In that case it doesn't provide any barrier semantics. See include/asm-ia64/atomic.h as an example. We should either change the implementation, or fix the docs. Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru> Acked-by: Nick Piggin <npiggin@suse.de> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2007-10-18bitops: introduce lock opsNick Piggin
Introduce test_and_set_bit_lock / clear_bit_unlock bitops with lock semantics. Convert all architectures to use the generic implementation. Signed-off-by: Nick Piggin <npiggin@suse.de> Acked-By: David Howells <dhowells@redhat.com> Cc: Richard Henderson <rth@twiddle.net> Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru> Cc: Russell King <rmk@arm.linux.org.uk> Cc: Haavard Skinnemoen <hskinnemoen@atmel.com> Cc: Bryan Wu <bryan.wu@analog.com> Cc: Mikael Starvik <starvik@axis.com> Cc: David Howells <dhowells@redhat.com> Cc: Yoshinori Sato <ysato@users.sourceforge.jp> Cc: "Luck, Tony" <tony.luck@intel.com> Cc: Hirokazu Takata <takata@linux-m32r.org> Cc: Geert Uytterhoeven <geert@linux-m68k.org> Cc: Roman Zippel <zippel@linux-m68k.org> Cc: Greg Ungerer <gerg@uclinux.org> Cc: Ralf Baechle <ralf@linux-mips.org> Cc: Kyle McMartin <kyle@mcmartin.ca> Cc: Matthew Wilcox <willy@debian.org> Cc: Paul Mackerras <paulus@samba.org> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: Heiko Carstens <heiko.carstens@de.ibm.com> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com> Cc: Paul Mundt <lethal@linux-sh.org> Cc: Kazumoto Kojima <kkojima@rr.iij4u.or.jp> Cc: Richard Curnow <rc@rc0.org.uk> Cc: William Lee Irwin III <wli@holomorphy.com> Cc: "David S. Miller" <davem@davemloft.net> Cc: Jeff Dike <jdike@addtoit.com> Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it> Cc: Miles Bader <uclinux-v850@lsi.nec.co.jp> Cc: Andi Kleen <ak@muc.de> Cc: Chris Zankel <chris@zankel.net> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2007-05-23Documentation/memory-barriers.txt: various fixesJarek Poplawski
Fix various grammatical issues in Documentation/memory-barriers.txt. Cc: "Robert P. J. Day" <rpjday@mindspring.com> Signed-off-by: Jarek Poplawski <jarkao2@o2.pl> Signed-off-by: David Howells <dhowells@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2006-11-30Fix typos in /Documentation : 'T''Matt LaPlante
This patch fixes typos in various Documentation txts. The patch addresses some +words starting with the letter 'T'. Signed-off-by: Matt LaPlante <kernel1@cyberdogtech.com> Acked-by: Randy Dunlap <rdunlap@xenotime.net> Signed-off-by: Adrian Bunk <bunk@stusta.de>
2006-11-08[PATCH] A minor fix for set_mb() in Documentation/memory-barriers.txtOleg Nesterov
set_mb() is used by set_current_state() which needs mb(), not wmb(). I think it would be right to assume that set_mb() implies mb(), all arches seem to do just this. Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru> Acked-by: David Howells <dhowells@redhat.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-10-20[PATCH] fix typo in memory barrier docsPaolo 'Blaisorblade' Giarrusso
Fix cut'n'paste typo - &a and &b are used in other examples, in this one the doc uses &u and &v. Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it> Acked-by: David Howells <dhowells@redhat.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-10-03Documentation: remove duplicated wordsPaolo Ornati
Remove many duplicated words under Documentation/ and do other small cleanups. Examples: "and and" --> "and" "in in" --> "in" "the the" --> "the" "the the" --> "to the" ... Signed-off-by: Paolo Ornati <ornati@fastwebnet.it> Signed-off-by: Adrian Bunk <bunk@stusta.de>
2006-10-03Fix some typos in Documentation/: 'A'Matt LaPlante
This patch fixes typos in various Documentation txts. This patch addresses some words starting with the letter 'A'. Signed-off-by: Matt LaPlante <kernel1@cyberdogtech.com> Acked-by: Randy Dunlap <rdunlap@xenotime.net> Acked-by: Alan Cox <alan@redhat.com> Signed-off-by: Adrian Bunk <bunk@stusta.de>
2006-07-14[PATCH] remove set_wmb - doc updateSteven Rostedt
This patch removes the reference to set_wmb from memory-barriers.txt since it shouldn't be used. Signed-off-by: Steven Rostedt <rostedt@goodmis.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-06-30typo fixes: occuring -> occurringAdrian Bunk
Signed-off-by: Adrian Bunk <bunk@stusta.de>
2006-06-25[PATCH] Corrections to memory barrier docDavid Howells
Apply some small corrections to the memory barrier document, as contributed by: Christoph Lameter <clameter@sgi.com> Kirill Smelkov <kirr@mns.spb.ru> Randy Dunlap <rdunlap@xenotime.net> Signed-off-by: David Howells <dhowells@redhat.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-06-25[PATCH] Another couple of alterations to the memory barrier docDavid Howells
Make another couple of alterations to the memory barrier document following suggestions by Alan Stern and in co-operation with Paul McKenney: (*) Rework the point of introduction of memory barriers and the description of what they are to reiterate why they're needed. (*) Modify a statement about the use of data dependency barriers to note that other barriers can be used instead (as they imply DD-barriers). Signed-off-by: David Howells <dhowells@redhat.com> Acked-By: Paul E. McKenney <paulmck@us.ibm.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-06-10[PATCH] Further alterations for memory barrier documentDavid Howells
From: David Howells <dhowells@redhat.com> Apply some alterations to the memory barrier document that I worked out with Paul McKenney of IBM, plus some of the alterations suggested by Alan Stern. The following changes were made: (*) One of the examples given for what can happen with overlapping memory barriers was wrong. (*) The description of general memory barriers said that a general barrier is a combination of a read barrier and a write barrier. This isn't entirely true: it implies both, but is more than a combination of both. (*) The first example in the "SMP Barrier Pairing" section was wrong: the loads around the read barrier need to touch the memory locations in the opposite order to the stores around the write barrier. (*) Added a note to make explicit that the loads should be in reverse order to the stores. (*) Adjusted the diagrams in the "Examples Of Memory Barrier Sequences" section to make them clearer. Added a couple of diagrams to make it more clear as to how it could go wrong without the barrier. (*) Added a section on memory speculation. (*) Dropped any references to memory allocation routines doing memory barriers. They may do sometimes, but it can't be relied on. This may be worthy of further documentation later. (*) Made the fact that a LOCK followed by an UNLOCK should not be considered a full memory barrier more explicit and gave an example. Signed-off-by: David Howells <dhowells@redhat.com> Acked-by: Paul E. McKenney <paulmck@us.ibm.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-05-15[PATCH] Fix typos in Documentation/memory-barriers.txtAneesh Kumar
Fix some typos in Documentation/memory-barriers.txt Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@gmail.com> Cc: David Howells <dhowells@redhat.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-04-11[PATCH] Improve data-dependency memory barrier example in documentationDavid Howells
In the memory barrier document, improve the example of the data dependency barrier situation by: (1) showing the initial values of the variables involved; and (2) repeating the instruction sequence description, this time with the data dependency barrier actually shown to make it clear what the revised sequence actually is. Signed-off-by: David Howells <dhowells@redhat.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-04-11[PATCH] Fix memory barrier docs wrt atomic opsDavid Howells
Fix the memory barrier documentation to attempt to describe atomic ops correctly. atomic_t ops that return a value _do_ imply smp_mb() either side, and so don't actually require smp_mb__*_atomic_*() special barriers. Also explains why special barriers exist in addition to normal barriers. Further fix the memory barrier documents to portray bitwise operation memory barrier effects correctly following Nick Piggin's comments. It makes the point that any atomic op that both modifies some state in memory and returns information on that state implies memory barriers on both sides. Signed-off-by: David Howells <dhowells@redhat.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-03-31[PATCH] Document Linux's memory barriers [try #7]David Howells
The attached patch documents the Linux kernel's memory barriers. I've updated it from the comments I've been given. The per-arch notes sections are gone because it's clear that there are so many exceptions, that it's not worth having them. I've added a list of references to other documents. I've tried to get rid of the concept of memory accesses appearing on the bus; what matters is apparent behaviour with respect to other observers in the system. Interrupts barrier effects are now considered to be non-existent. They may be there, but you may not rely on them. I've added a couple of definition sections at the top of the document: one to specify the minimum execution model that may be assumed, the other to specify what this document refers to by the term "memory". I've made greater mention of the use of mmiowb(). I've adjusted the way in which caches are described, and described the fun that can be had with cache coherence maintenance being unordered and data dependency not being necessarily implicit. I've described (smp_)read_barrier_depends(). I've rearranged the order of the sections, so that memory barriers are discussed in abstract first, and then described the memory barrier facilities available on Linux, before going on to more real-world discussions and examples. I've added information about the lack of memory barriering effects with atomic ops and bitops. I've added information about control dependencies. I've added more diagrams to illustrate caching interactions between CPUs. Signed-off-by: David Howells <dhowells@redhat.com> Signed-off-by: Linus Torvalds <torvalds@osdl.org>