Examples introducing neccesity of RMB+WMP pair reads as
A=3 READ B
www rrrrrr
B=4 READ A
Note the opposite order of reads vs writes.
But the first example without barriers reads as
A=3 READ A
B=4 READ B
There are 4 outcomes in the first example.
But if someone new to the concept tries to insert barriers like this:
A=3 READ A
www rrrrrr
B=4 READ B
he will still get all 4 possible outcomes, because "READ A" is first.
All this can be utterly confusing because barrier pair seems to be
superfluous. In short, fixup first example to match latter examples
with barriers.
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Update the documentation to reflect the change of barrier primitives.
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: David Howells <dhowells@redhat.com>
Link: http://lkml.kernel.org/n/tip-xslfehiga1twbk5uk94rij1e@git.kernel.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Fix double words "the the" in various files
within Documentations.
Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
This commit fixes a couple of typos and clarifies what happens when
the CPU chooses to execute a later lock acquisition before a prior
lock release, in particular, why deadlock is avoided.
Reported-by: Peter Hurley <peter@hurleysoftware.com>
Reported-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Reported-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Current compilers can "speculate" stores in the case where both legs
of the "if" statement start with identical stores. Because the stores
are identical, the compiler knows that the store will unconditionally
execute regardless of the "if" condition, and so the compiler is within
its rights to hoist the store to precede the condition. Such hoisting
destroys the control-dependency ordering. This ordering can be restored
by placing a barrier() at the beginning of each leg of the "if" statement.
This commit adds this requirement to the control-dependencies section.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
A control dependency consists of a load, a conditional that depends on
that load, and a store. This commit emphasizes this point in the
summary.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
The ACCESS_ONCE() primitive provides cache coherence, but the
documentation does not clearly state this. This commit therefore upgrades
the documentation.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
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>
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>
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>
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>
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>
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>
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>
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>
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>
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>
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>
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>
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>
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>
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>
(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>
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>
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>
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>
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>
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>
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>
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>
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>
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>
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>
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>
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>
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>
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>
Alexey Dobriyan
Peter Zijlstra
Linus Torvalds
Masanari Iida
Paul E. McKenney
Ingo Molnar
Richard Braun
Paul Bolle
David Howells
Nick Piggin
Randy Dunlap
Oleg Nesterov
Jarek Poplawski
Matt LaPlante
Paolo 'Blaisorblade' Giarrusso
Paolo Ornati
Steven Rostedt
Adrian Bunk
Aneesh Kumar