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tools/memory-model: Remove rb-dep, smp_read_barrier_depends, and lockless_dereference

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Since commit 76ebbe78f7 ("locking/barriers: Add implicit
smp_read_barrier_depends() to READ_ONCE()") was merged for the 4.15
kernel, it has not been necessary to use smp_read_barrier_depends().
Similarly, commit 59ecbbe7b3 ("locking/barriers: Kill
lockless_dereference()") removed lockless_dereference() from the
kernel.

Since these primitives are no longer part of the kernel, they do not
belong in the Linux Kernel Memory Consistency Model.  This patch
removes them, along with the internal rb-dep relation, and updates the
revelant documentation.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: akiyks@gmail.com
Cc: boqun.feng@gmail.com
Cc: dhowells@redhat.com
Cc: j.alglave@ucl.ac.uk
Cc: linux-arch@vger.kernel.org
Cc: luc.maranget@inria.fr
Cc: nborisov@suse.com
Cc: npiggin@gmail.com
Cc: parri.andrea@gmail.com
Cc: will.deacon@arm.com
Link: http://lkml.kernel.org/r/1519169112-20593-12-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Alan Stern 2018-02-20 15:25:12 -08:00 committed by Ingo Molnar
parent cac79a39f2
commit bf28ae5627
5 changed files with 46 additions and 48 deletions
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3
tools/memory-model/Documentation/cheatsheet.txt
View File

@ -6,8 +6,7 @@
Store, e.g., WRITE_ONCE() Y Y
Load, e.g., READ_ONCE() Y Y Y
Unsuccessful RMW operation Y Y Y
smp_read_barrier_depends() Y Y Y
*_dereference() Y Y Y Y
rcu_dereference() Y Y Y Y
Successful *_acquire() R Y Y Y Y Y Y
Successful *_release() C Y Y Y W Y
smp_rmb() Y R Y Y R

81
tools/memory-model/Documentation/explanation.txt
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@ -1,5 +1,5 @@
Explanation of the Linux-Kernel Memory Model
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Explanation of the Linux-Kernel Memory Consistency Model
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
:Author: Alan Stern <stern@rowland.harvard.edu>
:Created: October 2017
@ -35,25 +35,24 @@ Explanation of the Linux-Kernel Memory Model
INTRODUCTION
------------
The Linux-kernel memory model (LKMM) is rather complex and obscure.
This is particularly evident if you read through the linux-kernel.bell
and linux-kernel.cat files that make up the formal version of the
memory model; they are extremely terse and their meanings are far from
clear.
The Linux-kernel memory consistency model (LKMM) is rather complex and
obscure. This is particularly evident if you read through the
linux-kernel.bell and linux-kernel.cat files that make up the formal
version of the model; they are extremely terse and their meanings are
far from clear.
This document describes the ideas underlying the LKMM. It is meant
for people who want to understand how the memory model was designed.
It does not go into the details of the code in the .bell and .cat
files; rather, it explains in English what the code expresses
symbolically.
for people who want to understand how the model was designed. It does
not go into the details of the code in the .bell and .cat files;
rather, it explains in English what the code expresses symbolically.
Sections 2 (BACKGROUND) through 5 (ORDERING AND CYCLES) are aimed
toward beginners; they explain what memory models are and the basic
notions shared by all such models. People already familiar with these
concepts can skim or skip over them. Sections 6 (EVENTS) through 12
(THE FROM_READS RELATION) describe the fundamental relations used in
many memory models. Starting in Section 13 (AN OPERATIONAL MODEL),
the workings of the LKMM itself are covered.
toward beginners; they explain what memory consistency models are and
the basic notions shared by all such models. People already familiar
with these concepts can skim or skip over them. Sections 6 (EVENTS)
through 12 (THE FROM_READS RELATION) describe the fundamental
relations used in many models. Starting in Section 13 (AN OPERATIONAL
MODEL), the workings of the LKMM itself are covered.
Warning: The code examples in this document are not written in the
proper format for litmus tests. They don't include a header line, the
@ -827,8 +826,8 @@ A-cumulative; they only affect the propagation of stores that are
executed on C before the fence (i.e., those which precede the fence in
program order).
smp_read_barrier_depends(), rcu_read_lock(), rcu_read_unlock(), and
synchronize_rcu() fences have other properties which we discuss later.
read_lock(), rcu_read_unlock(), and synchronize_rcu() fences have
other properties which we discuss later.
PROPAGATION ORDER RELATION: cumul-fence
@ -988,8 +987,8 @@ Another possibility, not mentioned earlier but discussed in the next
section, is:
X and Y are both loads, X ->addr Y (i.e., there is an address
dependency from X to Y), and an smp_read_barrier_depends()
fence occurs between them.
dependency from X to Y), and X is a READ_ONCE() or an atomic
access.
Dependencies can also cause instructions to be executed in program
order. This is uncontroversial when the second instruction is a
@ -1015,9 +1014,9 @@ After all, a CPU cannot ask the memory subsystem to load a value from
a particular location before it knows what that location is. However,
the split-cache design used by Alpha can cause it to behave in a way
that looks as if the loads were executed out of order (see the next
section for more details). For this reason, the LKMM does not include
address dependencies between read events in the ppo relation unless an
smp_read_barrier_depends() fence is present.
section for more details). The kernel includes a workaround for this
problem when the loads come from READ_ONCE(), and therefore the LKMM
includes address dependencies to loads in the ppo relation.
On the other hand, dependencies can indirectly affect the ordering of
two loads. This happens when there is a dependency from a load to a
@ -1114,11 +1113,12 @@ code such as the following:
int *r1;
int r2;
r1 = READ_ONCE(ptr);
r1 = ptr;
r2 = READ_ONCE(*r1);
}
can malfunction on Alpha systems. It is quite possible that r1 = &x
can malfunction on Alpha systems (notice that P1 uses an ordinary load
to read ptr instead of READ_ONCE()). It is quite possible that r1 = &x
and r2 = 0 at the end, in spite of the address dependency.
At first glance this doesn't seem to make sense. We know that the
@ -1141,11 +1141,15 @@ This could not have happened if the local cache had processed the
incoming stores in FIFO order. In constrast, other architectures
maintain at least the appearance of FIFO order.
In practice, this difficulty is solved by inserting an
smp_read_barrier_depends() fence between P1's two loads. The effect
of this fence is to cause the CPU not to execute any po-later
instructions until after the local cache has finished processing all
the stores it has already received. Thus, if the code was changed to:
In practice, this difficulty is solved by inserting a special fence
between P1's two loads when the kernel is compiled for the Alpha
architecture. In fact, as of version 4.15, the kernel automatically
adds this fence (called smp_read_barrier_depends() and defined as
nothing at all on non-Alpha builds) after every READ_ONCE() and atomic
load. The effect of the fence is to cause the CPU not to execute any
po-later instructions until after the local cache has finished
processing all the stores it has already received. Thus, if the code
was changed to:
P1()
{
@ -1153,13 +1157,15 @@ the stores it has already received. Thus, if the code was changed to:
int r2;
r1 = READ_ONCE(ptr);
smp_read_barrier_depends();
r2 = READ_ONCE(*r1);
}
then we would never get r1 = &x and r2 = 0. By the time P1 executed
its second load, the x = 1 store would already be fully processed by
the local cache and available for satisfying the read request.
the local cache and available for satisfying the read request. Thus
we have yet another reason why shared data should always be read with
READ_ONCE() or another synchronization primitive rather than accessed
directly.
The LKMM requires that smp_rmb(), acquire fences, and strong fences
share this property with smp_read_barrier_depends(): They do not allow
@ -1751,11 +1757,10 @@ no further involvement from the CPU. Since the CPU doesn't ever read
the value of x, there is nothing for the smp_rmb() fence to act on.
The LKMM defines a few extra synchronization operations in terms of
things we have already covered. In particular, rcu_dereference() and
lockless_dereference() are both treated as a READ_ONCE() followed by
smp_read_barrier_depends() -- which also happens to be how they are
defined in include/linux/rcupdate.h and include/linux/compiler.h,
respectively.
things we have already covered. In particular, rcu_dereference() is
treated as READ_ONCE() and rcu_assign_pointer() is treated as
smp_store_release() -- which is basically how the Linux kernel treats
them.
There are a few oddball fences which need special treatment:
smp_mb__before_atomic(), smp_mb__after_atomic(), and

1
tools/memory-model/linux-kernel.bell
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@ -24,7 +24,6 @@ instructions RMW[{'once,'acquire,'release}]
enum Barriers = 'wmb (*smp_wmb*) ||
'rmb (*smp_rmb*) ||
'mb (*smp_mb*) ||
'rb_dep (*smp_read_barrier_depends*) ||
'rcu-lock (*rcu_read_lock*) ||
'rcu-unlock (*rcu_read_unlock*) ||
'sync-rcu (*synchronize_rcu*) ||

7
tools/memory-model/linux-kernel.cat
View File

@ -25,7 +25,6 @@ include "lock.cat"
(*******************)
(* Fences *)
let rb-dep = [R] ; fencerel(Rb_dep) ; [R]
let rmb = [R \ Noreturn] ; fencerel(Rmb) ; [R \ Noreturn]
let wmb = [W] ; fencerel(Wmb) ; [W]
let mb = ([M] ; fencerel(Mb) ; [M]) |
@ -61,11 +60,9 @@ let dep = addr | data
let rwdep = (dep | ctrl) ; [W]
let overwrite = co | fr
let to-w = rwdep | (overwrite & int)
let rrdep = addr | (dep ; rfi)
let strong-rrdep = rrdep+ & rb-dep
let to-r = strong-rrdep | rfi-rel-acq
let to-r = addr | (dep ; rfi) | rfi-rel-acq
let fence = strong-fence | wmb | po-rel | rmb | acq-po
let ppo = rrdep* ; (to-r | to-w | fence)
let ppo = to-r | to-w | fence
(* Propagation: Ordering from release operations and strong fences. *)
let A-cumul(r) = rfe? ; r

2
tools/memory-model/linux-kernel.def
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@ -13,14 +13,12 @@ WRITE_ONCE(X,V) { __store{once}(X,V); }
smp_store_release(X,V) { __store{release}(*X,V); }
smp_load_acquire(X) __load{acquire}(*X)
rcu_assign_pointer(X,V) { __store{release}(X,V); }
lockless_dereference(X) __load{lderef}(X)
rcu_dereference(X) __load{deref}(X)
// Fences
smp_mb() { __fence{mb} ; }
smp_rmb() { __fence{rmb} ; }
smp_wmb() { __fence{wmb} ; }
smp_read_barrier_depends() { __fence{rb_dep}; }
smp_mb__before_atomic() { __fence{before-atomic} ; }
smp_mb__after_atomic() { __fence{after-atomic} ; }
smp_mb__after_spinlock() { __fence{after-spinlock} ; }