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diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt index 867e0ea69b6d..dae8b8cb2ad3 100644 --- a/tools/memory-model/Documentation/explanation.txt +++ b/tools/memory-model/Documentation/explanation.txt @@ -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 |