On Sat, 29 Jun 2019, Andrea Parri wrote: > The formal memory consistency model has added support for plain accesses > (and data races). While updating the informal documentation to describe > this addition to the model is highly desirable and important future work, > update the informal documentation to at least acknowledge such addition. > > Signed-off-by: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx> > Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> > Cc: Will Deacon <will.deacon@xxxxxxx> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > Cc: Boqun Feng <boqun.feng@xxxxxxxxx> > Cc: Nicholas Piggin <npiggin@xxxxxxxxx> > Cc: David Howells <dhowells@xxxxxxxxxx> > Cc: Jade Alglave <j.alglave@xxxxxxxxx> > Cc: Luc Maranget <luc.maranget@xxxxxxxx> > Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxx> > Cc: Akira Yokosawa <akiyks@xxxxxxxxx> > Cc: Daniel Lustig <dlustig@xxxxxxxxxx> > --- Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> > tools/memory-model/Documentation/explanation.txt | 47 +++++++++++------------- > tools/memory-model/README | 18 ++++----- > 2 files changed, 30 insertions(+), 35 deletions(-) > > diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt > index 68caa9a976d0c..b42f7cd718242 100644 > --- a/tools/memory-model/Documentation/explanation.txt > +++ b/tools/memory-model/Documentation/explanation.txt > @@ -42,7 +42,8 @@ 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 > +This document describes the ideas underlying the LKMM, but excluding > +the modeling of bare C (or plain) shared memory accesses. It is meant > 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. > @@ -354,31 +355,25 @@ be extremely complex. > Optimizing compilers have great freedom in the way they translate > source code to object code. They are allowed to apply transformations > that add memory accesses, eliminate accesses, combine them, split them > -into pieces, or move them around. Faced with all these possibilities, > -the LKMM basically gives up. It insists that the code it analyzes > -must contain no ordinary accesses to shared memory; all accesses must > -be performed using READ_ONCE(), WRITE_ONCE(), or one of the other > -atomic or synchronization primitives. These primitives prevent a > -large number of compiler optimizations. In particular, it is > -guaranteed that the compiler will not remove such accesses from the > -generated code (unless it can prove the accesses will never be > -executed), it will not change the order in which they occur in the > -code (within limits imposed by the C standard), and it will not > -introduce extraneous accesses. > - > -This explains why the MP and SB examples above used READ_ONCE() and > -WRITE_ONCE() rather than ordinary memory accesses. Thanks to this > -usage, we can be certain that in the MP example, P0's write event to > -buf really is po-before its write event to flag, and similarly for the > -other shared memory accesses in the examples. > - > -Private variables are not subject to this restriction. Since they are > -not shared between CPUs, they can be accessed normally without > -READ_ONCE() or WRITE_ONCE(), and there will be no ill effects. In > -fact, they need not even be stored in normal memory at all -- in > -principle a private variable could be stored in a CPU register (hence > -the convention that these variables have names starting with the > -letter 'r'). > +into pieces, or move them around. The use of READ_ONCE(), WRITE_ONCE(), > +or one of the other atomic or synchronization primitives prevents a > +large number of compiler optimizations. In particular, it is guaranteed > +that the compiler will not remove such accesses from the generated code > +(unless it can prove the accesses will never be executed), it will not > +change the order in which they occur in the code (within limits imposed > +by the C standard), and it will not introduce extraneous accesses. > + > +The MP and SB examples above used READ_ONCE() and WRITE_ONCE() rather > +than ordinary memory accesses. Thanks to this usage, we can be certain > +that in the MP example, the compiler won't reorder P0's write event to > +buf and P0's write event to flag, and similarly for the other shared > +memory accesses in the examples. > + > +Since private variables are not shared between CPUs, they can be > +accessed normally without READ_ONCE() or WRITE_ONCE(). In fact, they > +need not even be stored in normal memory at all -- in principle a > +private variable could be stored in a CPU register (hence the convention > +that these variables have names starting with the letter 'r'). > > > A WARNING > diff --git a/tools/memory-model/README b/tools/memory-model/README > index 2b87f3971548c..fc07b52f20286 100644 > --- a/tools/memory-model/README > +++ b/tools/memory-model/README > @@ -167,15 +167,15 @@ scripts Various scripts, see scripts/README. > LIMITATIONS > =========== > > -The Linux-kernel memory model has the following limitations: > - > -1. Compiler optimizations are not modeled. Of course, the use > - of READ_ONCE() and WRITE_ONCE() limits the compiler's ability > - to optimize, but there is Linux-kernel code that uses bare C > - memory accesses. Handling this code is on the to-do list. > - For more information, see Documentation/explanation.txt (in > - particular, the "THE PROGRAM ORDER RELATION: po AND po-loc" > - and "A WARNING" sections). > +The Linux-kernel memory model (LKMM) has the following limitations: > + > +1. Compiler optimizations are not accurately modeled. Of course, > + the use of READ_ONCE() and WRITE_ONCE() limits the compiler's > + ability to optimize, but under some circumstances it is possible > + for the compiler to undermine the memory model. For more > + information, see Documentation/explanation.txt (in particular, > + the "THE PROGRAM ORDER RELATION: po AND po-loc" and "A WARNING" > + sections). > > Note that this limitation in turn limits LKMM's ability to > accurately model address, control, and data dependencies. >
