Messages in this thread | | | Date | Mon, 2 Mar 2020 11:49:07 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v3] tools/memory-model/Documentation: Fix "conflict" definition |
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On Mon, Mar 02, 2020 at 07:52:16PM +0100, Andrea Parri wrote: > On Mon, Mar 02, 2020 at 06:21:01PM +0100, Marco Elver wrote: > > The definition of "conflict" should not include the type of access nor > > whether the accesses are concurrent or not, which this patch addresses. > > The definition of "data race" remains unchanged. > > > > The definition of "conflict" as we know it and is cited by various > > papers on memory consistency models appeared in [1]: "Two accesses to > > the same variable conflict if at least one is a write; two operations > > conflict if they execute conflicting accesses." > > > > The LKMM as well as the C11 memory model are adaptations of > > data-race-free, which are based on the work in [2]. Necessarily, we need > > both conflicting data operations (plain) and synchronization operations > > (marked). For example, C11's definition is based on [3], which defines a > > "data race" as: "Two memory operations conflict if they access the same > > memory location, and at least one of them is a store, atomic store, or > > atomic read-modify-write operation. In a sequentially consistent > > execution, two memory operations from different threads form a type 1 > > data race if they conflict, at least one of them is a data operation, > > and they are adjacent in <T (i.e., they may be executed concurrently)." > > > > [1] D. Shasha, M. Snir, "Efficient and Correct Execution of Parallel > > Programs that Share Memory", 1988. > > URL: http://snir.cs.illinois.edu/listed/J21.pdf > > > > [2] S. Adve, "Designing Memory Consistency Models for Shared-Memory > > Multiprocessors", 1993. > > URL: http://sadve.cs.illinois.edu/Publications/thesis.pdf > > > > [3] H.-J. Boehm, S. Adve, "Foundations of the C++ Concurrency Memory > > Model", 2008. > > URL: https://www.hpl.hp.com/techreports/2008/HPL-2008-56.pdf > > > > Signed-off-by: Marco Elver <elver@google.com> > > Co-developed-by: Alan Stern <stern@rowland.harvard.edu> > > Signed-off-by: Alan Stern <stern@rowland.harvard.edu> > > LGTM: > > Acked-by: Andrea Parri <parri.andrea@gmail.com>
Applied, and thank you as well!
Thanx, Paul
> Thank you both, > > Andrea > > > > --- > > v3: > > * Apply Alan's suggestion. > > * s/two race candidates/race candidates/ > > > > v2: http://lkml.kernel.org/r/20200302141819.40270-1-elver@google.com > > * Apply Alan's suggested version. > > - Move "from different CPUs (or threads)" from "conflict" to "data > > race" definition. Update "race candidate" accordingly. > > * Add citations to commit message. > > > > v1: http://lkml.kernel.org/r/20200228164621.87523-1-elver@google.com > > --- > > .../Documentation/explanation.txt | 83 ++++++++++--------- > > 1 file changed, 45 insertions(+), 38 deletions(-) > > > > diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt > > index e91a2eb19592a..993f800659c6a 100644 > > --- a/tools/memory-model/Documentation/explanation.txt > > +++ b/tools/memory-model/Documentation/explanation.txt > > @@ -1987,28 +1987,36 @@ outcome undefined. > > > > In technical terms, the compiler is allowed to assume that when the > > program executes, there will not be any data races. A "data race" > > -occurs when two conflicting memory accesses execute concurrently; > > -two memory accesses "conflict" if: > > +occurs when there are two memory accesses such that: > > > > - they access the same location, > > +1. they access the same location, > > > > - they occur on different CPUs (or in different threads on the > > - same CPU), > > +2. at least one of them is a store, > > > > - at least one of them is a plain access, > > +3. at least one of them is plain, > > > > - and at least one of them is a store. > > +4. they occur on different CPUs (or in different threads on the > > + same CPU), and > > > > -The LKMM tries to determine whether a program contains two conflicting > > -accesses which may execute concurrently; if it does then the LKMM says > > -there is a potential data race and makes no predictions about the > > -program's outcome. > > +5. they execute concurrently. > > > > -Determining whether two accesses conflict is easy; you can see that > > -all the concepts involved in the definition above are already part of > > -the memory model. The hard part is telling whether they may execute > > -concurrently. The LKMM takes a conservative attitude, assuming that > > -accesses may be concurrent unless it can prove they cannot. > > +In the literature, two accesses are said to "conflict" if they satisfy > > +1 and 2 above. We'll go a little farther and say that two accesses > > +are "race candidates" if they satisfy 1 - 4. Thus, whether or not two > > +race candidates actually do race in a given execution depends on > > +whether they are concurrent. > > + > > +The LKMM tries to determine whether a program contains race candidates > > +which may execute concurrently; if it does then the LKMM says there is > > +a potential data race and makes no predictions about the program's > > +outcome. > > + > > +Determining whether two accesses are race candidates is easy; you can > > +see that all the concepts involved in the definition above are already > > +part of the memory model. The hard part is telling whether they may > > +execute concurrently. The LKMM takes a conservative attitude, > > +assuming that accesses may be concurrent unless it can prove they > > +are not. > > > > If two memory accesses aren't concurrent then one must execute before > > the other. Therefore the LKMM decides two accesses aren't concurrent > > @@ -2171,8 +2179,8 @@ again, now using plain accesses for buf: > > } > > > > This program does not contain a data race. Although the U and V > > -accesses conflict, the LKMM can prove they are not concurrent as > > -follows: > > +accesses are race candidates, the LKMM can prove they are not > > +concurrent as follows: > > > > The smp_wmb() fence in P0 is both a compiler barrier and a > > cumul-fence. It guarantees that no matter what hash of > > @@ -2326,12 +2334,11 @@ could now perform the load of x before the load of ptr (there might be > > a control dependency but no address dependency at the machine level). > > > > Finally, it turns out there is a situation in which a plain write does > > -not need to be w-post-bounded: when it is separated from the > > -conflicting access by a fence. At first glance this may seem > > -impossible. After all, to be conflicting the second access has to be > > -on a different CPU from the first, and fences don't link events on > > -different CPUs. Well, normal fences don't -- but rcu-fence can! > > -Here's an example: > > +not need to be w-post-bounded: when it is separated from the other > > +race-candidate access by a fence. At first glance this may seem > > +impossible. After all, to be race candidates the two accesses must > > +be on different CPUs, and fences don't link events on different CPUs. > > +Well, normal fences don't -- but rcu-fence can! Here's an example: > > > > int x, y; > > > > @@ -2367,7 +2374,7 @@ concurrent and there is no race, even though P1's plain store to y > > isn't w-post-bounded by any marked accesses. > > > > Putting all this material together yields the following picture. For > > -two conflicting stores W and W', where W ->co W', the LKMM says the > > +race-candidate stores W and W', where W ->co W', the LKMM says the > > stores don't race if W can be linked to W' by a > > > > w-post-bounded ; vis ; w-pre-bounded > > @@ -2380,8 +2387,8 @@ sequence, and if W' is plain then they also have to be linked by a > > > > w-post-bounded ; vis ; r-pre-bounded > > > > -sequence. For a conflicting load R and store W, the LKMM says the two > > -accesses don't race if R can be linked to W by an > > +sequence. For race-candidate load R and store W, the LKMM says the > > +two accesses don't race if R can be linked to W by an > > > > r-post-bounded ; xb* ; w-pre-bounded > > > > @@ -2413,20 +2420,20 @@ is, the rules governing the memory subsystem's choice of a store to > > satisfy a load request and its determination of where a store will > > fall in the coherence order): > > > > - If R and W conflict and it is possible to link R to W by one > > - of the xb* sequences listed above, then W ->rfe R is not > > - allowed (i.e., a load cannot read from a store that it > > + If R and W are race candidates and it is possible to link R to > > + W by one of the xb* sequences listed above, then W ->rfe R is > > + not allowed (i.e., a load cannot read from a store that it > > executes before, even if one or both is plain). > > > > - If W and R conflict and it is possible to link W to R by one > > - of the vis sequences listed above, then R ->fre W is not > > - allowed (i.e., if a store is visible to a load then the load > > - must read from that store or one coherence-after it). > > + If W and R are race candidates and it is possible to link W to > > + R by one of the vis sequences listed above, then R ->fre W is > > + not allowed (i.e., if a store is visible to a load then the > > + load must read from that store or one coherence-after it). > > > > - If W and W' conflict and it is possible to link W to W' by one > > - of the vis sequences listed above, then W' ->co W is not > > - allowed (i.e., if one store is visible to a second then the > > - second must come after the first in the coherence order). > > + If W and W' are race candidates and it is possible to link W > > + to W' by one of the vis sequences listed above, then W' ->co W > > + is not allowed (i.e., if one store is visible to a second then > > + the second must come after the first in the coherence order). > > > > This is the extent to which the LKMM deals with plain accesses. > > Perhaps it could say more (for example, plain accesses might > > -- > > 2.25.0.265.gbab2e86ba0-goog > >
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