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| -rw-r--r-- | tools/memory-model/Documentation/explanation.txt | 53 |
1 files changed, 36 insertions, 17 deletions
diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt index 1b5264559cd6..ecf6cccea5c3 100644 --- a/tools/memory-model/Documentation/explanation.txt +++ b/tools/memory-model/Documentation/explanation.txt @@ -27,7 +27,7 @@ Explanation of the Linux-Kernel Memory Consistency Model 19. AND THEN THERE WAS ALPHA 20. THE HAPPENS-BEFORE RELATION: hb 21. THE PROPAGATES-BEFORE RELATION: pb - 22. RCU RELATIONS: rcu-link, rcu-gp, rcu-rscsi, rcu-fence, and rb + 22. RCU RELATIONS: rcu-link, rcu-gp, rcu-rscsi, rcu-order, rcu-fence, and rb 23. LOCKING 24. ODDS AND ENDS @@ -1425,8 +1425,8 @@ they execute means that it cannot have cycles. This requirement is the content of the LKMM's "propagation" axiom. -RCU RELATIONS: rcu-link, rcu-gp, rcu-rscsi, rcu-fence, and rb -------------------------------------------------------------- +RCU RELATIONS: rcu-link, rcu-gp, rcu-rscsi, rcu-order, rcu-fence, and rb +------------------------------------------------------------------------ RCU (Read-Copy-Update) is a powerful synchronization mechanism. It rests on two concepts: grace periods and read-side critical sections. @@ -1536,29 +1536,29 @@ Z's CPU before Z begins but doesn't propagate to some other CPU until after X ends.) Similarly, X ->rcu-rscsi Y ->rcu-link Z says that X is the end of a critical section which starts before Z begins. -The LKMM goes on to define the rcu-fence relation as a sequence of +The LKMM goes on to define the rcu-order relation as a sequence of rcu-gp and rcu-rscsi links separated by rcu-link links, in which the number of rcu-gp links is >= the number of rcu-rscsi links. For example: X ->rcu-gp Y ->rcu-link Z ->rcu-rscsi T ->rcu-link U ->rcu-gp V -would imply that X ->rcu-fence V, because this sequence contains two +would imply that X ->rcu-order V, because this sequence contains two rcu-gp links and one rcu-rscsi link. (It also implies that -X ->rcu-fence T and Z ->rcu-fence V.) On the other hand: +X ->rcu-order T and Z ->rcu-order V.) On the other hand: X ->rcu-rscsi Y ->rcu-link Z ->rcu-rscsi T ->rcu-link U ->rcu-gp V -does not imply X ->rcu-fence V, because the sequence contains only +does not imply X ->rcu-order V, because the sequence contains only one rcu-gp link but two rcu-rscsi links. -The rcu-fence relation is important because the Grace Period Guarantee -means that rcu-fence acts kind of like a strong fence. In particular, -E ->rcu-fence F implies not only that E begins before F ends, but also -that any write po-before E will propagate to every CPU before any -instruction po-after F can execute. (However, it does not imply that -E must execute before F; in fact, each synchronize_rcu() fence event -is linked to itself by rcu-fence as a degenerate case.) +The rcu-order relation is important because the Grace Period Guarantee +means that rcu-order links act kind of like strong fences. In +particular, E ->rcu-order F implies not only that E begins before F +ends, but also that any write po-before E will propagate to every CPU +before any instruction po-after F can execute. (However, it does not +imply that E must execute before F; in fact, each synchronize_rcu() +fence event is linked to itself by rcu-order as a degenerate case.) To prove this in full generality requires some intellectual effort. We'll consider just a very simple case: @@ -1585,7 +1585,26 @@ G's CPU before G starts must propagate to every CPU before C starts. In particular, the write propagates to every CPU before F finishes executing and hence before any instruction po-after F can execute. This sort of reasoning can be extended to handle all the situations -covered by rcu-fence. +covered by rcu-order. + +The rcu-fence relation is a simple extension of rcu-order. While +rcu-order only links certain fence events (calls to synchronize_rcu(), +rcu_read_lock(), or rcu_read_unlock()), rcu-fence links any events +that are separated by an rcu-order link. This is analogous to the way +the strong-fence relation links events that are separated by an +smp_mb() fence event (as mentioned above, rcu-order links act kind of +like strong fences). Written symbolically, X ->rcu-fence Y means +there are fence events E and F such that: + + X ->po E ->rcu-order F ->po Y. + +From the discussion above, we see this implies not only that X +executes before Y, but also (if X is a store) that X propagates to +every CPU before Y executes. Thus rcu-fence is sort of a +"super-strong" fence: Unlike the original strong fences (smp_mb() and +synchronize_rcu()), rcu-fence is able to link events on different +CPUs. (Perhaps this fact should lead us to say that rcu-fence isn't +really a fence at all!) Finally, the LKMM defines the RCU-before (rb) relation in terms of rcu-fence. This is done in essentially the same way as the pb @@ -1596,7 +1615,7 @@ before F, just as E ->pb F does (and for much the same reasons). Putting this all together, the LKMM expresses the Grace Period Guarantee by requiring that the rb relation does not contain a cycle. Equivalently, this "rcu" axiom requires that there are no events E -and F with E ->rcu-link F ->rcu-fence E. Or to put it a third way, +and F with E ->rcu-link F ->rcu-order E. Or to put it a third way, the axiom requires that there are no cycles consisting of rcu-gp and rcu-rscsi alternating with rcu-link, where the number of rcu-gp links is >= the number of rcu-rscsi links. @@ -1750,7 +1769,7 @@ addition to normal RCU. The ideas involved are much the same as above, with new relations srcu-gp and srcu-rscsi added to represent SRCU grace periods and read-side critical sections. There is a restriction on the srcu-gp and srcu-rscsi links that can appear in an -rcu-fence sequence (the srcu-rscsi links must be paired with srcu-gp +rcu-order sequence (the srcu-rscsi links must be paired with srcu-gp links having the same SRCU domain with proper nesting); the details are relatively unimportant. |