diff options
Diffstat (limited to 'tools/memory-model')
| -rw-r--r-- | tools/memory-model/Documentation/explanation.txt | 83 | ||||
| -rw-r--r-- | tools/memory-model/Documentation/recipes.txt | 2 | ||||
| -rw-r--r-- | tools/memory-model/Documentation/references.txt | 21 | ||||
| -rw-r--r-- | tools/memory-model/README | 40 |
4 files changed, 100 insertions, 46 deletions
diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt index 01adf9e0ebac..f9d610d5a1a4 100644 --- a/tools/memory-model/Documentation/explanation.txt +++ b/tools/memory-model/Documentation/explanation.txt @@ -1985,28 +1985,36 @@ 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 @@ -2169,8 +2177,8 @@ 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 @@ -2324,12 +2332,11 @@ 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; @@ -2365,7 +2372,7 @@ 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 @@ -2378,8 +2385,8 @@ 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 @@ -2411,20 +2418,20 @@ 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 diff --git a/tools/memory-model/Documentation/recipes.txt b/tools/memory-model/Documentation/recipes.txt index 7fe8d7aa3029..63c4adfed884 100644 --- a/tools/memory-model/Documentation/recipes.txt +++ b/tools/memory-model/Documentation/recipes.txt @@ -126,7 +126,7 @@ However, it is not necessarily the case that accesses ordered by locking will be seen as ordered by CPUs not holding that lock. Consider this example: - /* See Z6.0+pooncerelease+poacquirerelease+fencembonceonce.litmus. */ + /* See Z6.0+pooncelock+pooncelock+pombonce.litmus. */ void CPU0(void) { spin_lock(&mylock); diff --git a/tools/memory-model/Documentation/references.txt b/tools/memory-model/Documentation/references.txt index b177f3e4a614..ecbbaa5396d4 100644 --- a/tools/memory-model/Documentation/references.txt +++ b/tools/memory-model/Documentation/references.txt @@ -73,6 +73,18 @@ o Christopher Pulte, Shaked Flur, Will Deacon, Jon French, Linux-kernel memory model ========================= +o Jade Alglave, Will Deacon, Boqun Feng, David Howells, Daniel + Lustig, Luc Maranget, Paul E. McKenney, Andrea Parri, Nicholas + Piggin, Alan Stern, Akira Yokosawa, and Peter Zijlstra. + 2019. "Calibrating your fear of big bad optimizing compilers" + Linux Weekly News. https://lwn.net/Articles/799218/ + +o Jade Alglave, Will Deacon, Boqun Feng, David Howells, Daniel + Lustig, Luc Maranget, Paul E. McKenney, Andrea Parri, Nicholas + Piggin, Alan Stern, Akira Yokosawa, and Peter Zijlstra. + 2019. "Who's afraid of a big bad optimizing compiler?" + Linux Weekly News. https://lwn.net/Articles/793253/ + o Jade Alglave, Luc Maranget, Paul E. McKenney, Andrea Parri, and Alan Stern. 2018. "Frightening small children and disconcerting grown-ups: Concurrency in the Linux kernel". In Proceedings of @@ -88,6 +100,11 @@ o Jade Alglave, Luc Maranget, Paul E. McKenney, Andrea Parri, and Alan Stern. 2017. "A formal kernel memory-ordering model (part 2)" Linux Weekly News. https://lwn.net/Articles/720550/ +o Jade Alglave, Luc Maranget, Paul E. McKenney, Andrea Parri, and + Alan Stern. 2017-2019. "A Formal Model of Linux-Kernel Memory + Ordering" (backup material for the LWN articles) + https://mirrors.edge.kernel.org/pub/linux/kernel/people/paulmck/LWNLinuxMM/ + Memory-model tooling ==================== @@ -110,5 +127,5 @@ Memory-model comparisons ======================== o Paul E. McKenney, Ulrich Weigand, Andrea Parri, and Boqun - Feng. 2016. "Linux-Kernel Memory Model". (6 June 2016). - http://open-std.org/JTC1/SC22/WG21/docs/papers/2016/p0124r2.html. + Feng. 2018. "Linux-Kernel Memory Model". (27 September 2018). + http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0124r6.html. diff --git a/tools/memory-model/README b/tools/memory-model/README index fc07b52f2028..ecb7385376bf 100644 --- a/tools/memory-model/README +++ b/tools/memory-model/README @@ -28,8 +28,34 @@ downloaded separately: See "herdtools7/INSTALL.md" for installation instructions. Note that although these tools usually provide backwards compatibility, -this is not absolutely guaranteed. Therefore, if a later version does -not work, please try using the exact version called out above. +this is not absolutely guaranteed. + +For example, a future version of herd7 might not work with the model +in this release. A compatible model will likely be made available in +a later release of Linux kernel. + +If you absolutely need to run the model in this particular release, +please try using the exact version called out above. + +klitmus7 is independent of the model provided here. It has its own +dependency on a target kernel release where converted code is built +and executed. Any change in kernel APIs essential to klitmus7 will +necessitate an upgrade of klitmus7. + +If you find any compatibility issues in klitmus7, please inform the +memory model maintainers. + +klitmus7 Compatibility Table +---------------------------- + + ============ ========== + target Linux herdtools7 + ------------ ---------- + -- 4.18 7.48 -- + 4.15 -- 4.19 7.49 -- + 4.20 -- 5.5 7.54 -- + 5.6 -- 7.56 -- + ============ ========== ================== @@ -207,11 +233,15 @@ The Linux-kernel memory model (LKMM) has the following limitations: case as a store release. b. The "unless" RMW operations are not currently modeled: - atomic_long_add_unless(), atomic_add_unless(), - atomic_inc_unless_negative(), and - atomic_dec_unless_positive(). These can be emulated + atomic_long_add_unless(), atomic_inc_unless_negative(), + and atomic_dec_unless_positive(). These can be emulated in litmus tests, for example, by using atomic_cmpxchg(). + One exception of this limitation is atomic_add_unless(), + which is provided directly by herd7 (so no corresponding + definition in linux-kernel.def). atomic_add_unless() is + modeled by herd7 therefore it can be used in litmus tests. + c. The call_rcu() function is not modeled. It can be emulated in litmus tests by adding another process that invokes synchronize_rcu() and the body of the callback |