diff options
Diffstat (limited to 'tools/memory-model/Documentation/litmus-tests.txt')
| -rw-r--r-- | tools/memory-model/Documentation/litmus-tests.txt | 64 |
1 files changed, 28 insertions, 36 deletions
diff --git a/tools/memory-model/Documentation/litmus-tests.txt b/tools/memory-model/Documentation/litmus-tests.txt index 8a9d5d2787f9..acac527328a1 100644 --- a/tools/memory-model/Documentation/litmus-tests.txt +++ b/tools/memory-model/Documentation/litmus-tests.txt @@ -946,22 +946,39 @@ Limitations of the Linux-kernel memory model (LKMM) include: carrying a dependency, then the compiler can break that dependency by substituting a constant of that value. - Conversely, LKMM sometimes doesn't recognize that a particular - optimization is not allowed, and as a result, thinks that a - dependency is not present (because the optimization would break it). - The memory model misses some pretty obvious control dependencies - because of this limitation. A simple example is: + Conversely, LKMM will sometimes overestimate the amount of + reordering compilers and CPUs can carry out, leading it to miss + some pretty obvious cases of ordering. A simple example is: r1 = READ_ONCE(x); if (r1 == 0) smp_mb(); WRITE_ONCE(y, 1); - There is a control dependency from the READ_ONCE to the WRITE_ONCE, - even when r1 is nonzero, but LKMM doesn't realize this and thinks - that the write may execute before the read if r1 != 0. (Yes, that - doesn't make sense if you think about it, but the memory model's - intelligence is limited.) + The WRITE_ONCE() does not depend on the READ_ONCE(), and as a + result, LKMM does not claim ordering. However, even though no + dependency is present, the WRITE_ONCE() will not be executed before + the READ_ONCE(). There are two reasons for this: + + The presence of the smp_mb() in one of the branches + prevents the compiler from moving the WRITE_ONCE() + up before the "if" statement, since the compiler has + to assume that r1 will sometimes be 0 (but see the + comment below); + + CPUs do not execute stores before po-earlier conditional + branches, even in cases where the store occurs after the + two arms of the branch have recombined. + + It is clear that it is not dangerous in the slightest for LKMM to + make weaker guarantees than architectures. In fact, it is + desirable, as it gives compilers room for making optimizations. + For instance, suppose that a 0 value in r1 would trigger undefined + behavior elsewhere. Then a clever compiler might deduce that r1 + can never be 0 in the if condition. As a result, said clever + compiler might deem it safe to optimize away the smp_mb(), + eliminating the branch and any ordering an architecture would + guarantee otherwise. 2. Multiple access sizes for a single variable are not supported, and neither are misaligned or partially overlapping accesses. @@ -1011,32 +1028,7 @@ Limitations of the Linux-kernel memory model (LKMM) include: additional call_rcu() process to the site of the emulated rcu-barrier(). - e. Although sleepable RCU (SRCU) is now modeled, there - are some subtle differences between its semantics and - those in the Linux kernel. For example, the kernel - might interpret the following sequence as two partially - overlapping SRCU read-side critical sections: - - 1 r1 = srcu_read_lock(&my_srcu); - 2 do_something_1(); - 3 r2 = srcu_read_lock(&my_srcu); - 4 do_something_2(); - 5 srcu_read_unlock(&my_srcu, r1); - 6 do_something_3(); - 7 srcu_read_unlock(&my_srcu, r2); - - In contrast, LKMM will interpret this as a nested pair of - SRCU read-side critical sections, with the outer critical - section spanning lines 1-7 and the inner critical section - spanning lines 3-5. - - This difference would be more of a concern had anyone - identified a reasonable use case for partially overlapping - SRCU read-side critical sections. For more information - on the trickiness of such overlapping, please see: - https://paulmck.livejournal.com/40593.html - - f. Reader-writer locking is not modeled. It can be + e. Reader-writer locking is not modeled. It can be emulated in litmus tests using atomic read-modify-write operations. |