semaphore: semaphore_units: return units when reassigned #1466
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bhalevy
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In v2 (ca8bd17)
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The test case fails with g++ (GCC) 12.2.1 20221121 (Red Hat 12.2.1-4) ``` tests/unit/semaphore_test.cc(0): Entering test case "test_semaphore_units_splitting" src/testing/seastar_test.cc(43): info: check true has passed tests/unit/semaphore_test.cc(269): info: check units.count() == 3 has passed tests/unit/semaphore_test.cc(270): info: check sm.available_units() == 0 has passed tests/unit/semaphore_test.cc(272): info: check sm.available_units() == 0 has passed tests/unit/semaphore_test.cc(273): info: check units.count() == 2 has passed tests/unit/semaphore_test.cc(274): info: check split.count() == 1 has passed tests/unit/semaphore_test.cc(276): info: check sm.available_units() == 1 has passed tests/unit/semaphore_test.cc(279): fatal error: in "test_semaphore_units_splitting": critical check sm.available_units() == 0 has failed [2 != 0] ``` Avoid reassigning `units` after it is destroyed. Split the existing test sub-cases into test cases of their own. Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
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@xemul can you please review? |
avikivity
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Jul 25, 2023
include/seastar/core/semaphore.hh
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| @@ -482,6 +482,7 @@ public: | |||
| semaphore_units(semaphore_units&& o) noexcept : _sem(o._sem), _n(std::exchange(o._n, 0)) { | |||
| } | |||
| semaphore_units& operator=(semaphore_units&& o) noexcept { | |||
| return_all(); | |||
| _sem = o._sem; | |||
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This doesn't work when this == &o.
It technically doesn't need to work (passing a temporary contains a promise not to use it other than to invoke the destructor), but the damage is so great it's better to protect against it.
When semaphore_units are (move-) reassigned with other units, the held units aren't currently returned to the semaphore. Call return_all before reassigning the semaphore_units object. Add a unit test reproducer. Fixes scylladb#1465 Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
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In v4 (c4733e5): protect against self-assignment in move assignment operator. |
avikivity
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Jul 25, 2023
| if (this != &o) { | ||
| return_all(); | ||
| _sem = o._sem; | ||
| _n = std::exchange(o._n, 0); |
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Note: the following change might also have fixed it:
std::swap(_sem, o,_sem);
std::swap(_n, o._n);
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When semaphore_units are (move-) reassigned with
other units, the held units aren't currently
returned to the semaphore.
This change implements the move-reassignment operator by swapping the semaphore and the units counter
so that when the moved-from semaphore_units object is destroyed, the units will be returned to the semaphore via
~semaphore_units->return_all.Add a unit test reproducer.
Fixes #1465
Signed-off-by: Benny Halevy bhalevy@scylladb.com