Add force_unlock() to allocator_storage. #195
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added 2 commits
August 27, 2025 11:13
This feature is almost never needed. However, when it is needed, it is badly needed. Consider the case where a program forks. The allowable operations in the child (before calling exec) are very limited. However, we might want to use a special purpose allocator. But, if that allocator can be used my multiple threads, then it is possible that one of those threads is holding the lock when fork is called. This means that the child can't acquire the lock because the thread holding it will never get to run and release the lock within the child process. In this case, the child must be able to unlock the allocator. This is, admittedly, a very exceptional use case. However, it has been added in such a way that it will only appear in the API if the provided Mutex type supports `force_unlock`, which is unlikely for most use cases. The only other way of accomplishing this task is using nasty techniques to subvert the visibility access rules of the language since the mutex is one of multiple a private base classes. Another small change to support this feature was to add a member access through pointer operator to the mutex storage classes.
allocator_storage takes a Mutex type supports force_unlock() but when the allocator is stateless, the actual mutex type will be no_mutex instead of the supported one. We could SFINAE on teh passed-in mutex type, and then detect the no_mutex type and turn it into a no-op. However, it doesn't seem like a bad thing to just add force_unlock() to the no_mutex types, since there is no locking going on anyway.
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This feature is almost never needed. However, when it is needed, it is badly needed.
Consider the case where a program forks. The allowable operations in the child (before calling exec) are very limited. However, we might want to use a special purpose allocator. But, if that allocator can be used my multiple threads, then it is possible that one of those threads is holding the lock when fork is called. This means that the child can't acquire the lock because the thread holding it will never get to run and release the lock within the child process.
In this case, the child must be able to unlock the allocator.
This is, admittedly, a very exceptional use case. However, it has been added in such a way that it will only appear in the API if the provided Mutex type supports
force_unlock, which is unlikely for most use cases.The only other way of accomplishing this task is using nasty techniques to subvert the visibility access rules of the language since the mutex is one of multiple a private base classes.
Another small change to support this feature was to add a member access through pointer operator to the mutex storage classes.