• Kirill Smelkov's avatar
    bigfile/virtmem: Big Virtmem lock · d53271b9
    Kirill Smelkov authored
    At present several threads running can corrupt internal virtmem
    datastructures (e.g. ram->lru_list, fileh->pagemap, etc).
    
    This can happen even if we have zope instances only with 1 worker thread
    - because there are other "system" thread, and python garbage collection
    can trigger at any thread, so if a virtmem object, e.g. VMA or FileH was
    there sitting at GC queue to be collected, their collection, and thus
    e.g. vma_unmap() and fileh_close() will be called from
    different-from-worker thread.
    
    Because of that virtmem just has to be aware of threads not to allow
    internal datastructure corruption.
    
    On the other hand, the idea of introducing userspace virtual memory
    manager turned out to be not so good from performance and complexity
    point of view, and thus the plan is to try to move it back into the
    kernel. This way it does not make sense to do a well-optimised locking
    implementation for userspace version.
    
    So we do just a simple single "protect-all" big lock for virtmem.
    
    Of a particular note is interaction with Python's GIL - any long-lived
    lock has to be taken with GIL released, because else it can deadlock:
    
        t1  t2
    
        G
        V   G
       !G   V
        G
    
    so we introduce helpers to make sure the GIL is not taken, and to retake
    it back if we were holding it initially.
    
    Those helpers (py_gil_ensure_unlocked / py_gil_retake_if_waslocked) are
    symmetrical opposites to what Python provides to make sure the GIL is
    locked (via PyGILState_Ensure / PyGILState_Release).
    
    Otherwise, the patch is more-or-less straightforward application for
    one-big-lock to protect everything idea.
    d53271b9
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