• Jeff King's avatar
    tempfile: use list.h for linked list · 24d82185
    Jeff King authored
    The tempfile API keeps to-be-cleaned tempfiles in a
    singly-linked list and never removes items from the list.  A
    future patch would like to start removing items, but removal
    from a singly linked list is O(n), as we have to walk the
    list to find the predecessor element. This means that a
    process which takes "n" simultaneous lockfiles (for example,
    an atomic transaction on "n" refs) may end up quadratic in
    "n".
    
    Before we start allowing items to be removed, it would be
    nice to have a way to cover this case in linear time.
    
    The simplest solution is to make an assumption about the
    order in which tempfiles are added and removed from the
    list. If both operations iterate over the tempfiles in the
    same order, then by putting new items at the end of the list
    our removal search will always find its items at the
    beginning of the list. And indeed, that would work for the
    case of refs. But it creates a hidden dependency between
    unrelated parts of the code. If anybody changes the ref code
    (or if we add a new caller that opens multiple simultaneous
    tempfiles) they may unknowingly introduce a performance
    regression.
    
    Another solution is to use a better data structure. A
    doubly-linked list works fine, and we already have an
    implementation in list.h. But there's one snag: the elements
    of "struct tempfile" are all marked as "volatile", since a
    signal handler may interrupt us and iterate over the list at
    any moment (even if we were in the middle of adding a new
    entry).
    
    We can declare a "volatile struct list_head", but we can't
    actually use it with the normal list functions. The compiler
    complains about passing a pointer-to-volatile via a regular
    pointer argument. And rightfully so, as the sub-function
    would potentially need different code to deal with the
    volatile case.
    
    That leaves us with a few options:
    
      1. Drop the "volatile" modifier for the list items.
    
         This is probably a bad idea. I checked the assembly
         output from "gcc -O2", and the "volatile" really does
         impact the order in which it updates memory.
    
      2. Use macros instead of inline functions. The irony here
         is that list.h is entirely implemented as trivial
         inline functions. So we basically are already
         generating custom code for each call. But sadly there's no
         way in C to declare the inline function to take a more
         generic type.
    
         We could do so by switching the inline functions to
         macros, but it does make the end result harder to read.
         And it doesn't fully solve the problem (for instance,
         the declaration of list_head needs to change so that
         its "prev" and "next" pointers point to other volatile
         structs).
    
      3. Don't use list.h, and just make our own ad-hoc
         doubly-linked list. It's not that much code to
         implement the basics that we need here. But if we're
         going to do so, why not add the few extra lines
         required to model it after the actual list.h interface?
         We can even reuse a few of the macro helpers.
    
    So this patch takes option 3, but actually implements a
    parallel "volatile list" interface in list.h, where it could
    potentially be reused by other code. This implements just
    enough for tempfile.c's use, though we could easily port
    other functions later if need be.
    Signed-off-by: 's avatarJeff King <peff@peff.net>
    Signed-off-by: 's avatarJunio C Hamano <gitster@pobox.com>
    24d82185
list.h 5.56 KB