pack-objects: document about thread synchronization
These extra comments should be make it easier to understand how to use locks in pack-objects delta search code. For reference, see 8ecce684a3 (basic threaded delta search - 2007-09-06) 384b32c09b (pack-objects: fix threaded load balancing - 2007-12-08) 50f22ada52 (threaded pack-objects: Use condition... - 2007-12-16) Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Nguyễn Thái Ngọc Duy committed
Jul 29, 2018 at 17:36 UTC
ffbd51cc609eebfd3ce9dd360ef145cfafc2e088
1 file changed
+19
builtin/pack-objects.c
+19
@@ -1852,18 +1852,30 @@ static int delta_cacheable(unsigned long src_size, unsigned long trg_size,
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#ifndef NO_PTHREADS
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+/* Protect access to object database */
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static pthread_mutex_t read_mutex;
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#define read_lock() pthread_mutex_lock(&read_mutex)
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#define read_unlock() pthread_mutex_unlock(&read_mutex)
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+/* Protect delta_cache_size */
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static pthread_mutex_t cache_mutex;
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#define cache_lock() pthread_mutex_lock(&cache_mutex)
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#define cache_unlock() pthread_mutex_unlock(&cache_mutex)
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+/*
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+ * Protect object list partitioning (e.g. struct thread_param) and
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+ * progress_state
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+ */
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static pthread_mutex_t progress_mutex;
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#define progress_lock() pthread_mutex_lock(&progress_mutex)
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#define progress_unlock() pthread_mutex_unlock(&progress_mutex)
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+/*
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+ * Access to struct object_entry is unprotected since each thread owns
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+ * a portion of the main object list. Just don't access object entries
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+ * ahead in the list because they can be stolen and would need
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+ * progress_mutex for protection.
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+ */
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#else
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#define read_lock() (void)0
@@ -2245,12 +2257,19 @@ static void try_to_free_from_threads(size_t size)
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static try_to_free_t old_try_to_free_routine;
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/*
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+ * The main object list is split into smaller lists, each is handed to
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+ * one worker.
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+ *
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* The main thread waits on the condition that (at least) one of the workers
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* has stopped working (which is indicated in the .working member of
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* struct thread_params).
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+ *
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* When a work thread has completed its work, it sets .working to 0 and
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* signals the main thread and waits on the condition that .data_ready
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* becomes 1.
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+ *
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+ * The main thread steals half of the work from the worker that has
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+ * most work left to hand it to the idle worker.
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*/
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struct thread_params {