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@xxxxxxxxx>
---
builtin/pack-objects.c | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)
diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index ebc8cefb53..6270f74c0b 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -1852,18 +1852,30 @@ static int delta_cacheable(unsigned long src_size, unsigned long trg_size,
#ifndef NO_PTHREADS
+/* Protect access to object database */
static pthread_mutex_t read_mutex;
#define read_lock() pthread_mutex_lock(&read_mutex)
#define read_unlock() pthread_mutex_unlock(&read_mutex)
+/* Protect delta_cache_size */
static pthread_mutex_t cache_mutex;
#define cache_lock() pthread_mutex_lock(&cache_mutex)
#define cache_unlock() pthread_mutex_unlock(&cache_mutex)
+/*
+ * Protect object list partitioning (e.g. struct thread_param) and
+ * progress_state
+ */
static pthread_mutex_t progress_mutex;
#define progress_lock() pthread_mutex_lock(&progress_mutex)
#define progress_unlock() pthread_mutex_unlock(&progress_mutex)
+/*
+ * Access to struct object_entry is unprotected since each thread owns
+ * a portion of the main object list. Just don't access object entries
+ * ahead in the list because they can be stolen and would need
+ * progress_mutex for protection.
+ */
#else
#define read_lock() (void)0
@@ -2245,12 +2257,19 @@ static void try_to_free_from_threads(size_t size)
static try_to_free_t old_try_to_free_routine;
/*
+ * The main object list is split into smaller lists, each is handed to
+ * one worker.
+ *
* The main thread waits on the condition that (at least) one of the workers
* has stopped working (which is indicated in the .working member of
* struct thread_params).
+ *
* When a work thread has completed its work, it sets .working to 0 and
* signals the main thread and waits on the condition that .data_ready
* becomes 1.
+ *
+ * The main thread steals half of the work from the worker that has
+ * most work left to hand it to the idle worker.
*/
struct thread_params {
--
2.18.0.656.gda699b98b3
