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pack-objects: document about thread synchronization

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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>
This commit is contained in:
Nguyễn Thái Ngọc Duy 2018-07-29 17:36:05 +02:00 committed by Junio C Hamano
parent 53f9a3e157
commit ffbd51cc60
1 changed files with 19 additions and 0 deletions
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19
builtin/pack-objects.c
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@ -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 {