Re: [PATCH] index-pack: protect deepest_delta in multithread code

Nguyễn Thái Ngọc Duy [off-list ref] writes:

deepest_delta is a global variable but is updated without protection
in resolve_delta(), a multithreaded function. Add a new mutex for it,
but only protect and update when it's actually used (i.e. show_stat is
non-zero).

Another variable that will not be updated is delta_depth in "struct
object_entry" as it's only useful when show_stat is 1. Putting it in
"if (show_stat)" makes it clearer.

The local variable "stat" is renamed to "show_stat" after moving to
global scope because the name "stat" conflicts with stat(2) syscall.

Looks good to me, too.  However, I think it would still be less magical
if we had the below too.  It also silences helgrind.

-- >8 --
Subject: [PATCH] index-pack: guard nr_resolved_deltas reads by lock

The threaded parts of index-pack increment the number of resolved
deltas in nr_resolved_deltas guarded by counter_mutex.  However, the
per-thread outer loop accessed nr_resolved_deltas without any locks.

This is not wrong as such, since it doesn't matter all that much
whether we get an outdated value.  However, unless someone proves that
this one lock makes all the performance difference, it would be much
cleaner to guard _all_ accesses to the variable with the lock.

Signed-off-by: Thomas Rast <redacted>
---
 builtin/index-pack.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index b3fee45..6be99e2 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c

@@ -968,7 +968,9 @@ static void *threaded_second_pass(void *data)
 	for (;;) {
 		int i;
 		work_lock();
+		counter_lock();
 		display_progress(progress, nr_resolved_deltas);
+		counter_unlock();
 		while (nr_dispatched < nr_objects &&
 		       is_delta_type(objects[nr_dispatched].type))
 			nr_dispatched++;

-- 
1.8.2.487.g725f6bb


-- 
Thomas Rast
trast@{inf,student}.ethz.ch