Fwd: [PATCH] pack-objects.c: Initialize read mutex in cmd_pack_objects

On Fri, Jan 18, 2019 at 4:21 AM Duy Nguyen [off-list ref] wrote:

On Fri, Jan 18, 2019 at 9:28 AM Patrick Hogg [off-list ref] wrote:

quoted

ac77d0c37 ("pack-objects: shrink size field in struct object_entry",
2018-04-14) added an extra usage of read_lock/read_unlock in the newly
introduced oe_get_size_slow for thread safety in parallel calls to
try_delta(). Unfortunately oe_get_size_slow is also used in serial
code, some of which is called before the first invocation of
ll_find_deltas. As such the read mutex is not guaranteed to be
initialized.

This must be the SIZE() macros in type_size_sort(), isn't it? I think
we hit the same problem (use of uninitialized mutex) in this same code
not long ago. I wonder if there's anyway we can reliably test and
catch this.

It was actually the SET_SIZE macro in check_object, at least for the
repo at my company that hits this issue.  I took a look at the call
tree for oe_get_size_slow and found that it's used in many places
outside of ll_find_deltas, so there are many potential call sites
where this could crop up:
oe_get_size_slow
    oe_size (SIZE macro)
        write_reuse_object
            write_object
                write_one
                    write_pack_file
                        cmd_pack_objects
        type_size_sort
            prepare_pack
                cmd_pack_objects
        try_delta
            find_deltas
                threaded_find_deltas
                    ll_find_deltas
                        prepare_pack
                            cmd_pack_objects
                ll_find_deltas
                    prepare_pack
                        cmd_pack_objects
        free_unpacked
            find_deltas
                threaded_find_deltas
                    ll_find_deltas
                        prepare_pack
                            cmd_pack_objects
                ll_find_deltas
                    prepare_pack
                        cmd_pack_objects
    oe_size_less_than
        prepare_pack
            cmd_pack_objects
    oe_size_greater_than
        write_no_reuse_object
            write_reuse_object
                write_object
                    write_one
                        write_pack_file
                            cmd_pack_objects
            write_object
                write_one
                    write_pack_file
                        cmd_pack_objects
        get_object_details
            prepare_pack
                cmd_pack_objects
    oe_set_size (SET_SIZE macro)
        check_object
            get_object_details
                prepare_pack
                    cmd_pack_objects
        drop_reused_delta
            break_delta_chains
                get_object_details
                    prepare_pack
                        cmd_pack_objects
(Sorry if this is redundant for those who know the code better)

quoted

Resolve this by splitting off the read mutex initialization from
init_threaded_search. Instead initialize (and clean up) the read
mutex in cmd_pack_objects.

Maybe move the mutex to 'struct packing_data' and initialize it in
prepare_packing_data(), so we centralize mutex at two locations:
generic ones go there, command-specific mutexes stay here in
init_threaded_search(). We could also move oe_get_size_slow() back to
pack-objects.c (the one outside builtin/).

I was already thinking that generic mutexes should be separated from
command specific ones (that's why I introduced init_read_mutex and
cleanup_read_mutex, but that may well not be the right exposure.)
I'll try my hand at this tonight (just moving the mutex to struct
packing_data and initializing it in prepare_packing_data, I'll leave
large code moves to the experts) and see how it turns out.

quoted

Signed-off-by: Patrick Hogg <redacted>
---
 builtin/pack-objects.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 411aefd68..9084bef02 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c

@@ -2381,22 +2381,30 @@ static pthread_cond_t progress_cond;
  */
 static void init_threaded_search(void)
 {
-       init_recursive_mutex(&read_mutex);
        pthread_mutex_init(&cache_mutex, NULL);
        pthread_mutex_init(&progress_mutex, NULL);
        pthread_cond_init(&progress_cond, NULL);
        old_try_to_free_routine = set_try_to_free_routine(try_to_free_from_threads);
 }

+static void init_read_mutex(void)
+{
+       init_recursive_mutex(&read_mutex);
+}
+
 static void cleanup_threaded_search(void)
 {
        set_try_to_free_routine(old_try_to_free_routine);
        pthread_cond_destroy(&progress_cond);
-       pthread_mutex_destroy(&read_mutex);
        pthread_mutex_destroy(&cache_mutex);
        pthread_mutex_destroy(&progress_mutex);
 }

+static void cleanup_read_mutex(void)
+{
+       pthread_mutex_destroy(&read_mutex);
+}
+
 static void *threaded_find_deltas(void *arg)
 {
        struct thread_params *me = arg;

@@ -3319,6 +3327,8 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
                OPT_END(),
        };

+       init_read_mutex();
+
        if (DFS_NUM_STATES > (1 << OE_DFS_STATE_BITS))
                BUG("too many dfs states, increase OE_DFS_STATE_BITS");

@@ -3495,5 +3505,7 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
                           _("Total %"PRIu32" (delta %"PRIu32"),"
                             " reused %"PRIu32" (delta %"PRIu32")"),
                           written, written_delta, reused, reused_delta);
+
+       cleanup_read_mutex();
        return 0;
 }
--

2.20.1.windows.1

--
Duy