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conf: Add thread-safe global tree reference

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Most of open functions in alsa-lib have the call pattern:
  snd_config_update();
  return snd_xxx_open(x, snd_config, ...);

This means that the toplevel config gets updated, and passed to a
local open function.  Although snd_config_update() itself has a
pthread mutex to be thread safe, the whole procedure above isn't
thread safe.  Namely, the global snd_config tree may be deleted and
recreated at any time while the open function is being processed.
This may lead to a data corruption and crash of the program.

For avoiding the corruption, this patch introduces a refcount to
config tree object.  A few new helper functions are introduced as
well:
- snd_config_update_ref() does update and take the refcount of the
  toplevel tree.   The obtained config tree has to be freed via
  snd_config_unref() below.
- snd_config_ref() and snd_config_unref() manage the refcount of the
  config object.  The latter eventually deletes the object when all
  references are gone.

Along with these additions, the caller of snd_config_update() and
snd_config global tree in alsa-lib are replaced with the new helpers.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
This commit is contained in:
Takashi Iwai 2016-05-17 15:39:07 +02:00
parent 5fb3fe1724
commit c9a0d7d601
9 changed files with 125 additions and 14 deletions
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4
include/conf.h
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@ -94,6 +94,10 @@ int snd_config_update_r(snd_config_t **top, snd_config_update_t **update, const
int snd_config_update_free(snd_config_update_t *update);
int snd_config_update_free_global(void);
int snd_config_update_ref(snd_config_t **top);
void snd_config_ref(snd_config_t *top);
void snd_config_unref(snd_config_t *top);
int snd_config_search(snd_config_t *config, const char *key,
snd_config_t **result);
int snd_config_searchv(snd_config_t *config,