ALSA: hda - Fix possible race on regmap bypass flip

HD-audio driver uses regmap cache bypass feature for reading a raw
value without the cache.  But this is racy since both the cached and
the uncached reads may occur concurrently.  The former is done via the
normal control API access while the latter comes from the proc file
read.

Even though the regmap itself has the protection against the
concurrent accesses, the flag set/reset is done without the
protection, so it may lead to inconsistent state of bypass flag that
doesn't match with the current read and occasionally result in a
kernel WARNING like:
  WARNING: CPU: 3 PID: 2731 at drivers/base/regmap/regcache.c:499 regcache_cache_only+0x78/0x93

One way to work around such a problem is to wrap with a mutex.  But in
this case, the solution is simpler: for the uncached read, we just
skip the regmap and directly calls its accessor.  The verb execution
there is protected by itself, so basically it's safe to call
individually.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=116171
Signed-off-by: Takashi Iwai <tiwai@suse.de>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/sound/hda_regmap.h b/include/sound/hda_regmap.h
index 2767c55a641e..ca64f0f50b45 100644
--- a/include/sound/hda_regmap.h
+++ b/include/sound/hda_regmap.h
@@ -17,6 +17,8 @@ int snd_hdac_regmap_add_vendor_verb(struct hdac_device *codec,
 				    unsigned int verb);
 int snd_hdac_regmap_read_raw(struct hdac_device *codec, unsigned int reg,
 			     unsigned int *val);
+int snd_hdac_regmap_read_raw_uncached(struct hdac_device *codec,
+				      unsigned int reg, unsigned int *val);
 int snd_hdac_regmap_write_raw(struct hdac_device *codec, unsigned int reg,
 			      unsigned int val);
 int snd_hdac_regmap_update_raw(struct hdac_device *codec, unsigned int reg,