[alsa-devel] Linux 2.6.38 freeze because of sound/core/pcm_lib.c commit 59ff878ffb26bc0be812ca8295799164f413ae88

Kelly Anderson kelly at silka.with-linux.com
Fri Apr 1 11:10:27 CEST 2011

On 04/01/11 01:47, Clemens Ladisch wrote:
> Kelly, to apply this patch I need a Signed-off-by tag from you (see
> Documentation/SubmittingPatches).
It works fine Clemens.


ALSA: pcm: fix infinite loop in snd_pcm_update_hw_ptr0()

When period interrupts are disabled, snd_pcm_update_hw_ptr0() compares
the current time against the time estimated for the current hardware
pointer to detect xruns.  The somewhat fuzzy threshold in the while loop
makes it possible that hdelta becomes negative; the comparison being
done with unsigned types then makes the loop go through the entire 263
negative range, and, depending on the value, never reaching an unsigned
value that is small enough to stop the loop.  Doing this with interrupts
disabled results in the machine locking up.

To prevent this, ensure that the loop condition uses signed types for
both operands so that the comparison is correctly done.

Many thanks to Kelly Anderson for debugging this.

Reported-by: Nix <nix at esperi.org.uk>
Reported-by: "Christopher K." <c.krooss at googlemail.com>
Reported-by: Kelly Anderson <kelly at silka.with-linux.com>
[cl: remove unneeded casts; use a temp variable]
Signed-off-by: Clemens Ladisch <clemens at ladisch.de>
Signed-off-by: Kelly Anderson <kelly at silka.with-linux.com>

--- a/sound/core/pcm_lib.c
+++ b/sound/core/pcm_lib.c
@@ -375,6 +375,7 @@ static int snd_pcm_update_hw_ptr0(struct 
snd_pcm_substream *substream,

      if (runtime->no_period_wakeup) {
+        snd_pcm_sframes_t xrun_threshold;
           * Without regular period interrupts, we have to check
           * the elapsed time to detect xruns.
@@ -383,7 +384,8 @@ static int snd_pcm_update_hw_ptr0(struct 
snd_pcm_substream *substream,
          if (jdelta < runtime->hw_ptr_buffer_jiffies / 2)
              goto no_delta_check;
          hdelta = jdelta - delta * HZ / runtime->rate;
+        xrun_threshold = runtime->hw_ptr_buffer_jiffies / 2 + 1;
-        while (hdelta > runtime->hw_ptr_buffer_jiffies / 2 + 1) {
+        while (hdelta > xrun_threshold) {
              delta += runtime->buffer_size;
              hw_base += runtime->buffer_size;
              if (hw_base >= runtime->boundary)

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