Re: oops in :snd_pcm_oss:resample_expand+0x19c/0x1f0

Takashi Iwai
2006-09-27T14:38:08Z

At Sun, 24 Sep 2006 19:01:27 +0200 (CEST),
Jean-Marc Saffroy wrote:
>
> The kernel is vanilla 2.6.18 on Debian etch, the box is a dual core Athlon
> in x86-64 mode (in case you didn't notice the long pointers ;-). The
> offending process is a chroot'ed IA32 Firefox. At first I suspected a
> problem with 32-bit compatibility, but stack traces below and source code
> suggest the problem could be different:
>
> - CPU #0 crashes in resample_expand() at line 116:
>
> 116 *dst = val;
>
> The target address is at ffffc200100e4466, which looks like a vmalloc'ed
> buffer (according to Documentation/x86_64/mm.txt).
>
> - CPU #1 is somewhere down vfree() called by snd_pcm_oss_change_params(),
> at line 1010:
>
> 1010 vfree(runtime->oss.buffer);
> 1011 runtime->oss.buffer = vmalloc(runtime->oss.period_bytes);
>
> Now it really looks like it *could* be a race between two threads using
> the same device, maybe the same buffers somehow, but I'm getting lost in
> the data structures, so help from knowledgeable people would be welcome.

Thanks for the detailed analysis. It looks indeed like a race between
two threads that we didn't consider much for OSS emulation (although
ALSA parts are coded to be thread-safe).

Could you try the patch below? It simply adds mutex around the parts
handling buffers concurrently.

Takashi

diff -r 7cc57ec28195 core/oss/pcm_oss.c
--- a/core/oss/pcm_oss.c Tue Sep 26 15:32:35 2006 +0200
+++ b/core/oss/pcm_oss.c Wed Sep 27 16:29:36 2006 +0200
@@ -810,6 +810,8 @@ static int snd_pcm_oss_change_params(str
struct snd_mask sformat_mask;
struct snd_mask mask;

+ if (mutex_lock_interruptible(&runtime->oss.params_lock))
+ return -EINTR;
sw_params = kmalloc(sizeof(*sw_params), GFP_KERNEL);
params = kmalloc(sizeof(*params), GFP_KERNEL);
sparams = kmalloc(sizeof(*sparams), GFP_KERNEL);
@@ -1020,6 +1022,7 @@ failure:
kfree(sw_params);
kfree(params);
kfree(sparams);
+ mutex_unlock(&runtime->oss.params_lock);
return err;
}

@@ -1307,14 +1310,17 @@ static ssize_t snd_pcm_oss_write1(struct

if ((tmp = snd_pcm_oss_make_ready(substream)) < 0)
return tmp;
+ mutex_lock(&runtime->oss.params_lock);
while (bytes > 0) {
if (bytes < runtime->oss.period_bytes || runtime->oss.buffer_used > 0) {
tmp = bytes;
if (tmp + runtime->oss.buffer_used > runtime->oss.period_bytes)
tmp = runtime->oss.period_bytes - runtime->oss.buffer_used;
if (tmp > 0) {
- if (copy_from_user(runtime->oss.buffer + runtime->oss.buffer_used, buf, tmp))
- return xfer > 0 ? (snd_pcm_sframes_t)xfer : -EFAULT;
+ if (copy_from_user(runtime->oss.buffer + runtime->oss.buffer_used, buf, tmp)) {
+ tmp = -EFAULT;
+ goto err;
+ }
}
runtime->oss.buffer_used += tmp;
buf += tmp;
@@ -1325,22 +1331,24 @@ static ssize_t snd_pcm_oss_write1(struct
tmp = snd_pcm_oss_write2(substream, runtime->oss.buffer + runtime->oss.period_ptr,
runtime->oss.buffer_used - runtime->oss.period_ptr, 1);
if (tmp <= 0)
- return xfer > 0 ? (snd_pcm_sframes_t)xfer : tmp;
+ goto err;
runtime->oss.bytes += tmp;
runtime->oss.period_ptr += tmp;
runtime->oss.period_ptr %= runtime->oss.period_bytes;
if (runtime->oss.period_ptr == 0 ||
runtime->oss.period_ptr == runtime->oss.buffer_used)
runtime->oss.buffer_used = 0;
- else if ((substream->f_flags & O_NONBLOCK) != 0)
- return xfer > 0 ? xfer : -EAGAIN;
+ else if ((substream->f_flags & O_NONBLOCK) != 0) {
+ tmp = -EAGAIN;
+ goto err;
+ }
}
} else {
tmp = snd_pcm_oss_write2(substream,
(const char __force *)buf,
runtime->oss.period_bytes, 0);
if (tmp <= 0)
- return xfer > 0 ? (snd_pcm_sframes_t)xfer : tmp;
+ goto err;
runtime->oss.bytes += tmp;
buf += tmp;
bytes -= tmp;
@@ -1350,7 +1358,12 @@ static ssize_t snd_pcm_oss_write1(struct
break;
}
}
+ mutex_unlock(&runtime->oss.params_lock);
return xfer;
+
+ err:
+ mutex_unlock(&runtime->oss.params_lock);
+ return xfer > 0 ? (snd_pcm_sframes_t)xfer : tmp;
}

static ssize_t snd_pcm_oss_read2(struct snd_pcm_substream *substream, char *buf, size_t bytes, int in_kernel)
@@ -1397,12 +1410,13 @@ static ssize_t snd_pcm_oss_read1(struct

if ((tmp = snd_pcm_oss_make_ready(substream)) < 0)
return tmp;
+ mutex_lock(&runtime->oss.params_lock);
while (bytes > 0) {
if (bytes < runtime->oss.period_bytes || runtime->oss.buffer_used > 0) {
if (runtime->oss.buffer_used == 0) {
tmp = snd_pcm_oss_read2(substream, runtime->oss.buffer, runtime->oss.period_bytes, 1);
if (tmp <= 0)
- return xfer > 0 ? (snd_pcm_sframes_t)xfer : tmp;
+ goto err;
runtime->oss.bytes += tmp;
runtime->oss.period_ptr = tmp;
runtime->oss.buffer_used = tmp;
@@ -1410,8 +1424,10 @@ static ssize_t snd_pcm_oss_read1(struct
tmp = bytes;
if ((size_t) tmp > runtime->oss.buffer_used)
tmp = runtime->oss.buffer_used;
- if (copy_to_user(buf, runtime->oss.buffer + (runtime->oss.period_ptr - runtime->oss.buffer_used), tmp))
- return xfer > 0 ? (snd_pcm_sframes_t)xfer : -EFAULT;
+ if (copy_to_user(buf, runtime->oss.buffer + (runtime->oss.period_ptr - runtime->oss.buffer_used), tmp)) {
+ tmp = -EFAULT;
+ goto err;
+ }
buf += tmp;
bytes -= tmp;
xfer += tmp;
@@ -1420,14 +1436,19 @@ static ssize_t snd_pcm_oss_read1(struct
tmp = snd_pcm_oss_read2(substream, (char __force *)buf,
runtime->oss.period_bytes, 0);
if (tmp <= 0)
- return xfer > 0 ? (snd_pcm_sframes_t)xfer : tmp;
+ goto err;
runtime->oss.bytes += tmp;
buf += tmp;
bytes -= tmp;
xfer += tmp;
}
}
+ mutex_unlock(&runtime->oss.params_lock);
return xfer;
+
+ err:
+ mutex_unlock(&runtime->oss.params_lock);
+ return xfer > 0 ? (snd_pcm_sframes_t)xfer : tmp;
}

static int snd_pcm_oss_reset(struct snd_pcm_oss_file *pcm_oss_file)
@@ -1528,6 +1549,7 @@ static int snd_pcm_oss_sync(struct snd_p
return err;
format = snd_pcm_oss_format_from(runtime->oss.format);
width = snd_pcm_format_physical_width(format);
+ mutex_lock(&runtime->oss.params_lock);
if (runtime->oss.buffer_used > 0) {
#ifdef OSS_DEBUG
printk("sync: buffer_used\n");
@@ -1537,8 +1559,10 @@ static int snd_pcm_oss_sync(struct snd_p
runtime->oss.buffer + runtime->oss.buffer_used,
size);
err = snd_pcm_oss_sync1(substream, runtime->oss.period_bytes);
- if (err < 0)
+ if (err < 0) {
+ mutex_unlock(&runtime->oss.params_lock);
return err;
+ }
} else if (runtime->oss.period_ptr > 0) {
#ifdef OSS_DEBUG
printk("sync: period_ptr\n");
@@ -1548,8 +1572,10 @@ static int snd_pcm_oss_sync(struct snd_p
runtime->oss.buffer,
size * 8 / width);
err = snd_pcm_oss_sync1(substream, size);
- if (err < 0)
+ if (err < 0) {
+ mutex_unlock(&runtime->oss.params_lock);
return err;
+ }
}
/*
* The ALSA's period might be a bit large than OSS one.
@@ -1579,6 +1605,7 @@ static int snd_pcm_oss_sync(struct snd_p
snd_pcm_lib_writev(substream, buffers, size);
}
}
+ mutex_unlock(&runtime->oss.params_lock);
/*
* finish sync: drain the buffer
*/
@@ -2172,6 +2199,7 @@ static void snd_pcm_oss_init_substream(s
runtime->oss.params = 1;
runtime->oss.trigger = 1;
runtime->oss.rate = 8000;
+ mutex_init(&runtime->oss.params_lock);
switch (SNDRV_MINOR_OSS_DEVICE(minor)) {
case SNDRV_MINOR_OSS_PCM_8:
runtime->oss.format = AFMT_U8;
diff -r 7cc57ec28195 include/pcm_oss.h
--- a/include/pcm_oss.h Tue Sep 26 15:32:35 2006 +0200
+++ b/include/pcm_oss.h Wed Sep 27 16:28:28 2006 +0200
@@ -56,6 +56,7 @@ struct snd_pcm_oss_runtime {
size_t mmap_bytes;
char *buffer; /* vmallocated period */
size_t buffer_used; /* used length from period buffer */
+ struct mutex params_lock;
#ifdef CONFIG_SND_PCM_OSS_PLUGINS
struct snd_pcm_plugin *plugin_first;
struct snd_pcm_plugin *plugin_last;