* Re: [linux-dvb] Re: dvb-kernel crasher
[not found] ` <200501051404.43182.adq_dvb@lidskialf.net>
@ 2005-01-06 16:26 ` Olaf Titz
0 siblings, 0 replies; only message in thread
From: Olaf Titz @ 2005-01-06 16:26 UTC (permalink / raw)
To: Andrew de Quincey; +Cc: linux-dvb, linux-kernel
Hello,
[CC to linux-kernel]
After more debugging I found the most likely suspect to be GCC 3.3.3.
Compiling with GCC 2.95 makes the problem go away.
Here's more analysis:
1. I tried with preempt off since the fops_get path gets much more
complex with preempt. Same result, i.e. crashes. The following is with
preempt off.
2. Inserted debug into try_module_get. The crash happens before
try_module_get is called.
The relevant routines look now like the following (no functional
changes, only debug output):
static inline int try_module_get(struct module *module)
{
int ret = 1;
if (module) {
unsigned int cpu = get_cpu();
#ifdef DEBUG_DVB
printk(KERN_DEBUG
"try_module_get(%s): cpu=%d state=%d count=%d ",
module->name, module->state,
module->ref[cpu].count);
#endif
if (likely(module_is_live(module)))
local_inc(&module->ref[cpu].count);
else
ret = 0;
put_cpu();
#ifdef DEBUG_DVB
printk("ret=%d\n", ret);
#endif
}
return ret;
}
static int dvb_device_open(struct inode *inode, struct file *file)
{
struct dvb_device *dvbdev;
dvbdev = dvbdev_find_device (iminor(inode));
if (dvbdev_debug) {
printk(KERN_DEBUG "dvb_device_open: i_rdev=%x ",
inode->i_rdev);
if (dvbdev) {
print_symbol("dvbdev=%s ", dvbdev);
print_symbol("fops=%s ", dvbdev ? dvbdev->fops : 0);
} else {
printk("dvbdev=NULL ");
}
printk("file=%p ", file);
print_symbol("f_op=%s ", file->f_op);
printk("f_flags=%x private_data=%x\n",
file->f_flags, file->private_data);
}
if (dvbdev && dvbdev->fops) {
int err = 0;
struct file_operations *old_fops;
file->private_data = dvbdev; /**** see below ****/
old_fops = file->f_op; /**** see below ****/
file->f_op = fops_get(dvbdev->fops);
if (dvbdev_debug)
print_symbol("dvb_device_open: open=%s ",
file->f_op->open);
if(file->f_op->open)
err = file->f_op->open(inode,file);
if (err) {
fops_put(file->f_op);
file->f_op = fops_get(old_fops);
}
fops_put(old_fops);
dprintk("result=%d\n", err);
return err;
}
dprintk(KERN_DEBUG "dvb_device_open: result=ENODEV\n");
return -ENODEV;
}
This gives output like this in normal operation:
Jan 6 11:15:18 bigred kernel: dvb_device_open: i_rdev=d400004
dvbdev=0xcfff41e0 fops=dvb_demux_fops+0x0/0xffff81e0 [dvb_core]
file=c0505560 f_op=dvb_device_fops+0x0/0xffff8280 [dvb_core] f_flags=802
private_data=0
Jan 6 11:15:18 bigred kernel: try_module_get(budget_ci): cpu=0 state=16
count=1 ret=1
Jan 6 11:15:18 bigred kernel: dvb_device_open:
open=dvb_demux_open+0x0/0x110 [dvb_core] result=0
3. The crash now looks like this:
Jan 6 11:15:19 bigred kernel: dvb_device_open: i_rdev=d400003
dvbdev=0xcfff4620 fops=dvb_frontend_fops+0x0/0xffff7ca0 [dvb_core]
file=c0a4bb00 f_op=dvb_device_fops+0x0/0xffff8280 [dvb_core] f_flags=0
private_data=0
Jan 6 11:15:19 bigred kernel: Unable to handle kernel paging request at
virtual address d0a97580
Jan 6 11:15:19 bigred kernel: printing eip:
Jan 6 11:15:19 bigred kernel: d0aa20d8
Jan 6 11:15:19 bigred kernel: *pde = 0ffcb067
Jan 6 11:15:19 bigred kernel: *pte = 00000000
Jan 6 11:15:19 bigred kernel: Oops: 0000 [#1]
4. Further digging with a disassembler into the module suggests that the
crash happens at one of the statements marked "see below". The debug
output, however, shows that the "file" pointer is most likely _not_
corrupted. This leaves not many possible reasons.
I've not exactly understood what the compiler makes of these statements
though, so I tried another compiler, and voila - it works.
----
So can some kernel guru perhaps look into what GCC 3.3.3 makes of
fops_get()? Perhaps we have a compiler bug or some sort of unfortunate
interaction with the nontrivial macroized code...
Thanks for your help anyway,
Olaf
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