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Subjectmultiple bread()s causes oops
> I'm still trying to read physical sectors in some partitioning code in
> linux/fs/partitions, and have made progress. Thanks for the pointers on
> set_blocksize(), that helps.
My ultimate goal is to read blocks 1, 2-34, n, n-32..n-1, where n is the
number of hardware sectors on the disk. However, when I sequentially read
blocks, I get the oopses below. (kernel 2.4.0-test9-final on i686 SMP and
UP, IA-64 SMP fails similarly.)

dev is a SCSI disk, /dev/sdd or the like.
set_blocksize(dev, 512)
bread(dev, 1, 512) - succeeds. - Use the data to figure out how many more
blocks to read, and where they are.
bread(dev, 2, 512) - succeeds.
bread(dev, 3, 512) - succeeds.
bread(dev, 4, 512) - succeeds (most of the time).
bread(dev, 5, 512) - oops, kernel dereferences NULL.

If I insert a delay between the breads {while (jiffies < j + HZ)
schedule();}, I can read more blocks, but it fails after 9-10 blocks rather
than 4-5. Or, if I insert a delay in bread() between calling ll_rw_block()
and wait_on_block(), it fails after 9-10 blocks.

It seems that the I/O completion is actually causing the NULL dereference.
It's neither in ll_rw_block() or wait_on_block() when it bombs, from what I
can tell.

I'm not acquiring any locks in my code before calling bread(). Should I?
I've tried getting the BKL first, no difference.

This has tripped me up for about a week now, so I'd really appreciate any

Matt Domsch
Dell Enterprise Systems Group
Linux Development Team

SMP oops.txt
Unable to handle kernel NULL pointer dereference at virtual address 00000000
*pde = 37b11001
Oops: 0000
CPU: 0
EIP: 0010:[<00000000>]
Using defaults from ksymoops -t elf32-i386 -a i386
EFLAGS: 00010202
eax: 00000000 ebx: c211dba0 ecx: 00000000 edx: c029bfa8
esi: 00000000 edi: 00000001 ebp: 0000001f esp: c029bf60
ds: 0018 es: 0018 ss: 0018
Process swapper (pid: 0, stackpage=c029b000)
Stack: c010d4c0 0000001f 00000000 c029bfa8 c029bfa8 c02d8be0 0000001f
c010d6c1 0000001f c029bfa8 c211dba0 00000000 c029a000 c0109bf0
ffffe000 c010bbf8 c029a000 c029a000 00000000 c0109bf0 c029a000
Call Trace: [<c010d4c0>] [<c010d6c1>] [<c0109bf0>] [<c010bbf8>] [<c0109bf0>]
[<c0109c1e>] [<c0109ca2>]
Code: Bad EIP value.

>>EIP; 00000000 Before first symbol
Trace; c010d4c0 <handle_IRQ_event+60/90>
Trace; c010d6c1 <do_IRQ+a1/100>
Trace; c0109bf0 <default_idle+0/40>
Trace; c010bbf8 <ret_from_intr+0/20>
Trace; c0109bf0 <default_idle+0/40>
Trace; c0109c1e <default_idle+2e/40>
Trace; c0109ca2 <cpu_idle+52/70>

UP oops.txt
ksymoops 2.3.4 on i686 2.4.0-test9. Options used
-V (default)
-k /proc/ksyms (default)
-l /proc/modules (default)
-o /lib/modules/2.4.0-test9/ (default)
-m /root/linux/ (specified)

Warning (expand_objects): object /lib/aic7xxx.o for module aic7xxx has
changed since load
Unable to handle kernel paging request at virtual address fffffffc
*pde = 00001063
Oops: 0000
CPU: 0
EIP: 0010:[<c016d49b>]
Using defaults from ksymoops -t elf32-i386 -a i386
EFLAGS: 00010286
eax: ffffffff ebx: ffffffff ecx: 00000000 edx: 00000000
esi: ffffffff edi: 0000021c ebp: c1ef52e0 esp: f7adb77c
ds: 0018 es: 0018 ss: 0018
Process insmod (pid: 7, stackpage=f7adb000)
Stack: 00000001 c1ef52e0 00000000 00000000 c025cc00 c016d68c c1ef52e0
00000002 00000000 c02ae580 00000000 c01204da c1ef52e0 c02c6600
c02c6600 c011d5d8 00000001 00000001 c02ae5a0 0000000e c011d4dd
Call Trace: [<c016d68c>] [<c01204da>] [<c011d6cc>] [<c011d5d8>] [<c011d4dd>]
[<c010bc81>] [<c010a8a0>]
[<c0119e9a>] [<c014e8e7>] [<c021bd24>] [<c014ea8c>] [<c021bede>]
[<c0119ea9>] [<c014ebb4>] [<c014ed6b>]
[<c014edf7>] [<c014ee79>] [<c014f14d>] [<f881abcd>] [<f881a5a0>]
[<f881ad58>] [<f881b053>] [<c019b7e1>]
[<c01a2cec>] [<c0117751>] [<c0250040>] [<c01209c0>] [<c0120cd3>]
[<c0120876>] [<c0120a84>] [<c011d6cc>]
[<c011d5d8>] [<c011d4dd>] [<c010bc81>] [<c010a8a0>] [<c0119ea9>]
[<c0132040>] [<c0217440>] [<c014f371>]
[<c014f383>] [<c014e5f0>] [<c0119ea9>] [<c014e04c>] [<c01a69fa>]
[<c014e134>] [<c01a710a>] [<f8825260>]
[<c019cf30>] [<f8804000>] [<f8804050>] [<f881e257>] [<f8825260>]
[<f8825260>] [<c011a7d2>] [<f8804048>]
Code: 8b 0c 82 89 d8 31 cf 8b 4d 18 01 c8 21 f0 8b 0c 82 89 d8 31

>>EIP; c016d49b <add_entropy_words+5b/d0> <=====
Trace; c016d68c <batch_entropy_process+3c/c0>
Trace; c01204da <tqueue_bh+3a/50>
Trace; c011d6cc <bh_action+1c/60>
Trace; c011d5d8 <tasklet_hi_action+38/60>
Trace; c011d4dd <do_softirq+5d/80>
Trace; c010bc81 <do_IRQ+a1/b0>
Trace; c010a8a0 <ret_from_intr+0/20>
Trace; c0119e9a <printk+15a/180>
Trace; c014e8e7 <unparse_buffer_head+27/140>
Trace; c021bd24 <tvecs+7e40/fba8>
Trace; c014ea8c <my_bread+8c/c0> (this is my function, bread + extra
Trace; c021bede <tvecs+7ffa/fba8>
Trace; c0119ea9 <printk+169/180>
Trace; c014ebb4 <ReadLBA+f4/170> (this is my function, calls my_bread())
Trace; c014ed6b <ReadGuidPartitionEntries+4b/70> (These next 4 are my
functions too, they don't touch the disk at all).
Trace; c014edf7 <ReadGuidPartitionTableHeader+67/a0>
Trace; c014ee79 <IsGuidPartitionTableValid+49/1b0>
Trace; c014f14d <add_gpt_partitions+9d/250>
Trace; f881abcd <[aic7xxx]aic7xxx_build_negotiation_cmnd+1cd/1e0>
Trace; f881a5a0 <[aic7xxx]aic7xxx_negotiation_complete+0/460>
Trace; f881ad58 <[aic7xxx]aic7xxx_buildscb+178/340>
Trace; f881b053 <[aic7xxx]aic7xxx_queue+133/140>
Trace; c019b7e1 <scsi_dispatch_cmd+1b1/230>
Trace; c01a2cec <scsi_request_fn+2ec/330>
Trace; c0117751 <schedule+281/410>
Trace; c0250040 <llc_oui+121e/143e>
Trace; c01209c0 <update_process_times+20/80>
Trace; c0120cd3 <do_timer+23/90>
Trace; c0120876 <update_wall_time+16/50>
Trace; c0120a84 <timer_bh+24/250>
Trace; c011d6cc <bh_action+1c/60>
Trace; c011d5d8 <tasklet_hi_action+38/60>
Trace; c011d4dd <do_softirq+5d/80>
Trace; c010bc81 <do_IRQ+a1/b0>
Trace; c010a8a0 <ret_from_intr+0/20>
Trace; c0119ea9 <printk+169/180>
Trace; c0132040 <set_blocksize+1a0/1e0>
Trace; c0217440 <tvecs+355c/fba8>
Trace; c014f371 <efi_partition+71/a0> (this is my function, called by
Trace; c014f383 <efi_partition+83/a0>
Trace; c014e5f0 <msdos_partition+200/3b0>
Trace; c0119ea9 <printk+169/180>
Trace; c014e04c <check_partition+8c/d0>
Trace; c01a69fa <sd_init_onedisk+71a/730>
Trace; c014e134 <grok_partitions+64/b0>
Trace; c01a710a <sd_finish+14a/1e0>
Trace; f8825260 <[aic7xxx]driver_template+0/6b>
Trace; c019cf30 <scsi_register_host+280/2b0>
Trace; f8804000 <_end+38527350/385273b0>
Trace; f8804050 <_end+385273a0/385273b0>
Trace; f881e257 <[aic7xxx]init_this_scsi_driver+17/40>
Trace; f8825260 <[aic7xxx]driver_template+0/6b>
Trace; f8825260 <[aic7xxx]driver_template+0/6b>
Trace; c011a7d2 <sys_init_module+422/4a0>
Trace; f8804048 <_end+38527398/385273b0>
Trace; c010a7f7 <system_call+33/38>
Code; c016d49b <add_entropy_words+5b/d0>
00000000 <_EIP>:
Code; c016d49b <add_entropy_words+5b/d0> <=====
0: 8b 0c 82 mov (%edx,%eax,4),%ecx <=====
Code; c016d49e <add_entropy_words+5e/d0>
3: 89 d8 mov %ebx,%eax
Code; c016d4a0 <add_entropy_words+60/d0>
5: 31 cf xor %ecx,%edi
Code; c016d4a2 <add_entropy_words+62/d0>
7: 8b 4d 18 mov 0x18(%ebp),%ecx
Code; c016d4a5 <add_entropy_words+65/d0>
a: 01 c8 add %ecx,%eax
Code; c016d4a7 <add_entropy_words+67/d0>
c: 21 f0 and %esi,%eax
Code; c016d4a9 <add_entropy_words+69/d0>
e: 8b 0c 82 mov (%edx,%eax,4),%ecx
Code; c016d4ac <add_entropy_words+6c/d0>
11: 89 d8 mov %ebx,%eax
Code; c016d4ae <add_entropy_words+6e/d0>
13: 31 00 xor %eax,(%eax)

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