Re: files_lock deadlock?

[Martin Wilck <martin.wilck@fujitsu-siemens.com>]

Alexander Nyberg wrote:
> spin_lock_irqsave is only needed when a lock is taken both in normal
> context and in interrupt context. Clearly this lock is not intended to
> be taken in interrupt context. 

Further investigation has shown that there was indeed a deadlock, at 
least until 2.6.10.

See this patch:
http://linux.bkbits.net:8080/linux-2.6/cset@1.1982.120.2?nav=index.html|src/|src/net|src/net/ipv6|related/net/ipv6/addrconf.c
("[IPV6]: Unregister per-device snmp6 proc entry earlier.")

That patch (indirectly) moves a call to remove_proc_entry() from
a softirq execution path, thus removing the files_lock deadlock thgat 
can be seen in the trace below. However, I think there may be other 
paths like this, and even if there aren't people may very easily add 
calls to (say) remove_prc_entry() to softirq paths in the future, which 
will conjur up the same problem again. So my plea is to make files_lock 
protected by spin_lock_bh().

Regards
Martin


Kernel panic - not syncing: fs/proc/generic.c:521: 
spin_lock(fs/file_table.c:ffffffff80420280) already locked by fs/file_tabl
e.c/204
Kernel BUG at panic:74
invalid operand: 0000 [1]
CPU 0
Modules linked in: md5 ipv6 mptctl smbus(U) ipmi(U) sunrpc scsi_dump 
diskdump button battery ac uhci_hcd ehci_hcd tg3 sg dm_s
napshot dm_zero dm_mirror ext3 jbd dm_mod mptscsih mptbase sd_mod scsi_mod
Pid: 3974, comm: ifdown Not tainted 2.6.9-11.EL
RIP: 0010:[<ffffffff80137d42>] <ffffffff80137d42>{panic+211}
RSP: 0018:ffffffff8048cce8  EFLAGS: 00010286
RAX: 0000000000000089 RBX: ffffffff80358c9a RCX: 0000000000003c22
RDX: 00000000ffffff01 RSI: 0000000000003c22 RDI: ffffffff804189e0
RBP: 000001020b8b0dc0 R08: 0000000000000002 R09: 000000000000000d
R10: 000000000000000e R11: 0000ffff80436560 R12: 00000100efec3400
R13: 0000000000000004 R14: 000001020f52b6d8 R15: 000001000dfbf800
FS:  0000002a955843e0(0000) GS:ffffffff8051e980(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 000000000044f3c0 CR3: 0000000000101000 CR4: 00000000000006e0
Process ifdown (pid: 3974, threadinfo 000001020d816000, task 
000001020b039110)
Stack: 0000003000000030 ffffffff8048cdd0 ffffffff8048cd08 000001000827a6e0
       ffffffff8048cd58 ffffffff8036314e 0000000000000209 ffffffff80361bcb
       ffffffff80420280 ffffffff80361bcb
Call Trace:
<IRQ>   <ffffffff8027f373>{cfq_next_request+59}
        <ffffffff80275481>{elv_next_request+238}
        <ffffffff801cd6be>{remove_proc_entry+288}
        <ffffffffa0174422>{:ipv6:snmp6_unregister_dev+54}
        <ffffffffa015390b>{:ipv6:in6_dev_finish_destroy+132}
        <ffffffff802e6bc7>{dst_destroy+121}
        <ffffffff802e6cef>{dst_run_gc+212}
        <ffffffff801426a5>{run_timer_softirq+591}
        <ffffffff8013e34c>{__do_softirq+76}
        <ffffffff8013e3d3>{do_softirq+49}
        <ffffffff80113987>{do_IRQ+664}
        <ffffffff80110e8f>{ret_from_intr+0}  <EOI>
        <ffffffff801ec510>{dummy_inode_permission+0}
        <ffffffff801ec510>{dummy_inode_permission+0}
        <ffffffff80187041>{file_move+204}
        <ffffffff801ec510>{dummy_inode_permission+0}
        <ffffffff80184c6f>{dentry_open+179}
        <ffffffff80192b70>{open_exec+140}
        <ffffffff80162cec>{__generic_file_aio_read+390}
        <ffffffff80185c53>{vfs_read+248}
        <ffffffff801bd3ce>{load_elf_binary+784}
        <ffffffff80185b2f>{do_sync_read+173}
        <ffffffff801924b1>{copy_strings+451}
        <ffffffff801945f3>{search_binary_handler+128}
        <ffffffff801948e7>{do_execve+393}
        <ffffffff801108c6>{system_call+126}
        <ffffffff8010ece5>{sys_execve+56}
        <ffffffff80110cea>{stub_execve+106}

Code: 0f 0b 33 a2 35 80 ff ff ff ff 4a 00 31 ff e8 67 bf fe ff 31


--
Martin Wilck                Phone: +49 5251 8 15113
Fujitsu Siemens Computers   Fax:   +49 5251 8 20409
Heinz-Nixdorf-Ring 1        mailto:Martin.Wilck@Fujitsu-Siemens.com
D-33106 Paderborn           http://www.fujitsu-siemens.com/primergy
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