[Devel] Re: [PATCH RESEND 2/2] Fix some kallsyms_lookup() vs rmmod races

[Paulo Marques]

Andrew Morton wrote:
> On Fri, 16 Mar 2007 17:16:39 +0000 Paulo Marques <pmarques at grupopie.com> wrote:
> 
>> Does freeze_processes() / unfreeze_processes() solve this by only 
>> freezing processes that have voluntarily scheduled (opposed to just 
>> being preempted)?
> 
> It goes much much further than that.  Those processes need to actually
> perform an explicit call to try_to_freeze().

Ok, I've just done a few tests with the attached patch. It basically 
creates a freeze_machine_run function that is equivalent in interface to 
stop_machine_run, but uses freeze_processes / thaw_processes to stop the 
machine.

This is more of a proof of concept than an actual patch. At the very 
least "freeze_machine_run" should be moved to kernel/power/process.c and 
declared at include/linux/freezer.h so that it could be treated as a 
more general purpose function and not something that is module specific.

Anyway, I then tested it by running a modprobe/rmmod loop while running 
a "cat /proc/kallsyms" loop.

On the first run I forgot to remove the mutex_lock(module_mutex) from 
the /proc/kallsyms read path and the freezer was unable to freeze the 
"cat" process that was waiting for the same mutex that the freezer 
process was holding :P

After removing the module_mutex locking from "module_get_kallsym" 
everything was going fine (at least I got no oopses) until I got this:

kernel: Stopping user space processes timed out after 20 seconds (1 
tasks refusing to freeze):
kernel:  kbluetoothd
kernel: Restarting tasks ... <4> Strange, kseriod not stopped
kernel:  Strange, pdflush not stopped
kernel:  Strange, pdflush not stopped
kernel:  Strange, kswapd0 not stopped
kernel:  Strange, cifsoplockd not stopped
kernel:  Strange, cifsdnotifyd not stopped
kernel:  Strange, jfsIO not stopped
kernel:  Strange, jfsCommit not stopped
kernel:  Strange, jfsCommit not stopped
kernel:  Strange, jfsSync not stopped
kernel:  Strange, xfslogd/0 not stopped
kernel:  Strange, xfslogd/1 not stopped
kernel:  Strange, xfsdatad/0 not stopped
kernel:  Strange, xfsdatad/1 not stopped
kernel:  Strange, kjournald not stopped
kernel:  Strange, khubd not stopped
kernel:  Strange, khelper not stopped
kernel:  Strange, kbluetoothd not stopped
kernel: done.

I repeated the test and did a Alt+SysRq+T to try to find out what 
kbluetoothd was doing and got this:

kernel: kbluetoothd   D 79A11860     0 19156      1               19142 
(NOTLB)
kernel: 9a269e4c 00000082 00000001 79a11860 00000000 79a09860 c7018030 
00000003
kernel: 9a269e71 78475100 c7ebe000 c6730e40 00000000 00000001 00000001 
00000001
kernel: 00000000 9a2d7570 79a11860 c7018140 00000000 00001832 42430d03 
000000ab
kernel: Call Trace:
kernel:  [<7845dba3>] wait_for_completion+0x7d/0xb7
kernel:  [<781190ba>] default_wake_function+0x0/0xc
kernel:  [<781190ba>] default_wake_function+0x0/0xc
kernel:  [<7812c759>] call_usermodehelper_keys+0xd1/0xf1
kernel:  [<7812c41e>] request_module+0x96/0xd9
kernel:  [<783e30fe>] sock_alloc_inode+0x20/0x4e
kernel:  [<78172559>] alloc_inode+0x15/0x115
kernel:  [<78172d87>] new_inode+0x24/0x81
kernel:  [<783e4003>] __sock_create+0x111/0x199
kernel:  [<783e40a3>] sock_create+0x18/0x1d
kernel:  [<783e40e1>] sys_socket+0x1c/0x43
kernel:  [<783e51da>] sys_socketcall+0x247/0x24c
kernel:  [<78121b2d>] sys_gettimeofday+0x2c/0x65
kernel:  [<78103f10>] sysenter_past_esp+0x5d/0x81

And this was as far as I got...

This actually seems like a better approach than to hold module_mutex 
everywhere to account for an operation that should be "rare" (module 
loading/unloading). If something like this goes in, there are probably a 
few more places inside module.c where we can drop the locking completely.

However, it still has a few gotchas. Apart from the problem above (which 
may still be me doing something wrong) it makes module loading / 
unloading depend on CONFIG_PM which is somewhat unexpected for the user.

Would it make sense to separate the process freezing / thawing API from 
actual power management and create a new config option (CONFIG_FREEZER?) 
that was automatically selected by the systems that used it (CONFIG_PM, 
CONFIG_MODULES, etc.)? or is that overkill?

-- 
Paulo Marques - www.grupopie.com

"Nostalgia isn't what it used to be."

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