[RFC PATCH 0/5] module: Remove stop_machine from module unloading

From: Masami Hiramatsu
Date: Mon Aug 25 2014 - 06:55:35 EST


Here is a series of patches which remove stop_machine() from
module unloading.

Currently, each module unloading calls stop_machine()s 2 times.
One is for safely removing module from lists and one is to
check the reference counter. However, both are not necessary
for those purposes (required by current implementation).

First, we can use RCU for the list operation, we just need
a synchronize_rcu right before cleaning up.
Second, the reference counter can be checked atomically by
using atomic_t, instead of per-cpu module_ref. Of course,
for BIG SMP machines, atomic operation is not efficient.
However, they usually don't need to remove most of modules too.

In this series, I just fixed to use RCU for the module(and
bugs) list for the first stop_machine.
And for the second one, I replaced module_ref with atomic_t
and introduced a "module lockup" module load option, which
makes a module un-removable (lock up the module in kernel).
The lockup modules can not be removed except forced, and the
kernel skips module refcounting on those modules. Thus we
can minimize the performance impact on the BIG SMP machines.

BTW, of course this requires to update libkmod to support
new MODULE_INIT_LOCKUP_MODULE flag. I'm not sure where is
good to send the patches I have. Sould I better sending
kmod patches on LKML?

Thank you,


Masami Hiramatsu (5):
module: Wait for RCU synchronizing before releasing a module
module: Unlink module with RCU synchronizing instead of stop_machine
lib/bug: Use RCU list ops for module_bug_list
module: Lock up a module when loading with a LOCLUP flag
module: Remove stop_machine from module unloading

include/linux/module.h | 22 ++----
include/trace/events/module.h | 2 -
include/uapi/linux/module.h | 1
kernel/module.c | 147 +++++++++++++++++------------------------
lib/bug.c | 20 ++++--
5 files changed, 85 insertions(+), 107 deletions(-)


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