Here's the FAQ again. Note that the init stuff is not currently true
(you can enter a module doing init, with possibly bad results),
because Linus patched my code. It's on my TODO list to revisit this
> > > (So you can't unload agpgart whilst you've a 3d using app (like X)
> > > open). This seems quite sensible, but these days you can't unload
> > > agpgart.ko anyway because the chipset module (via-agp.ko in your
> > > case) already has it 'in use', so I'm tempted to drop those bits.
> > If this is true (it usually is), you can simply drop them.
> I'll need 'something' in the chipset drivers. The first thing that
> jumps to mind is to give the chipset drivers an 'acquire' op
> which does the locking much like the old agp_backend_acquire() does.
The problem is, a module basically can't lock itself. Hence you
should expose an "owner" field when you register an interface, and
have the caller do the locking. There are other possibly solutions,
but none of them were as simple as "lock down before you call in".
> > There are other cases where the caller is not grabbing references, so
> > MOD_INC_USE_COUNT is better than nothing (should the warning stay for
> > 2.6? Good question).
> Why exactly isn't it safe any more? If there's documentation on this,
> I'd love to read it. If there isn't, there really should be.
Consider the fictitious my_module.c:
static void my_function_called_through_ptr(void)
If you get preempted before the MOD_INC_USE_COUNT, an unload could
occur, boom. If you get preempted after the MOD_DEC_USE_COUNT, but
before the return, and an unload occurs, boom.
If it were not called through a pointer, (ie. called by name) then it
means the module calling it has a reference (this is done
automatically in symbol resolution, or symbol_get()), so no problem.
[ As an implementation detail, this is *not* a problem on SMP without
preemption. But that's an implementation detail, and it wasn't true
with the previous module implementation. ]
Hope this helps,
-- Anyone who quotes me in their sig is an idiot. -- Rusty Russell.
Golden Rule: If you are calling though a function pointer into a (different) module, you must hold a reference to that module. Otherwise you risk sleeping in the module while it is unloaded.
Q: How do I get a reference to a module? A: Usually, a successful call to try_module_get(owner). You don't need to check for owner != NULL, BTW.
Q: When does try_module_get(owner) fail? A: When the module is not ready to be entered (ie. still in init_module) or it is being removed. This prevents you entering the module as it is being discarded (init might fail, or it's being removed).
Q: But the modules' init routine calls my register() routine which wants to call back into one of the function pointers immediately, and so try_module_get() fails! (because the module is not finished initializing yet) A: You're being called from the module, so someone already has a reference (unless there's a bug), so you don't need a try_module_get().
This does mean that if you were to register a structure for *another* module (does anyone do this?) you'd need to have a reference to it.
Q: How do I put the reference back? A: Using module_put(owner) (owner == NULL is OK).
Q: Do I really need to put try_module_get() before every function ptr call? A: If the function does not sleep (any cannot be preempted) ie. is called in softirq or hardirq context, you can omit this step, since you obviously won't sleep inside the module.
Also, most structs have clear "start" and "stop" functions (eg. mount/umount), so you only need one try_module_get() on start, and module_put() on stop.
Q: Is it safe to call try_module_get() and module_put() from an interrtupt / softirq? A: Yes.
Q: My code use "MOD_INC_USE_COUNT". Do I still need to adjust my module count when someone calls one of my functions? A: No, you never need to adjust your own module count. There are five ways a function in your module can get called: firstly, it could be your module_init() function, in which case the module code holds a reference. It could be another module using one of your EXPORT_SYMBOL'ed functions, in which case you cannot be removed since they would have to be removed first. It could be a module which found an EXPORT_SYMBOL'ed function using symbol_get(), in which case they hold a reference count. It could be through a function pointer which your module gave out previously, which is discussed above. Finally, it could be from within your own module, in which case someone must already hold a reference.
Q: My code uses "__MOD_INC_USE_COUNT(reg->owner)", but now I get a warning at runtime that it is unsafe. What do I need to do? A: You need to use try_module_get(), and not call into the module if it fails (act as if it hasn't registered yet). Note that you no longer need to check for NULL yourself, try_module_get() does that.
Q: My code used "GET_USE_COUNT(module)" to get the reference count. A: Don't do that. If module unloading is disabled, there is no reference count, and there is never a single value you can assign to.
Q: My code used "try_inc_mod_count(module)" to get the reference count. Should I change it? A: No hurry. try_module_get() is exactly the same: the new name reflects that this is now the only way to get a reference.
Q: How does the code in try_module_get() work? A: It disables preemption for a moment, checks the live flag, and then increments a per-cpu counter if the module is live. This is even lighter-weight (in icache and cycles) than using a brlock, but has the same effect. If CONFIG_MODULE_UNLOAD=n, it just becomes a check that the module is live.
Q: How does the module remove code work? A: It stops the machine by scheduling threads for every other CPU, then they all disable interrupts. At this stage we know that noone is in try_module_get(), so we can reliably read the counter. If zero, or the rmmod user specified --wait, we set the live flag to false. After this, the reference count should not increase, and each module_put() will wake us up, so we can check the counter again.
Q: Are these changes all so you could implement an in-kernel module linker? A: No, they were to prevent load and unload races without altering every module, nor introducing drastic new requirements.
Q: Doesn't putting linking code into the kernel just add bloat? A: The total linking code is about 200 generic lines, 100 x86-specific lines. The ia64 linking code is about 500 lines (it's the most complex). Richard Henderson has a great suggestion for simplifying it furthur, which I'm implementing now. "insmod" is now a single portable system call, meaning insmod can be written in about 20 lines of code.
The previous code had to implement the two module loading system calls, the module querying system call, and the /proc/ksyms output, required a little more code than the current x86 linker.
Q: Why not just fix the old code? A: Because having something so intimate with the kernel in userspace greatly restricts what changes the kernel can make. Moving into the kernel means I have implemented modversions, typesafe extensible module parameters and kallsyms without altering userspace in any way. Future extensions won't have to worry about the version of modutils problem.
Q: Why not implement two-stage insert / two-stage delete? A: Because I implemented it first and it sucked. And because this *is* two-stage insert and two-stage delete, without exposing it to the modules using it, with the added advantage that the second stage is atomic (activation/deactivation is simply changing mod->live, modulo locking implementation magic detailed above). This prevents the race between deactivating the module and finding that someone has starting using it as you are deactivating it. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to firstname.lastname@example.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/