Is try_module_get buggy?

Is try_module_get buggy?

[rmingming]

Hi,
 I have a problem about the try_module_get function, I don't know if someone
removed the module just AFTER line 372, then what happens? Because in this
situation, the variable module will be incorrect, and module_is_live
function will lead to unpredicatable behaviour.

368 static inline int try_module_get(struct module *module)
369 {
370         int ret = 1;
371
372         if (module) {
373                 unsigned int cpu = get_cpu();
374                 if (likely(module_is_live(module)))
375                         local_inc(&module->ref[cpu].count);
376                 else
377                         ret = 0;
378                 put_cpu();
379         }
380         return ret;
381 }

Best wishes to all,
rmingming

Re: Is try_module_get buggy?

[Rusty Russell]

On Saturday 12 January 2008 15:35:27 rmingming wrote:
> Hi,
>  I have a problem about the try_module_get function, I don't know if
> someone removed the module just AFTER line 372, then what happens? Because
> in this situation, the variable module will be incorrect, and
> module_is_live function will lead to unpredicatable behaviour.
>
> 368 static inline int try_module_get(struct module *module)
> 369 {
> 370         int ret = 1;
> 371
> 372         if (module) {
> 373                 unsigned int cpu = get_cpu();
> 374                 if (likely(module_is_live(module)))
> 375                         local_inc(&module->ref[cpu].count);
> 376                 else
> 377                         ret = 0;
> 378                 put_cpu();
> 379         }
> 380         return ret;
> 381 }

Hi rminming,

try_module_get is designed to ensure that you don't call a function inside a 
module without a reference.  Like any reference function, it cannot handle 
the case where the argument is invalid (or invalidated partway through the 
call).

In this case, the module pointer is usually inside a registered structure.  
The pointer will be valid until the structure is unregistered, which the 
calling code presumably prevents while it's doing a lookup.

Hope that clarifies,
Rusty.

Re: Is try_module_get buggy?

[rmingming]

> Hi rminming,
>
> try_module_get is designed to ensure that you don't call a function inside a
> module without a reference.  Like any reference function, it cannot handle
> the case where the argument is invalid (or invalidated partway through the
> call).
>
> In this case, the module pointer is usually inside a registered structure.
> The pointer will be valid until the structure is unregistered, which the
> calling code presumably prevents while it's doing a lookup.
>
> Hope that clarifies,
> Rusty.
>

Hi Rusty,

Thank you so much!
I think you mean that this situation can really happen, because this
is a reference function, but it is so seldom, and the common result of
this situation is a kernel oops, it's not very important. Am I right?

Sorry about later reply, thanks again.
Best wishes,

rmingming