Re: inode-based dlopen caching

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 08/06/2021 13:26, Soni L. wrote:
> 
> 
> On 2021-06-08 10:14 a.m., Adhemerval Zanella wrote:
>>
>>
>> On 07/06/2021 19:50, Soni L. wrote:
>>>
>>>
>>> On 2021-06-07 6:53 p.m., Adhemerval Zanella wrote:
>>>>
>>>>
>>>> On 05/06/2021 10:59, Soni L. via Libc-help wrote:
>>>>> Currently dlopen caching is based on filenames, it'd be nice if it was
>>>>> based on inodes to support better "re"loading (aka loading a new module
>>>>> with the same name because unloading modules with threads is never a
>>>>> good idea). This is good for stuff that deals with plugins.
>>>>>
>>>>
>>>> What do you mean by 'caching' in this scenario? glibc does not maintain
>>>> a cache of loaded libraries, different than other implementation it
>>>> does try to unload the library on dlclose (there are cases where it
>>>> is not readily possible due dependency chains). 
>>>>
>>>> And I am not seeing how inote-bases dlopen really helps here, if inode 
>>>> changes means that file was potentially changed (so you will need to 
>>>> proper dclose it).  I think using filenames is in fact the proper way
>>>> here, since Linux does the hard lifting of the inode cache and provide
>>>> fast file access and mmap support for shared libraries.
>>>>
>>> You can't unload a dlopen that uses threads, at least not safely. So for
>>> all intents and purposes you can't unload it. Instead you need to tell
>>> it you're done with it, but not unload it, and load the new one. But
>>> that's the problem - the filename-based stuff means you can't load the
>>> new one.
>>>
>>
>> Sure you can unload a dlopen library, the API makes the program responsible
>> to synchronize the access (since dlsym/dlvsym returns an function pointer).
>> [snip]
> If the shared library creates its own threads, those won't be killed
> when the shared library is closed. If the shared library is then
> unloaded, those threads will be running code from the void. That's a
> problem.
> 
> Being able to update the library, without getting rid of the original
> (i.e. by unlinking the original file first), allows the program to
> gracefully update loaded plugins without a full restart.
> 

Again, this is a library issue that should be dealt by the provided
API by the library (such as providing a cleanup handler to synchronize
or cancel the threads execution).

In this scenario you are describing you will end up with the library
loaded in two different mapping with potentially two different code
(since you are updating the library and dlclose might be a noop).
I really don't see this as an improvement, it is rather a potential
trigger to subtle issues, specially if the threads trying to sync
with process shared resources.