Re: [PATCH] nptl: Fix deadlock on atfork handler which calls dlclose (BZ#24595)

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

On 23/05/2019 14:31, Florian Weimer wrote:
> * Adhemerval Zanella:
> 
>> We can go back to old behaviour of using a lock-free registration on the
>> atfork lists, make a lock-free copy on fork, and handle the synchronization
>> with __unregister_atfork using locks plus futexes.  It would require all the
>> old complexity of mixing lock-free algorithms with locks accesses plus using 
>> an unbounded alloca on fork (maybe we could just use malloc to allocate
>> the backup list, my understanding it is avoid to to add another issue to
>> make fork async-signal-safe).
> 
> I think we can avoid the async-signal-safety issue (for now) by not
> using locks in single-threaded mode.  pthread_atfork is not required to
> be async-signal-safe, so we need not worry about fork/pthread_at_fork
> interactions in single-threaded processes, beyond the modification of
> the handler list from the handler itself.
> 
> Maybe we can avoid making the copy for every fork call, using some sort
> of copy-on-write list?  On the reader side, the protocol would look like
> this:
> 
>   lock the fork handler mutex
>     get pointer to the dynlist head from a global variable
>     increment the reference counter next to the dynlist head
>   unlock the fork handler mutex
> 
>   perform all the fork work, traversing the list as needed
>   (without any locking)
> 
>   lock the fork handler mutex
>     decrement the reference counter
>     if the counter is zero, delocate the entire data structure
>       (list and struct containing list head plus reference counter)
>   unlock the fork handler mutex
> 
> On the registration/unregistration side (pthread_atfork,
> __unregister_atfork), we would do this:
> 
>   lock the fork handler mutex
>     get pointer to the dynlist head from the global variable
>     if the reference counter is 1:
>       modify the list in place
>     else:
>       make a modified copy of the list
>       update the global variable to point to it
>       decrement the reference counter in the original list
>   unlock the fork handler mutex
> 
> (The reference counter in the quiet state would have to be 1, because
> the list is referenced from the global variable.)
> 
> I think this is still simpler than the original scheme.  It also ensures
> that we are not modifying the list during the traversal in fork.
> Instead, we update a list that will be used for future forks.
> 
> What do you think?

The solution sounds correct, but I don't have a strong opinion if this
is really an improvement over a recursive lock plus a linked list.  It 
potentially adds 'free' calls in fork for multithread mode if list needs 
to be deallocated. Also, since the locks is internal to register-atfork.c
we might have a better control to make the exported interfaces not 
deadlock.