[Bug dynamic-link/19329] dl-tls.c assert failure at concurrent pthread_create and dlopen

["nsz at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=19329

--- Comment #40 from Szabolcs Nagy <nsz at gcc dot gnu.org> ---
(In reply to xujing from comment #39)
> I'm sorry, I misled you. I think there is an ABBA deadlock issue in some
> scenarios.
> 
> If I have a c++ dynamic library(named libA.so) that contains a global
> object, the global object will call the post-constructor at initialization
> and hold it's own lock(named A_lock) when dlopen loads libA.so. Assume that
> two threads execute the following process：
>     Thread1:dlopen(libA.so) => hold dl_load_lock => load libA.so => init
> global 
>             object from libA.so => wait for hold A_lock
>     Thread2:my own code hold A_lock => pthread_create =>
> _dl_allocate_tls_init 
>             => wait for hold dl_load_lock
> In this case, an ABBA deadlock occurs. Is this a bug?

yes i think this should work (it is a lock ordering
issue between a user and libc internal lock, which
is only possible if user code is run while a libc
lock is held)

note that if you replace pthread_create with dlopen
that deadlocks too. so it's still bug 15686. but it
may be more common than i expected. i think we need
to look at fixing that bug. (fixing the dynamic tls
race of this bug without locks in pthread_create is
very hard, so i don't think we can revert the quoted
patch)

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