Comment 39 for bug 1863162

(In reply to xujing from comment #35)
> (In reply to <email address hidden> from comment #31)
> > The master branch has been updated by Szabolcs Nagy <email address hidden>:
> >
> > https://sourceware.org/git/gitweb.cgi?p=glibc.git;
> > h=1387ad6225c2222f027790e3f460e31aa5dd2c54
> >
> > commit 1387ad6225c2222f027790e3f460e31aa5dd2c54
> > Author: Szabolcs Nagy <email address hidden>
> > Date: Wed Dec 30 19:19:37 2020 +0000
> >
> > elf: Fix data races in pthread_create and TLS access [BZ #19329]
> >
> > DTV setup at thread creation (_dl_allocate_tls_init) is changed
> > to take the dlopen lock, GL(dl_load_lock). Avoiding data races
> > here without locks would require design changes: the map that is
> > accessed for static TLS initialization here may be concurrently
> > freed by dlclose. That use after free may be solved by only
> > locking around static TLS setup or by ensuring dlclose does not
> > free modules with static TLS, however currently every link map
> > with TLS has to be accessed at least to see if it needs static
> > TLS. And even if that's solved, still a lot of atomics would be
> > needed to synchronize DTV related globals without a lock. So fix
> > both bug 19329 and bug 27111 with a lock that prevents DTV setup
> > running concurrently with dlopen or dlclose.
> >
> > _dl_update_slotinfo at TLS access still does not use any locks
> > so CONCURRENCY NOTES are added to explain the synchronization.
> > The early exit from the slotinfo walk when max_modid is reached
> > is not strictly necessary, but does not hurt either.
> >
> > An incorrect acquire load was removed from _dl_resize_dtv: it
> > did not synchronize with any release store or fence and
> > synchronization is now handled separately at thread creation
> > and TLS access time.
> >
> > There are still a number of racy read accesses to globals that
> > will be changed to relaxed MO atomics in a followup patch. This
> > should not introduce regressions compared to existing behaviour
> > and avoid cluttering the main part of the fix.
> >
> > Not all TLS access related data races got fixed here: there are
> > additional races at lazy tlsdesc relocations see bug 27137.
> >
> > Reviewed-by: Adhemerval Zanella <email address hidden>
>
> this patch use dl_load_lock in _dl_allocate_tls_init, is there a problem
> when dlopen a dynamic library which will call pthread_create? I think it
> will cause dl_load_lock and dl_load_lock dead lock.

dlopen will hold on dl_load_lock, and pthread_create will call _dl_allocate_tls_init and then will hold on dl_load_lock. Finally, it will cause dead lock.