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

["xujing99 at huawei dot com" <sourceware-bugzilla@sourceware.org>]

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

--- Comment #36 from xujing <xujing99 at huawei dot com> ---
(In reply to xujing from comment #35)
> (In reply to cvs-commit@gcc.gnu.org from comment #31)
> > The master branch has been updated by Szabolcs Nagy <nsz@sourceware.org>:
> > 
> > https://sourceware.org/git/gitweb.cgi?p=glibc.git;
> > h=1387ad6225c2222f027790e3f460e31aa5dd2c54
> > 
> > commit 1387ad6225c2222f027790e3f460e31aa5dd2c54
> > Author: Szabolcs Nagy <szabolcs.nagy@arm.com>
> > 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  <adhemerval.zanella@linaro.org>
> 
> 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.

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