puzzled: fork +libthr

Andriy Gapon avg at FreeBSD.org
Sun Apr 17 15:56:58 UTC 2011

on 17/04/2011 18:21 Daniel Eischen said the following:
> On Sun, 17 Apr 2011, Andriy Gapon wrote:
>> on 16/04/2011 14:46 Andriy Gapon said the following:
>>> The second puzzle is the EPERM return value itself, on stable/8.
>>> From what I seem chromium does a bunch of forks before it gets to the place of
>>> interest.  My debugging shows that those forks are "single-threaded" (i.e. code
>>> in thr_fork.c is not called).  And then in a process/thread that makes that
>>> pthread_cond_wait call I see that libthr and kernel have different opinions
>>> about what current TID is.  Userland part uses what is actually a kernel TID of
>>> its parent thread (the one that called fork).  And given how the work is divided
>>> between userland and kernel in libthr, that mismatch leads to serious
>>> consequences.
>>> So my question is why libthr doesn't see its actual TID.  Maybe some
>>> initialization code is not invoked.  BTW, chromium is linked to both libc and
>>> libthr (per ldd).  But it seems that there are no pthread calls up the fork
>>> chain until that pthread_cond_wait call.
>> The second problem seems to be caused by chrome binary being linked to libc and
>> libthr in "incorrect order", libc comes before libthr in ldd output.  My
>> debugging shows that fork is resolved from libc, not from libthr.
>> Not sure what to blame here:
>> - build toolchain for putting libc before libthr
>> - rtld for not preferring libthr over libc
>> - libc/libthr for being split into two pieces in the current way
> - The build procedure for chromium.
> libc/[libc_r, libpthread, libthr] have always behaved that
> way since the libc/libc_r split.

Well, I wouldn't blame it so expressly: -pthread is the first option on the
linkage command line, there is -lc there also.  I would expect that that would
do the right thing, but it doesn't.  And that's a PITA for porting.

Andriy Gapon

More information about the freebsd-threads mailing list