puzzled: fork +libthr

[Alexander Kabaev]

On Sun, 17 Apr 2011 18:56:52 +0300
Andriy Gapon <avg at FreeBSD.org> wrote:

> 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.
> 

I would blame it, and expressly at that. -pthread is a shortcut for
{-lpthread -lc} instead of just {-lc} in the place where implied libc
is provided by the compiler driver and has no magic properties you want
it it to have. If chromium build infrastructure circumvents that, it is
only said build infrastructure to blame.

-- 
Alexander Kabaev
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