#1391: forkProcess() in Schedule.c with -threaded should initialize mutexes in
child process (POSIX)
----------------------------------+-----------------------------------------
Reporter: thorkilnaur | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Runtime System | Version: 6.7
Severity: normal | Keywords:
Difficulty: Unknown | Os: MacOS X
Testcase: forkprocess01(ghci) | Architecture: powerpc
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{{{forkProcess()}}} in {{{Schedule.c}}} implements
{{{System.Posix.Process.forkProcess}}} essentially by {{{fork()}}}'ing. In
http://www.gnu.org/software/libc/manual/html_node/Threads-and-Fork.html we
read that
It's not intuitively obvious what should happen when a multi-threaded
POSIX process calls fork. ...
fork duplicates the whole memory space, including mutexes in their
current locking state, but
only the calling thread: other threads are not running in the child
process. The mutexes are
not usable after the fork and must be initialized with
pthread_mutex_init in the child process.
Although a lot of things happen in {{{forkProcess()}}} in {{{Schedule.c}}}
in the child process after {{{fork()}}} returns, this initialization of
mutexes and related delicate matters are not done.
On my PPC Mac OS X 10.4.9, I have observed this to eventually result in
the child process getting a {{{SIGSEGV}}} (signal 11; segmentation fault)
when run via {{{ghc --interactive}}}. The failing test case
{{{forkprocess01(ghci)}}} for PPC Mac OS X is an example of this.
The case of {{{forkprocess01}}} failing when run via {{{ghci
--interactive}}} comes about because ghc itself is linked with
{{{-threaded}}}. With a ghc linked without {{{-threaded}}}, the
segmentation fault does not happen.
When executing {{{forkprocess01}}} compiled with {{{--make}}}
{{{-threaded}}}, the error again cannot be reproduced. But this seems to
be because {{{forkprocess01}}} itself does not use multiple threads. With
a slightly extended version of {{{forkprocess01}}} called
{{{forkprocess03}}}:
{{{
-- forkprocess03.hs:
-- Test that we can call exitFailure in a forked process, and have it
-- communicated properly to the parent.
-- Do this (forkprocess01) within a forkIO'ed child process.
import System.Exit
import System.Posix.Process
import Control.Concurrent
main0 = do
p <- forkProcess $ exitWith (ExitFailure 72)
r <- getProcessStatus True False p
print r
main = do
p <- forkIO $ main0
threadDelay 10000000
print p
}}}
the erroneous reaction can be observed when compiled with {{{-threaded}}}
using a fairly recent ghc HEAD:
{{{
$ /Users/thorkilnaur/tn/GHCDarcsRepository/ghc-HEAD-for-HughesPJ-wrong-
fill-indent-20070506_1304/ghc/compiler/stage2/ghc-inplace --version
The Glorious Glasgow Haskell Compilation System, version 6.7.20070513
$ touch forkprocess03.hs
$ /Users/thorkilnaur/tn/GHCDarcsRepository/ghc-HEAD-for-HughesPJ-wrong-
fill-indent-20070506_1304/ghc/compiler/stage2/ghc-inplace --make
forkprocess03 -threaded
[1 of 1] Compiling Main ( forkprocess03.hs, forkprocess03.o )
Linking forkprocess03 ...
$ ./forkprocess03
Just (Terminated 11)
ThreadId 4
$
}}}
Whereas without {{{-threaded}}}, the program seems to run fine:
{{{
$ touch forkprocess03.hs
$ /Users/thorkilnaur/tn/GHCDarcsRepository/ghc-HEAD-for-HughesPJ-wrong-
fill-indent-20070506_1304/ghc/compiler/stage2/ghc-inplace --make
forkprocess03
[1 of 1] Compiling Main ( forkprocess03.hs, forkprocess03.o )
Linking forkprocess03 ...
$ ./forkprocess03
Just (Exited (ExitFailure 72))
ThreadId 2
$
}}}
The repair, however, is not particularly obvious. The above reference
suggests using {{{pthread_atfork}}} to set up handlers to lock all mutexes
before {{{fork()}}}'ing and subsequently unlock them in the parent and
initializing them in the child. But even if this is chosen as the way
forward, additional matters need to be clarified, to ensure that such
handling plays well with the rest of the threaded runtime system and also
retains the Windows variant of things. Also, if anything is done about
this, #1185 should probably considered as well.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1391>
GHC <http://www.haskell.org/ghc/>
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