On Sat, Jul 25, 2009 at 19:25, Gregory P. Smith
On Thu, Jul 23, 2009 at 4:28 PM, Thomas Wouters
So attached (and at http://codereview.appspot.com/96125/show ) is a preliminary fix, correcting the problem with os.fork(), os.forkpty() and os.fork1(). This doesn't expose a general API for C code to use, for two reasons: it's not easy, and I need this fix more than I need the API
:-) (I actually need this fix myself for Python 2.4, but it applies fairly cleanly.)
This looks good to me.
Anyone else want to take a look at this before I check it in? I updated the patch (in Rietveld) to contain some documentation about the hazards of mixing fork and threads, which is the best we can do at the moment, at least without seriously overhauling the threading APIs (which, granted, is not that bad an idea, considering the mess they're in.) I've now thoroughly tested the patch, and for most platforms it's strictly better. On AIX it *may* behave differently (possibly 'incorrectly' for specific cases) if something other than os.fork() calls the C fork() and calls PyOS_AfterFork(), since on AIX it used to nuke the thread lock. *I* think the new behaviour (not nuking the lock) is the correct thing to do, but since most places that release the import lock don't bother to check if the lock was even held, the old behaviour may have been succesfully masking the bug on AIX systems. Perhaps for the backport to 2.6 (which I think is in order, and also in accordance with the guidelines) I should leave the AIX workaround in? Anyone think it should not be removed from 3.x/2.7?
Your idea of making this an API called a 'fork lock' or something sounds good (though I think it needs a better name. PyBeforeFork & PyAfterFork?). The subprocess module, for example, disables garbage collection before forking and restores it afterwards to avoid http://bugs.python.org/issue1336. That type of thing could also be done in such a function.
Unfortunately it's rather hard to make those functions work correctly with the current API -- we can't provide functions you can just use as arguments to pthread_atfork because the global interpreter lock is not re-entrant and we have no way of testing whether the current thread holds the GIL. I also get the creepy-crawlies when I look at the various thread_* implementations and see the horribly unsafe things they do (and also, for instance, the PendingCall stuff in ceval.c :S) Unfortunately there's no good way to fix these things without breaking API compatibility, let alone ABI compatibility.
Related to the above subprocess fork + gc bug.. It'd be nice for CPython to have code that does the fork+misc twiddling+exec all in one C call without needing to execute Python code in the child process prior to the exec(). Most of the inner body of subprocess's _execute_child() method could be done that way. (with the obvious exception of the preexec_fn)
To fix the mutex-across-fork problem correctly we should really acquire three locks (the import lock, the GIL and the TLS lock, in that order.)
import lock is re-entrant, and the TLS lock is tightly confined to the thread-local-storage lookup functions, but the GIL is neither re-entrant nor inspectable. That means we can't use pthread_atfork (we can't tell whether we have the GIL already or not, right before the fork), nor can we
convenient API for extension modules to use, really. The best we can do is provide three functions, pthread_atfork-style: a 'prepare' function, an 'after-fork-in-child' function, and an 'after-fork-in-parent' function. The 'prepare' function would start by releasing the GIL, then acquire the import lock, the GIL and the TLS lock in that order. It would also record *somewhere* the thread_ident of the current thread. The 'in-parent' function would simply release the TLS lock and the import lock, and the 'in-child' would release those locks, call the threading._at_fork() function, and fix up the TLS data, using the recorded thread ident to do lookups. The 'in-child' function would replace the current PyOS_AfterFork() function (which blindly reinitializes the GIL and the TLS lock, and calls threading._at_fork().)
Alternatively we could do what we do now, which is to ignore the fact
thread idents may change by fork() and thus that thread-local data may disappear, in which case the 'in-child' function could do a little less work. I'm suitably scared and disgusted of the TLS implementation, the threading implementations we support (including the pthread one) and the way we blindly type-pun a pthread_t to a long, that I'm not sure I want to
The provide a that try
and build something correct and reliable on top of it.
-- Thomas Wouters
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