Date: Tue, 12 Jan 2021 05:12:52 +0000 From: bugzilla-noreply@freebsd.org To: threads@FreeBSD.org Subject: [Bug 252579] fork() causes process to hang in rare circumstances. Message-ID: <bug-252579-13406-T7y83D9ziW@https.bugs.freebsd.org/bugzilla/> In-Reply-To: <bug-252579-13406@https.bugs.freebsd.org/bugzilla/> References: <bug-252579-13406@https.bugs.freebsd.org/bugzilla/>
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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=252579 Jason A. Harmening <jah@FreeBSD.org> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |jah@FreeBSD.org --- Comment #1 from Jason A. Harmening <jah@FreeBSD.org> --- I can reproduce this 100% of the time on a -current VM using the supplied test code. I noticed a few things: --for me, the parent process seems to be hanging during fork(); I see no evidence the child process is ever spawned. --wmesg for the process is 'umtxn', and ddb shows what looks like the main thread attempting to take a userspace lock, going through umtxq_lock(), and sleeping in sleepq_wait_sig() --I tried to write a smaller test program to reproduce the failure by simulating the locking done by the NS dispatcher and the pthread_create() issued by the stub, but this did not reproduce the hang. --However, if I just link the original test program against libpthread ('cc -o bug -pthread bug.c), then I can no longer reproduce the hang. This tells me the problem might have something to do with some bit of static umtx initialization that happens when linking against libpthread/libthr. If this initialization hasn't happened by the time the NS dispatcher (which loads the stub through dlopen()) is invoked, then fork() ends up stuck in a umtx wait that never gets signaled. It might also be related to the __isthreaded checks made by lib/libc/net/nsdispatch.c, which smell fishy to me. At the very least, it might be possible to make a smaller repro case by writing a test program (that does not link libpthread) which dlopen()s a simple library (which does link libpthread) and calls an entry point that spawns a thread. -- You are receiving this mail because: You are the assignee for the bug.help
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