Date: Fri, 13 Sep 2019 16:05:21 +0200 From: Mariusz Zaborski <oshogbo@freebsd.org> To: Ryan Stone <rysto32@gmail.com> Cc: FreeBSD Current <freebsd-current@freebsd.org> Subject: Re: Deadlock involving truss -f, pdfork() and wait4() Message-ID: <CAGOYWV-v1-F3dWQovdj5TZMZeBFifDm020AXnPYdZSHPKLG4SA@mail.gmail.com> In-Reply-To: <CAFMmRNwRWZsQhMSY7MwS2kU-qNJ0%2BPk676mi59_Y3QsbcEmPVQ@mail.gmail.com> References: <CAFMmRNwRWZsQhMSY7MwS2kU-qNJ0%2BPk676mi59_Y3QsbcEmPVQ@mail.gmail.com>
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Hello Ryan, Can you verify is this patch fix your issue: https://reviews.freebsd.org/D20362 Thanks, Mariusz On Thu, 12 Sep 2019 at 21:37, Ryan Stone <rysto32@gmail.com> wrote: > > I've hit an issue with a simple use of pdfork(). I have a process > that calls pdfork() and the parent immediately does a wait4() on the > child pid. This works fine under normal conditions, but if the parent > is run under truss -f, the three processes deadlock. If I switch out > pdfork() for fork(), the deadlock does not occur. > > This C file demonstrates the issue: > > https://people.freebsd.org/~rstone/pdfork.c > > If I run "truss -f ./pdfork", which uses fork(), it completes within a > second. If I run "truss -f ./pdfork -p", which uses pdfork(), the > processes deadlock. If I run "./pdfork -p" without truss, it > completes normally. > > procstat reports the following kernel stacks: > > 27572 102043 truss - mi_switch+0xe2 > sleepq_catch_signals+0x425 sleepq_wait_sig+0xf _sleep+0x1bf > kern_wait6+0x695 sys_wait6+0x9f amd64_syscall+0x36e > fast_syscall_common+0x101 > 27573 102469 pdfork - mi_switch+0xe2 > sleepq_catch_signals+0x425 sleepq_wait_sig+0xf _sleep+0x1bf > kern_wait6+0x695 sys_wait4+0x78 amd64_syscall+0x36e > fast_syscall_common+0x101 > 27574 102053 pdfork - mi_switch+0xe2 > thread_suspend_switch+0xd4 ptracestop+0x13b fork_return+0x14e > fork_exit+0x83 fork_trampoline+0xe > > As near as I can tell, truss is blocked waiting for ptrace events, the > parent process is blocked in wait4, and the child process is perhaps > waiting for its parent to exit the kernel so it can send the ptrace > event? > > I really don't see anything obvious in the pdfork() code path that > would cause this to happen when fork() doesn't have the problem. It > may be that pdfork() just changes the timing enough to expose a latent > bug. > > I'm seeing this on a recentish current (r351363). > _______________________________________________ > freebsd-current@freebsd.org mailing list > https://lists.freebsd.org/mailman/listinfo/freebsd-current > To unsubscribe, send any mail to "freebsd-current-unsubscribe@freebsd.org"
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