Date: Mon, 30 Nov 2020 11:44:17 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Alan Somers <asomers@freebsd.org> Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: Why don't lio_listio's signals get delivered? Message-ID: <X8S%2B8WrwZJlJEEhe@kib.kiev.ua> In-Reply-To: <CAOtMX2ixRRPvza4G85qTOjaDAp34L23AO-1UQQ5HYj79bfucPw@mail.gmail.com> References: <CAOtMX2ixRRPvza4G85qTOjaDAp34L23AO-1UQQ5HYj79bfucPw@mail.gmail.com>
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On Sun, Nov 29, 2020 at 05:54:32PM -0700, Alan Somers wrote: > I'm investigating https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=220398 > . What happens is that lio_listio sends a signal to the current process > (synchronously, from within the lio_listio syscall itself, not after some > I/O completes). With dtrace, I've verified that the signal isn't masked, > that it is caught, and that signotify and tdsigwakeup do get called. But > after lio_listio returns, the process never receives the signal. If it > registers a handler, the handler never gets called. If I set the signal to > SIGTERM, the process doesn't get terminated. It's as if the signal is > ignored. Asynchronously generated signals still work, however. > > What's going on? My knowledge of signal-delivery internals is running out, > and I would appreciate some help. > > Steps to reproduce: > $ cd /usr/tests/sys/aio > $ kyua debug lio_test:lio_listio_empty_nowait_signal > > Stack trace at the time the signal looks like it's being delivered: > kernel`tdsendsignal+0x4e1 > kernel`kern_lio_listio+0x493 > kernel`sys_lio_listio+0xc2 > kernel`amd64_syscall+0x12e > kernel`0xffffffff81007e1e > > lio_listio is attempting to send this signal via aio_sendsig, 24 lines from > the bottom of kern_lio_listio. Yes, the signal is queued to the current process, but then, several lines below it, it is dequeued. Look at the block with if (lj->lioj_count == 0) right above the finalization of error code. Problem is that ksi is embedded into aioliojob, and the later needs to be freed there when nitems == 0, because there is no callback. Perhaps, if you need signal to be delivered, we need to allocate ksi just in this case. BTW, I think there is a similar problem with knote, it is not delivered to userspace because knote is signalled but then deleted from kq before syscall returns.
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