Date: Tue, 11 Jan 2022 16:38:40 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Mark Johnston <markj@freebsd.org> Cc: Andriy Gapon <avg@freebsd.org>, Mateusz Guzik <mjguzik@gmail.com>, freebsd-hackers@freebsd.org Subject: Re: Debugging a (potentially?) ZFS-related panic, and discussion about large patchsets Message-ID: <Yd2WcDMxiintBwMh@kib.kiev.ua> In-Reply-To: <Yd2RTqQxO4JqJ5gr@nuc> References: <20220110221116.gustgfgfge6pb5fe@mutt-hbsd> <YdzCatNYBVDFi9So@nuc> <CAGudoHHuNbYNFsc3vqihFkQ0xA9Hojp21-QuHczVXvMVSHfQ5w@mail.gmail.com> <1bf39343-c9b2-353c-63e7-8604adc9d391@FreeBSD.org> <Yd2RTqQxO4JqJ5gr@nuc>
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On Tue, Jan 11, 2022 at 09:16:46AM -0500, Mark Johnston wrote: > On Tue, Jan 11, 2022 at 09:28:27AM +0200, Andriy Gapon wrote: > > On 11/01/2022 01:43, Mateusz Guzik wrote: > > > imo the kernel should be patched to obtain the trace on its own. As > > > the target has interrupts disabled it will have to do it with NMI, but > > > support for that got scrapped in > > > > > > commit 1c29da02798d968eb874b86221333a56393a94c3 > > > Author: Mark Johnston<markj@FreeBSD.org> > > > Date: Fri Jan 31 15:43:33 2020 +0000 > > > > > > Reimplement stack capture of running threads on i386 and amd64. > > > > This is an off-topic for the thread, but as far as I recall, even when the stack > > capture (e.g., for procstat -k) was implemented using NMI there was a piece of > > code in the corresponding NMI handler that skipped the stack tracing if > > interrupts were disabled. I don't recall / know why. > > You can see that in the removed stack_nmi_handler() that used to be in > > sys/x86/x86/stack_machdep.c. > > I think it may have been to avoid tracing threads in the middle of a > context switch, but I can't remember exactly which inconsistencies were > problematic. Thread stack can become unmapped any moment it went off cpu. You do not know which place in the context switch code was interrupted.
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