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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Dmitry Dolgov <9erthalion6@gmail.com>
Cc: bpf@vger.kernel.org, linux-rt-users@vger.kernel.org, ast@kernel.org,
	daniel@iogearbox.net, andrii@kernel.org
Subject: Re: bpf_ringbuf_reserve deadlock on rt kernels
Message-ID: <20240613104058.bwDKdWSV@linutronix.de>
References: <jxkyec5jd54r3cmel4e3pep4ebo3pd4xgedwtb7gj65fntf4s7@om5r3mowjknb>
 <20240612143223.BO0LMFEZ@linutronix.de>
 <mt2yblzo4ezlx4vjfmw3pul3cqgd27oddbaq2coqjkcp342cni@ob4gkaxfz7mg>
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On 2024-06-13 12:23:46 [+0200], Dmitry Dolgov wrote:
> > On Wed, Jun 12, 2024 at 04:32:23PM GMT, Sebastian Andrzej Siewior wrote:
> >
> > > The BPF program in question is attached to sched_switch. The issue se=
ems
> > > to be similar to a couple of syzkaller reports [1], [2], although the
> > > latter one is about nested progs, which seems to be not the case here.
> > > Talking about nested progs, applying a similar approach as in [3]
> > > reworked for bpf_ringbuf, elliminates the issue.
> > >
> > > Do I miss anything, is it a known issue? Any ideas how to address tha=
t?
> >
> > I haven't attached bpf program to trace-events so this new to me. But if
> > you BPF attach programs to trace-events then there might be more things
> > that can go wrong=E2=80=A6
>=20
> Things related to RT kernels, or something else?

Related to RT kernel. The trace-event is invoked with disabled
preemption. This means locking is limit to raw_spinlock_t and no memory
allocation are allowed. Otherwise the splat below will appear.

Sebastian