li.kr...@googlemail.com, vgu...@kernel.org, linux-...@vger.kernel.org, j...@joshtriplett.org, rost...@goodmis.org, r...@vger.kernel.org, b...@alien8.de, bc...@quicinc.com, tsbog...@alpha.franken.de, linux-par...@vger.kernel.org, sudeep.ho...@arm.com, shawn...@kernel.org, da...@davemloft.net, dal...@libc.org, t...@atomide.com, amakha...@vmware.com, bjorn.anders...@linaro.org, h...@zytor.com, sparcli...@vger.kernel.org, linux-hexa...@vger.kernel.org, linux-ri...@lists.infradead.org, anton.iva...@cambridgegreys.com, jo...@southpole.se, yury.no...@gmail.com, rich...@nod.at, x...@kernel.org, li...@armlinux.org.uk, mi...@redhat.com, a...@eecs.berkeley.edu, paul...@kernel.org, h...@linux.ibm.com, stefan.kristians...@saunalahti.fi, openr...@lists.librecores.org, paul.walms...@sifive.com, linux-te...@vger.kernel.org, namhy...@kernel.org, andriy.shevche...@linux.intel.com, jpoim...@kernel.org, jgr...@suse.com, mon...@monstr.eu, linux-m...@vger.kernel.org, pal...@dabbelt.com, a...@brainfault.org, ink@ jurassic.park.msu.ru, johan...@sipsolutions.net, linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+archive=mail-archive....@lists.ozlabs.org Sender: "Linuxppc-dev" <linuxppc-dev-bounces+archive=mail-archive....@lists.ozlabs.org>
On Thu, Jun 09, 2022 at 11:16:46AM +0200, Petr Mladek wrote: > On Wed 2022-06-08 16:27:47, Peter Zijlstra wrote: > > The problem, per commit fc98c3c8c9dc ("printk: use rcuidle console > > tracepoint"), was printk usage from the cpuidle path where RCU was > > already disabled. > > > > Per the patches earlier in this series, this is no longer the case. > > My understanding is that this series reduces a lot the amount > of code called with RCU disabled. As a result the particular printk() > call mentioned by commit fc98c3c8c9dc ("printk: use rcuidle console > tracepoint") is called with RCU enabled now. Hence this particular > problem is fixed better way now. > > But is this true in general? > Does this "prevent" calling printk() a safe way in code with > RCU disabled? On x86_64, yes. Other architectures, less so. Specifically, the objtool noinstr validation pass will warn at build time (DEBUG_ENTRY=y) if any noinstr/cpuidle code does a call to non-vetted code like printk(). At the same time; there's a few hacks that allow WARN to work, but mostly if you hit WARN in entry/noinstr you get to keep the pieces in any case. On other architecture we'll need to rely on runtime coverage with PROVE_RCU. That is, if a splat like in the above mentioned commit happens again, we'll need to fix it by adjusting the callchain, not by mucking about with RCU state. > I am not sure if anyone cares. printk() is the best effort > functionality because of the consoles code anyway. Also I wonder > if anyone uses this trace_console(). This is the tracepoint used to spool all of printk into ftrace, I suspect there's users, but I haven't used it myself. > Therefore if this patch allows to remove some tricky tracing > code then it might be worth it. But if trace_console_rcuidle() > variant is still going to be available then I would keep using it. My ultimate goal is to delete trace_.*_rcuidle() and RCU_NONIDLE() entirely. We're close, but not quite there yet.