From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx2.suse.de ([195.135.220.15]:50220 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726388AbgKCKkx (ORCPT ); Tue, 3 Nov 2020 05:40:53 -0500 Date: Tue, 3 Nov 2020 11:40:49 +0100 From: Petr Mladek Subject: Re: [PATCH 11/11 v2] ftrace: Add recording of functions that caused recursion Message-ID: <20201103104049.GN20201@alley> References: <20201030213142.096102821@goodmis.org> <20201030214014.801706340@goodmis.org> <20201102164147.GJ20201@alley> <20201102120907.457ad2f7@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201102120907.457ad2f7@gandalf.local.home> List-ID: To: Steven Rostedt Cc: linux-kernel@vger.kernel.org, Masami Hiramatsu , Andrew Morton , Peter Zijlstra , Ingo Molnar , Josh Poimboeuf , Jiri Kosina , Miroslav Benes , Jonathan Corbet , Guo Ren , "James E.J. Bottomley" , Helge Deller , Michael Ellerman , Benjamin Herrenschmidt , Paul Mackerras , Heiko Carstens , Vasily Gorbik , Christian Borntraeger , Thomas Gleixner , Borislav Petkov , x86@kernel.org, "H. Peter Anvin" , Kees Cook , Anton Vorontsov , Colin Cross , Tony Luck , Joe Lawrence , Kamalesh Babulal , Mauro Carvalho Chehab , Sebastian Andrzej Siewior , linux-doc@vger.kernel.org, linux-csky@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, live-patching@vger.kernel.org On Mon 2020-11-02 12:09:07, Steven Rostedt wrote: > On Mon, 2 Nov 2020 17:41:47 +0100 > Petr Mladek wrote: > > > On Fri 2020-10-30 17:31:53, Steven Rostedt wrote: > > > From: "Steven Rostedt (VMware)" > > > > > > This adds CONFIG_FTRACE_RECORD_RECURSION that will record to a file > > > "recursed_functions" all the functions that caused recursion while a > > > callback to the function tracer was running. > > > > > > > > --- /dev/null > > > +++ b/kernel/trace/trace_recursion_record.c > > > + if (index >= CONFIG_FTRACE_RECORD_RECURSION_SIZE) > > > + return; > > > + > > > + for (i = index - 1; i >= 0; i--) { > > > + if (recursed_functions[i].ip == ip) { > > > + cached_function = ip; > > > + return; > > > + } > > > + } > > > + > > > + cached_function = ip; > > > + > > > + /* > > > + * We only want to add a function if it hasn't been added before. > > > + * Add to the current location before incrementing the count. > > > + * If it fails to add, then increment the index (save in i) > > > + * and try again. > > > + */ > > > + old = cmpxchg(&recursed_functions[index].ip, 0, ip); > > > + if (old != 0) { > > > + /* Did something else already added this for us? */ > > > + if (old == ip) > > > + return; > > > + /* Try the next location (use i for the next index) */ > > > + i = index + 1; > > > > What about > > > > index++; > > > > We basically want to run the code again with index + 1 limit. > > But something else could update nr_records, and we want to use that if > nr_records is greater than i. > > Now, we could swap the use case, and have > > int index = 0; > > [..] > i = atomic_read(&nr_records); > if (i > index) > index = i; > > [..] > > index++; > goto again; > > > > > > Maybe, it even does not make sense to check the array again > > and we should just try to store the value into the next slot. > > We do this dance to prevent duplicates. I see. My code was wrong. It reserved slot for the new "ip" by cmpxchg on nr_records. The "ip" was stored later so that any parallel call need not see that it is a dumplicate. Your code reserves the slot by cmpxchg of "ip". Any parallel call would fail to take the slot and see the "ip" in the next iteration. Best Regards, Petr