linux-kernel - Re: [PATCH v4 28/39] unwind_user/deferred: Add deferred unwinding interface

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Message-ID: <20250123215806.GC969@noisy.programming.kicks-ass.net>
Date: Thu, 23 Jan 2025 22:58:06 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	linux-kernel@...r.kernel.org, Indu Bhagat <indu.bhagat@...cle.com>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	linux-perf-users@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	linux-toolchains@...r.kernel.org, Jordan Rome <jordalgo@...a.com>,
	Sam James <sam@...too.org>, linux-trace-kernel@...r.kernel.org,
	Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Jens Remus <jremus@...ux.ibm.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Florian Weimer <fweimer@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Weinan Liu <wnliu@...gle.com>
Subject: Re: [PATCH v4 28/39] unwind_user/deferred: Add deferred unwinding
 interface

On Thu, Jan 23, 2025 at 10:30:56AM -0800, Josh Poimboeuf wrote:
> On Thu, Jan 23, 2025 at 09:17:18AM +0100, Peter Zijlstra wrote:
> > On Wed, Jan 22, 2025 at 02:51:27PM -0800, Josh Poimboeuf wrote:
> > > On Wed, Jan 22, 2025 at 03:16:16PM +0100, Peter Zijlstra wrote:
> > > The ctx_ctr is always incremented before calling this, so 0 isn't a
> > > valid cookie.
> > 
> > Right, so that's the problem. You're considering 0 an invalid cookie,
> > but ctx_to_cookie(0, 1<<48) will be a 0 cookie.
> > 
> > That thing *will* wrap.
> 
> Well, yes, after N years of sustained very high syscall activity on CPU
> 0, with stack tracing enabled, in which multiple tracer unwind requests
> happen to occur in the same entry context where ctx_ctr wrapped, one of
> the tracers might get an invalid cookie.
> 
> I can double-increment the counter when it's (1UL << 48) - 1).  Or use
> some other bit for "cookie valid".

Right, steal one bit from counter and make it always 1. 47 bit wrap
around should be fine.