linux-kernel - Re: [PATCH v2 00/11] unwind, perf: sframe user space unwinding, deferred perf callchains

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Message-ID: <20241002223125.2d610510@gandalf.local.home>
Date: Wed, 2 Oct 2024 22:31:25 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, x86@...nel.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>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH v2 00/11] unwind, perf: sframe user space unwinding,
 deferred perf callchains

On Mon, 16 Sep 2024 17:39:53 +0200
Josh Poimboeuf <jpoimboe@...nel.org> wrote:

> Completely untested patch here that I hacked up today:

I can tell it wasn't tested ;-)

> 
>  git://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git sframe-2.1
> 
> To avoid races, unwind_user_deferred() can't be called from NMI.  The
> tracers will need to trigger irq_work to call it.

diff --git a/kernel/unwind/user.c b/kernel/unwind/user.c
index 7edb0833fe46..c56cf5d564df 100644
--- a/kernel/unwind/user.c
+++ b/kernel/unwind/user.c
@@ -177,7 +181,8 @@ int unwind_user_deferred(struct unwind_callback *callback, u64 *ctx_cookie)
 
 		cookie = __this_cpu_read(ctx_ctr);
 		cookie &= ((1UL << 48) - 1);
-		cookie |= ((cpu << 48) + 1);
+		cookie |= cpu << 48;
+		cookie++;
 		__this_cpu_write(ctx_ctr, cookie);
 
 		current->unwind_ctx_cookie = cookie;

As the cookie never got incremented.

That was just one issue. Things are still not working but I'll debug the
rest later.

-- Steve