From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 77D61EC9 for ; Thu, 6 Jul 2023 05:10:16 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9F034C433C7; Thu, 6 Jul 2023 05:10:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1688620215; bh=UmRuzOJt+ryrnFuwX+ytQQxkA5mGM4cYrZAvC81+VW0=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=nOiFQFT1HxSRxmkGVv0BKxN1bSNrGkM9k0rrQi/q8xjFFDS+FbE4O7QswwAJPr33l FONsFjjCfhtmpZYa+ePf2RdY8xMi6aQLoNroCJYDph2EOgj4mQsDEsqFsPZUVKlTOU Z1CylwK/DH6fzlHN01+ycm9TbCXXv3diLuSAot3AHa4zbdRaXjB5ZmFHMSHMaOJTn3 +hUPxvdYxyOZ01DMH2QkNms8f70Gt+ZlzhYN9vuT+JG70RumgxZoapzWR9ZvqB4Ycp VOoo8aHVc8sNWtYVUpNYtrecP94vjk8iKcJQX/VjqlbQ3o7tv+HBKzltzGA7aSroqr HdtdTpL6AKBOw== Date: Thu, 6 Jul 2023 14:10:12 +0900 From: Masami Hiramatsu (Google) To: Steven Rostedt Cc: Jiri Olsa , Mark Rutland , lkml , linux-trace-kernel@vger.kernel.org, bpf@vger.kernel.org Subject: Re: [PATCH] fprobe: Ensure running fprobe_exit_handler() finished before calling rethook_free() Message-Id: <20230706141012.c1a0ae0901e0fdec7b3078c7@kernel.org> In-Reply-To: <20230705212657.5968daf7@gandalf.local.home> References: <20230628012305.978e34d44f1a53fe20327fde@kernel.org> <168796344232.46347.7947681068822514750.stgit@devnote2> <20230705212657.5968daf7@gandalf.local.home> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Wed, 5 Jul 2023 21:26:57 -0400 Steven Rostedt wrote: > On Wed, 28 Jun 2023 23:44:02 +0900 > "Masami Hiramatsu (Google)" wrote: > > > From: Masami Hiramatsu (Google) > > > > Ensure running fprobe_exit_handler() has finished before > > calling rethook_free() in the unregister_fprobe() so that caller can free > > the fprobe right after unregister_fprobe(). > > > > unregister_fprobe() ensured that all running fprobe_entry/exit_handler() > > have finished by calling unregister_ftrace_function() which synchronizes > > RCU. But commit 5f81018753df ("fprobe: Release rethook after the > > ftrace_ops is unregistered") changed to call rethook_free() after > > unregister_ftrace_function(). So call rethook_stop() to make rethook > > disabled before unregister_ftrace_function() and ensure it again. > > I'm confused. I still don't understand why it is bad to call > unregister_ftrace_function() *before* rethook_free(). > > Can you show the race condition you are trying to avoid? Yes. This is ensuring all handlers exit when returning from unregister_fprobe() so that the caller can release the data which will be accessed from the handlers. The entry handler is safe because unregister_ftrace_function() waits for the ftrace handlers. But that is not enough for the exit handler. With only Jiri's patch, following flow can happen; ------ CPU1 CPU2 call unregister_fprobe() ... __fprobe_handler() rethook_hook() on probed function unregister_ftrace_function() return from probed function rethook hooks find rh->handler == fprobe_exit_handler call fprobe_exit_handler() rethook_free(): set rh->handler = NULL; return from unreigster_fprobe; call fp->exit_handler() <- (*) (*) In this point, the exit handler is called after returning from unregister_fprobe(). ------ So, this patch changes it as following; ------ CPU1 CPU2 call unregister_fprobe() ... rethook_stop(): set rh->handler = NULL; __fprobe_handler() rethook_hook() on probed function unregister_ftrace_function() return from probed function rethook hooks find rh->handler == NULL return from rethook rethook_free() return from unreigster_fprobe; ------ I can also just put a synchronize_sched_rcu() right after rethook_free() to wait for all running fprobe_exit_handler() too. Thank you, > > -- Steve > > > > > > > Fixes: 5f81018753df ("fprobe: Release rethook after the ftrace_ops is > > unregistered") Cc: stable@vger.kernel.org > > Signed-off-by: Masami Hiramatsu (Google) > > --- > > include/linux/rethook.h | 1 + > > kernel/trace/fprobe.c | 3 +++ > > kernel/trace/rethook.c | 13 +++++++++++++ > > 3 files changed, 17 insertions(+) > > > > diff --git a/include/linux/rethook.h b/include/linux/rethook.h > > index c8ac1e5afcd1..bdbe6717f45a 100644 > > --- a/include/linux/rethook.h > > +++ b/include/linux/rethook.h > > @@ -59,6 +59,7 @@ struct rethook_node { > > }; > > > > struct rethook *rethook_alloc(void *data, rethook_handler_t handler); > > +void rethook_stop(struct rethook *rh); > > void rethook_free(struct rethook *rh); > > void rethook_add_node(struct rethook *rh, struct rethook_node *node); > > struct rethook_node *rethook_try_get(struct rethook *rh); > > diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c > > index 0121e8c0d54e..75517667b54f 100644 > > --- a/kernel/trace/fprobe.c > > +++ b/kernel/trace/fprobe.c > > @@ -364,6 +364,9 @@ int unregister_fprobe(struct fprobe *fp) > > fp->ops.saved_func != fprobe_kprobe_handler)) > > return -EINVAL; > > > > + if (fp->rethook) > > + rethook_stop(fp->rethook); > > + > > ret = unregister_ftrace_function(&fp->ops); > > if (ret < 0) > > return ret; > > diff --git a/kernel/trace/rethook.c b/kernel/trace/rethook.c > > index 60f6cb2b486b..468006cce7ca 100644 > > --- a/kernel/trace/rethook.c > > +++ b/kernel/trace/rethook.c > > @@ -53,6 +53,19 @@ static void rethook_free_rcu(struct rcu_head *head) > > kfree(rh); > > } > > > > +/** > > + * rethook_stop() - Stop using a rethook. > > + * @rh: the struct rethook to stop. > > + * > > + * Stop using a rethook to prepare for freeing it. If you want to wait > > for > > + * all running rethook handler before calling rethook_free(), you need to > > + * call this first and wait RCU, and call rethook_free(). > > + */ > > +void rethook_stop(struct rethook *rh) > > +{ > > + WRITE_ONCE(rh->handler, NULL); > > +} > > + > > /** > > * rethook_free() - Free struct rethook. > > * @rh: the struct rethook to be freed. > -- Masami Hiramatsu (Google)