linux-kernel - Re: [PATCHv2 perf/core 01/22] uprobes: Remove breakpoint in unapply_uprobe under mmap_write

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Message-ID: <20250520141925.GA14203@redhat.com>
Date: Tue, 20 May 2025 16:19:26 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Masami Hiramatsu <mhiramat@...nel.org>
Cc: Jiri Olsa <jolsa@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
	Andrii Nakryiko <andrii@...nel.org>, bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	x86@...nel.org, Song Liu <songliubraving@...com>,
	Yonghong Song <yhs@...com>,
	John Fastabend <john.fastabend@...il.com>,
	Hao Luo <haoluo@...gle.com>, Steven Rostedt <rostedt@...dmis.org>,
	Alan Maguire <alan.maguire@...cle.com>,
	David Laight <David.Laight@...LAB.COM>,
	Thomas Weißschuh <thomas@...ch.de>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCHv2 perf/core 01/22] uprobes: Remove breakpoint in
 unapply_uprobe under mmap_write_lock

On 05/20, Masami Hiramatsu wrote:
>
> On Thu, 15 May 2025 14:10:58 +0200
> Jiri Olsa <jolsa@...nel.org> wrote:
>
> > Currently unapply_uprobe takes mmap_read_lock, but it might call
> > remove_breakpoint which eventually changes user pages.
> >
> > Current code writes either breakpoint or original instruction, so
> > it can probably go away with that, but with the upcoming change that
> > writes multiple instructions on the probed address we need to ensure
> > that any update to mm's pages is exclusive.
> >
>
> So, this is a bugfix, right?

No, mmap_read_lock() is fine.

To remind, this was already discussed with you, see
[PATCH 02/12] uprobes: grab write mmap lock in unapply_uprobe()
https://lore.kernel.org/all/20240625002144.3485799-3-andrii@kernel.org/

And you even reviewed this patch
[PATCH 1/2] uprobes: document the usage of mm->mmap_lock
https://lore.kernel.org/all/20240710140045.GA1084@redhat.com/

But, as the changelog explains, this patch is needed for the upcoming changes.

--------------------------------------------------------------------------
Just in case... I'll try to read this series tomorrow, but at first glance
this version addresses all my concerns.

Oleg.