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

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Message-ID: <aDTezSUdW6QvQ733@krava>
Date: Mon, 26 May 2025 23:36:13 +0200
From: Jiri Olsa <olsajiri@...il.com>
To: Masami Hiramatsu <mhiramat@...nel.org>
Cc: Oleg Nesterov <oleg@...hat.com>, 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 Thu, May 22, 2025 at 11:48:22PM +0900, Masami Hiramatsu wrote:
> On Tue, 20 May 2025 16:19:26 +0200
> Oleg Nesterov <oleg@...hat.com> wrote:
> 
> > 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.
> 
> Oops, OK. So current code is good with either mmap_read_lock() or mmap_write_lock().
> But the patch description is a bit confusing. If the point is an atomic (byte?)
> update or not, it should describe it.

ok, I'll try to make the changelog more detailed

thanks,
jirka

> 
> Thank you,
> 
> > 
> > --------------------------------------------------------------------------
> > Just in case... I'll try to read this series tomorrow, but at first glance
> > this version addresses all my concerns.
> > 
> > Oleg.
> > 
> 
> 
> -- 
> Masami Hiramatsu (Google) <mhiramat@...nel.org>