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

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Message-Id: <20250522234822.0410cabbbbfb58ef327805a9@kernel.org>
Date: Thu, 22 May 2025 23:48:22 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Oleg Nesterov <oleg@...hat.com>
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 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.

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>