linux-kernel - Re: [PATCH v2 tip/perf/core 5/5] uprobes: add speculative lockless VMA-to-inode-to-uprobe resolution

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Message-ID: <20241003093228.GA20733@redhat.com>
Date: Thu, 3 Oct 2024 11:32:29 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Andrii Nakryiko <andrii@...nel.org>, linux-trace-kernel@...r.kernel.org,
	peterz@...radead.org, rostedt@...dmis.org, mhiramat@...nel.org,
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org, jolsa@...nel.org,
	paulmck@...nel.org, willy@...radead.org, surenb@...gle.com,
	akpm@...ux-foundation.org, linux-mm@...ck.org, mjguzik@...il.com,
	brauner@...nel.org, jannh@...gle.com, mhocko@...nel.org,
	vbabka@...e.cz, mingo@...nel.org
Subject: Re: [PATCH v2 tip/perf/core 5/5] uprobes: add speculative lockless
 VMA-to-inode-to-uprobe resolution

On 10/02, Andrii Nakryiko wrote:
>
> On Wed, Oct 2, 2024 at 12:25 AM Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > > +     vm_file = READ_ONCE(vma->vm_file);
> > > +     if (!vm_file)
> > > +             return NULL;
> > > +
> > > +     offset = (loff_t)(vma->vm_pgoff << PAGE_SHIFT) + (bp_vaddr - vma->vm_start);
> >
> > LGTM. But perhaps vma->vm_pgoff and vma->vm_start need READ_ONCE() as well,
> > if nothing else to shut up KCSAN if this code races with, say, __split_vma() ?
>
> We keep going back and forth between reading directly, using
> READ_ONCE(), and annotating with data_race(). I don't think it matters
> in terms of correctness or performance, so I'm happy to add whatever
> incantations that will make everyone satisfied. Let's see what others
> think, and I'll incorporate that into the next revision.

OK, agreed...

And I guess I was wrong anyway, READ_ONCE() alone won't shutup KCSAN in
this case.

Oleg.