linux-kernel - Re: [Question] smp_wmb() in prepare

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Date:   Thu, 22 Nov 2018 16:05:24 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [Question] smp_wmb() in prepare_uprobe()

On 11/22, Andrea Parri wrote:
>
> > See 142b18ddc81439acda4bc4231b291e99fe67d507 ("uprobes: Fix handle_swbp()
> > vs unregister() + register() race") and the comment above this rmb().
>
> Mmh..., at first glance, this suggests me that the above set_bit() and
> test_bit() to/from uprobe->flags are among these memory accesses.  But
> this doesn't make sense to me: these accesses do not "alternate" (i.e.,
> they both appear after the corresponding barrier..); instead I'd expect
> something like (on top of the above diff):
>
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 2d29977522017..a75b9a08dee54 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -2178,10 +2178,18 @@ static void handle_swbp(struct pt_regs *regs)
>  	 * After we hit the bp, _unregister + _register can install the
>  	 * new and not-yet-analyzed uprobe at the same address, restart.
>  	 */
> -	smp_rmb(); /* pairs with the smp_wmb() in prepare_uprobe() */
>  	if (unlikely(!test_bit(UPROBE_COPY_INSN, &uprobe->flags)))
>  		goto out;
>
> +	/*
> +	 * Pairs with the smp_wmb() in prepare_uprobe().
> +	 *
> +	 * Guarantees that if we see the UPROBE_COPY_INSN bit set, then
> +	 * we must (can) also see the stores to &uprobe->arch performed
> +	 * by prepare_uprobe() (say).
> +	 */
> +	smp_rmb();

OOPS, you are right! Thanks.

Oleg.