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[109.80.100.85]) by smtp.gmail.com with ESMTPSA id x58sm13104893edm.10.2018.11.22.07.45.32 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Thu, 22 Nov 2018 07:45:33 -0800 (PST) Date: Thu, 22 Nov 2018 16:45:27 +0100 From: Andrea Parri To: Oleg Nesterov Cc: Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Alexander Shishkin , Jiri Olsa , Namhyung Kim , linux-kernel@vger.kernel.org Subject: Re: [Question] smp_wmb() in prepare_uprobe() Message-ID: <20181122154526.GA14616@andrea> References: <20181121224124.GB4016@andrea> <20181122123655.GD28270@redhat.com> <20181122134400.GA10327@andrea> <20181122150524.GF28270@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181122150524.GF28270@redhat.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Nov 22, 2018 at 04:05:24PM +0100, Oleg Nesterov wrote: > 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. Thank you for the clarification; I'll send a patch with the fix shortly. Andrea > > Oleg. >