From: "Paul E. McKenney" <paulmck@kernel.org>
To: Florian Weimer <fweimer@redhat.com>
Cc: "Paul E. McKenney via Libc-alpha" <libc-alpha@sourceware.org>,
Peter Zijlstra <peterz@infradead.org>,
Boqun Feng <boqun.feng@gmail.com>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/5] nptl: Add rseq registration
Date: Mon, 6 Dec 2021 13:08:13 -0800 [thread overview]
Message-ID: <20211206210813.GF641268@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <87k0ghlbsk.fsf@oldenburg.str.redhat.com>
On Mon, Dec 06, 2021 at 09:26:51PM +0100, Florian Weimer wrote:
> * Paul E. McKenney via Libc-alpha:
>
> >> The C memory model is broken and does not prevent out-of-thin-air
> >> values. As far as I know, this breaks single-copy atomicity. In
> >> practice, compilers will not exercise the latitude offered by the memory
> >> model. volatile does not ensure absence of data races.
> >
> > Within the confines of the standard, agreed, use of the volatile keyword
> > does not explicitly prevent data races.
> >
> > However, volatile accesses are (informally) defined to suffice for
> > device-driver memory accesses that communicate with devices, whether via
> > MMIO or DMA-style shared memory. The device-driver firmware is often
> > written in C or C++. So doesn't this informal device-driver guarantee
> > need to also do what is needed for userspace code that is communicating
> > with kernel code? If not, why not?
>
> The informal guarantee is probably good enough here, too. However, the
> actual accesses are behind macros, and those macros use either
> non-volatile plain reads or inline assembler (which use
> single-instruction naturally aligned reads).
Agreed, a non-volatile plain read is quite dangerous in this context.
Thanx, Paul
next prev parent reply other threads:[~2021-12-06 21:08 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-06 13:45 [PATCH 0/5] Extensible rseq support for glibc Florian Weimer
2021-12-06 13:46 ` [PATCH 1/5] nptl: Add <thread_pointer.h> for defining __thread_pointer Florian Weimer
2021-12-06 16:44 ` Mathieu Desnoyers
2021-12-06 17:01 ` Florian Weimer
2021-12-06 19:55 ` Florian Weimer
2021-12-06 13:46 ` [PATCH 2/5] nptl: Add rseq registration Florian Weimer
2021-12-06 16:53 ` Mathieu Desnoyers
2021-12-06 17:10 ` Florian Weimer
2021-12-06 16:59 ` Mathieu Desnoyers
2021-12-06 17:14 ` Florian Weimer
2021-12-06 18:52 ` Mathieu Desnoyers
2021-12-06 19:03 ` Florian Weimer
2021-12-06 20:11 ` Paul E. McKenney
2021-12-06 20:26 ` Florian Weimer
2021-12-06 21:08 ` Paul E. McKenney [this message]
2021-12-06 13:46 ` [PATCH 3/5] Linux: Use rseq to accelerate sched_getcpu Florian Weimer
2021-12-06 16:50 ` Szabolcs Nagy
2021-12-06 17:06 ` Florian Weimer
2021-12-06 17:45 ` Szabolcs Nagy
2021-12-07 15:48 ` Florian Weimer
2021-12-06 13:46 ` [PATCH 4/5] nptl: Add glibc.pthread.rseq tunable to control rseq registration Florian Weimer
2021-12-06 13:53 ` [PATCH 5/5] nptl: Add public rseq symbols and <sys/rseq.h> Florian Weimer
2021-12-07 11:01 ` Szabolcs Nagy
2021-12-07 11:28 ` Florian Weimer
2021-12-06 16:13 ` [PATCH 0/5] Extensible rseq support for glibc Mathieu Desnoyers
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