linux-kernel - Re: [PATCH 2/5] nptl: Add rseq registration

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Date:   Mon, 6 Dec 2021 13:08:13 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     "Paul E. McKenney via Libc-alpha" <libc-alpha@...rceware.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/5] nptl: Add rseq registration

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