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

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Date:   Mon, 06 Dec 2021 21:26:51 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     "Paul E. McKenney via Libc-alpha" <libc-alpha@...rceware.org>
Cc:     paulmck@...nel.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

* 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).

THanks,
Florian