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Date:   Thu, 14 Jun 2018 09:46:46 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Florian Weimer <fweimer@redhat.com>
Cc:     Pavel Machek <pavel@ucw.cz>, carlos <carlos@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        libc-alpha <libc-alpha@sourceware.org>
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Subject: Re: Restartable Sequences system call merged into Linux
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Thread-Topic: Restartable Sequences system call merged into Linux
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----- On Jun 14, 2018, at 9:32 AM, Florian Weimer fweimer@redhat.com wrote:

> On 06/14/2018 03:25 PM, Pavel Machek wrote:
> 
>> But the proposal wanted to add a syscall to thread creation, right?
>> And I believe that may be noticeable.
> 
> We already call set_robust_list, so we could just pass a larger area to
> that and the kernel could use it.  Then no additional system call would
> be needed in the common case (new kernel which recognizes the new area
> size).
> 
> But then we cannot use an initial-exec thread local variable for it
> (although the offset from the thread pointer will still be constant, of
> course).

I'm wondering whether we could turn the problem around: expose a new
system call allowing to register an array of pointers to per-thread data,
which would be used rather than set_robust_list when available. This way,
we could register both the robust list and rseq with a single system call,
e.g.:

enum linux_tls_area_type {
    LINUX_TLS_ROBUST_LIST,
    LINUX_TLS_RSEQ,
};

struct linux_tls_area_item {
    enum linux_tls_area_type type;
    void *p;
};

long sys_register_tls_areas(struct linux_tls_area_item *array, size_t nb)

This would allow registering various TLS data structures with a single
system call without hindering flexibility on the user-space side. For
instance, we could still use initial-exec and the __rseq_abi symbol for
rseq with this approach.

Thoughts ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com