linux-kernel - Re: [RFC PATCH v9 for 4.15 01/14] Restartable sequences system call

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Date:   Tue, 17 Oct 2017 17:48:04 +0000 (UTC)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Ben Maurer <bmaurer@...com>
Cc:     Andi Kleen <andi@...stfloor.org>, carlos <carlos@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        David Goldblatt <davidgoldblatt@...com>,
        Qi Wang <qiwang@...com>, Boqun Feng <boqun.feng@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Paul Turner <pjt@...gle.com>, Andrew Hunter <ahh@...gle.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>,
        Josh Triplett <josh@...htriplett.org>,
        Will Deacon <will.deacon@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Chris Lameter <cl@...ux.com>, Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-api <linux-api@...r.kernel.org>
Subject: Re: [RFC PATCH v9 for 4.15 01/14] Restartable sequences system call

----- On Oct 17, 2017, at 12:41 PM, Ben Maurer bmaurer@...com wrote:

>> I have a use-case for keeping the reference counting in place though. It's
>> use of rseq in signal handlers.
> 
> Would this be solved by saying that the rseq api will return an error if you
> register and there's already a block registered. In this case the signal
> handler would register the rseq abi area just as the non-signal code is trying
> to do the same. The non-signal code would see this error code and realize that
> its job had been done for it and then go on it's way.

Yes, that should work, as long as we return a specific error code, e.g. -EBUSY,
to tell the caller that rseq has actually been registered.

> 
> It would be unsafe for signal handler code to *unregister* the area, but I don't
> think that's necessary.

Right.

> 
> Basically we'd support a refcount of either 0 or 1, but nothing else.

Yep, I'll try this out.

> 
> If a signal handler registers the ABI area, how will it ensure the ABI is
> cleaned up at thread exit? I can't imagine pthread_key is signal safe.

You have a very good point there. This highlights a signal-safety issue
I have in liburcu-bp when used by lttng-ust. pthread_setspecific is
indeed not listed as being signal-safe: it can perform on-demand memory
allocation when a second level array is needed.

I'll have to scratch my head a bit to fix this one.

Thanks!

Mathieu

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