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

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Date:   Tue, 17 Oct 2017 16:19:41 +0000
From:   Ben Maurer <bmaurer@...com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Andi Kleen <andi@...stfloor.org>
CC:     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

Hey,

> So far the restrictions I see for libraries using this symbol are:
> - They should never be unloaded,
> - They should never be loaded with dlopen RTLD_LOCAL flag.

We talked a bit about this off-list but I wanted to state publicly that I think this model works well for our use case. Specifically,

(1) It reduces complexity by focusing on the common case -- long term we expect glibc to manage the process of using this feature and registering/deregistering threads for rseq. Unloading isn't a challenge in these situations, so why add the complexity for it?

(2) This still allows for early adopters to use rseq before there is glibc support. I believe the vast majority of real world applications meet these two criteria you've listed. If not, they can create a thin shared library that has the sole purpose of providing the weak symbol and that never gets unloaded

(3) This allows for applications to provide the __rseq_abi so that they can ensure it uses the initial_exec tls model and optimize in-application assembly code for it. This is a good optimization for server applications that tend to statically link.

If others agree with this, would it make sense to remove the concept of reference counting in the system call that defines and redefines the per-thread area? Seems like it would remove complexity.

-b