linux-kernel - Re: [RFC PATCH for 4.21 01/16] rseq/selftests: Add reference counter to coexist with glibc

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Date:   Thu, 11 Oct 2018 10:37:25 +0000
From:   Szabolcs Nagy <Szabolcs.Nagy@....com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>
CC:     nd <nd@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>,
        Ben Maurer <bmaurer@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Catalin Marinas <Catalin.Marinas@....com>,
        Will Deacon <Will.Deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Shuah Khan <shuah@...nel.org>,
        Carlos O'Donell <carlos@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        Joseph Myers <joseph@...esourcery.com>
Subject: Re: [RFC PATCH for 4.21 01/16] rseq/selftests: Add reference counter
 to coexist with glibc

On 10/10/18 20:19, Mathieu Desnoyers wrote:
> In order to integrate rseq into user-space applications, add a reference
> counter field after the struct rseq TLS ABI so many rseq users can be
> linked into the same application (e.g. librseq and glibc). The
> reference count ensures that rseq syscall registration/unregistration
> happens only for the most early/late user for each thread, thus ensuring
> that rseq is registered across the lifetime of all rseq users for a
> given thread.
...
> +__attribute__((visibility("hidden"))) __thread
> +volatile struct libc_rseq __lib_rseq_abi = {
...
> +extern __attribute__((weak, alias("__lib_rseq_abi"))) __thread
> +volatile struct rseq __rseq_abi;
...
> @@ -70,7 +86,7 @@ int rseq_register_current_thread(void)
>  	sigset_t oldset;
>  
>  	signal_off_save(&oldset);
> -	if (refcount++)
> +	if (__lib_rseq_abi.refcount++)
>  		goto end;
>  	rc = sys_rseq(&__rseq_abi, sizeof(struct rseq), 0, RSEQ_SIG);

why do you use a local refcounter instead of the __rseq_abi one?

what prevents calling rseq_register_current_thread more than 4G times?

why cant the kernel see that the same address is registered again and succeed?