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Re: [RFC PATCH v4 1/5] glibc: Perform rseq(2) registration at nptl init and thread creation
- From: Mathieu Desnoyers <mathieu dot desnoyers at efficios dot com>
- To: Rich Felker <dalias at libc dot org>
- Cc: Florian Weimer <fweimer at redhat dot com>, carlos <carlos at redhat dot com>, Joseph Myers <joseph at codesourcery dot com>, Szabolcs Nagy <szabolcs dot nagy at arm dot com>, libc-alpha <libc-alpha at sourceware dot org>, Thomas Gleixner <tglx at linutronix dot de>, Ben Maurer <bmaurer at fb dot com>, Peter Zijlstra <peterz at infradead dot org>, "Paul E. McKenney" <paulmck at linux dot vnet dot ibm dot com>, Boqun Feng <boqun dot feng at gmail dot com>, Will Deacon <will dot deacon at arm dot com>, Dave Watson <davejwatson at fb dot com>, Paul Turner <pjt at google dot com>, linux-kernel <linux-kernel at vger dot kernel dot org>, linux-api <linux-api at vger dot kernel dot org>
- Date: Mon, 26 Nov 2018 14:22:05 -0500 (EST)
- Subject: Re: [RFC PATCH v4 1/5] glibc: Perform rseq(2) registration at nptl init and thread creation
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----- On Nov 26, 2018, at 12:10 PM, Rich Felker firstname.lastname@example.org wrote:
> On Mon, Nov 26, 2018 at 05:03:02PM +0100, Florian Weimer wrote:
>> * Mathieu Desnoyers:
>> > So let's make __rseq_abi and __rseq_refcount strong symbols then ?
>> Yes, please. (But I'm still not sure we need the reference counter.)
> The reference counter is needed for out-of-libc implementations
> interacting and using the dtor hack. An in-libc implementation doesn't
> need to inspect/honor the reference counter, but it does seem to need
> to indicate that it has a reference, if you want it to be compatible
> with out-of-libc implementations, so that the out-of-libc one will not
> unregister the rseq before libc is done with it.
Let's consider two use-cases here: one (simpler) is use of rseq TLS
from thread context by out-of-libc implementations. The other is use of
rseq TLS from signal handler by out-of-libc implementations.
If we only care about users of rseq from thread context, then libc
could simply set the refcount value to 1 on thread start,
and should not care about the value on thread exit. The libc can
either directly call rseq unregister, or rely on thread calling exit
to implicitly unregister rseq, which depends on its own TLS life-time
guarantees. For instance, if the IE-model TLS is valid up until call
to exit, just calling the exit system call is fine. However, if a libc
has a window at thread exit during which the kernel can preempt the
thread with the IE-model TLS area being already reclaimed, then it
needs to explicitly call rseq unregister before freeing the TLS.
The second use-case is out-of-libc implementations using rseq from
signal handler. This one is trickier. First, pthread_key setspecific
is unfortunately not async-signal-safe. I can't find a good way to
seamlessly integrate rseq into out-of-libc signal handlers while
performing lazy registration without races on thread exit. If we
figure out a way to do this though, we should increment the refcount
at thread start in libc (rather than just set it to 1) in case a
signal handler gets nested immediately over the start of the thread
and registers rseq as well.
It looks like it's not the only issue I have with calling lttng-ust
instrumentation from signal handlers, here is the list I have so
* glibc global-dynamic TLS variables are not async-signal-safe,
and lttng-ust cannot use IE-model TLS because it is meant to be
* pthread_setspecific is not async-signal-safe,
There should be ways to eventually solve those issues, but it would
be nice if for now the way rseq is implemented in libc does not add
yet another limitation for signal handlers.
> Alternatively another protocol could be chosen for this purpose, but
> if has to be something stable and agreed upon, since things would
> break badly if libc and the library providing rseq disagreed.
Absolutely. We need to agree on that protocol before user-space
applications/libraries start using rseq.