From mboxrd@z Thu Jan  1 00:00:00 1970
From: Mathieu Desnoyers <mathieu.desnoyers-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
Subject: Re: [RFC PATCH v11 for 4.15 01/24] Restartable sequences system
 call
Date: Thu, 16 Nov 2017 18:49:49 +0000 (UTC)
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To: Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
Cc: "Paul E. McKenney" <paulmck-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>, Boqun Feng <boqun.feng-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>, Dave Watson <davejwatson-b10kYP2dOMg@public.gmane.org>, linux-kernel <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, linux-api <linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Paul Turner <pjt-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Russell King <linux-lFZ/pmaqli7XmaaqVzeoHQ@public.gmane.org>, Thomas Gleixner <tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>, Ingo Molnar <mingo-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, "H. Peter Anvin" <hpa-YMNOUZJC4hwAvxtiuMwx3w@public.gmane.org>, Andrew Hunter <ahh-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Andi Kleen <andi-Vw/NltI1exuRpAAqCnN02g@public.gmane.org>, Chris Lameter <cl-vYTEC60ixJUAvxtiuMwx3w@public.gmane.org>, Ben Maurer <bmaurer-b10kYP2dOMg@public.gmane.org>, rostedt <rostedt-nx8X9YLhiw1AfugRpC6u6w@public.gmane.org>, Josh Triplett <josh-iaAMLnmF4UmaiuxdJuQwMA@public.gmane.org>, Linus Torvalds <torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Catalin Marinas <catalin.marinas-5wv7dgnIgG8@public.gmane.org>, Will Deacon <will>
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----- On Nov 16, 2017, at 1:43 PM, Peter Zijlstra peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org wrote:

> On Tue, Nov 14, 2017 at 03:03:51PM -0500, Mathieu Desnoyers wrote:
>> +/*
>> + * If parent process has a registered restartable sequences area, the
>> + * child inherits. Only applies when forking a process, not a thread. In
>> + * case a parent fork() in the middle of a restartable sequence, set the
>> + * resume notifier to force the child to retry.
>> + */
>> +static inline void rseq_fork(struct task_struct *t, unsigned long clone_flags)
>> +{
>> +	if (clone_flags & CLONE_THREAD) {
>> +		t->rseq = NULL;
>> +		t->rseq_len = 0;
>> +		t->rseq_sig = 0;
>> +	} else {
>> +		t->rseq = current->rseq;
>> +		t->rseq_len = current->rseq_len;
>> +		t->rseq_sig = current->rseq_sig;
>> +		rseq_set_notify_resume(t);
>> +	}
>> +}
> 
> This hurts my brain... what happens if you fork a multi-threaded
> process?
> 
> Do we fully inherit the TLS state of the calling thread?

Yes, exactly. The user-space TLS should be inherited from that of
the calling thread.

At kernel-level, the only thing that's not inherited here is the
task struct rseq_event_mask, which tracks whether a restart is
needed. But this would only be relevant if fork() can be invoked
from a signal handler, or if fork() could be invoked from a
rseq critical section (which really makes little sense).

Should I copy the current->rseq_event_mask on process fork just to
be on the safe side though ?

Thanks,

Mathieu



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