Re: [RFC PATCH v11 for 4.15 01/24] Restartable sequences system call

[Mathieu Desnoyers <mathieu.desnoyers-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>]

----- 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