linux-kernel - Re: [RFC PATCH for 4.18 00/16] Restartable Sequences

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Message-ID: <1210024721.6363.1532785744879.JavaMail.zimbra@efficios.com>
Date:   Sat, 28 Jul 2018 09:49:04 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Pavel Machek <pavel@....cz>, Carlos O'Donell <carlos@...hat.com>,
        Florian Weimer <fweimer@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andrew Hunter <ahh@...gle.com>,
        Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>,
        Ben Maurer <bmaurer@...com>, 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>
Subject: Re: [RFC PATCH for 4.18 00/16] Restartable Sequences

----- On Jul 27, 2018, at 6:01 PM, Pavel Machek pavel@....cz wrote:

> Hi!
> 
>> So for instance, this turns:
>> 
>>   int cpu = rseq_per_cpu_lock(lock, target_cpu);
>>   [...]
>>   rseq_per_cpu_unlock(lock, cpu);
>> 
>> into
>> 
>>   int cpu = rseq_this_cpu_lock(lock);
>>   [...]
>>   rseq_per_cpu_unlock(lock, cpu);
>> 
>> and:
>> 
>>   per_cpu_list_push(list, node, target_cpu);
>>   [...]
>>   per_cpu_list_pop(list, node, target_cpu);
>> 
>> into
>> 
>>   this_cpu_list_push(list, node, &cpu);  /* cpu is an output parameter. */
>>   [...]
>>   node = this_cpu_list_pop(list, &cpu);  /* cpu is an output parameter. */
>> 
>> Eventually integrating cpu_opv or some alternative will allow passing
>> the cpu number as parameter rather than requiring the algorithm to work
>> on the current CPU.
>> 
>> The second effect of not having the cpu_opv fallback is that
>> line and instruction single-stepping with a debugger transforms rseq
>> critical sections based on retry loops into never-ending loops.
>> Debuggers need to use the __rseq_table section to skip those critical
>> sections in order to correctly behave when single-stepping a thread
>> which uses rseq in a retry loop. However, applications which use an
>> alternative fallback method rather than retrying on rseq fast-path abort
>> won't be affected by this kind of single-stepping issue.
>> 
>> Thanks for your feedback!
> 
> Would it make sense to include Documentation/ patch? I guess at least
> manpage describing the syscall will be needed....

Hi Pavel,

Documentation-wise, I have posted a rseq man page rfc here:

https://lkml.kernel.org/r/20180616195803.29877-1-mathieu.desnoyers@efficios.com

comments are welcome!

It does not include any details about user-space library APIs though, as
this is not the purpose of the syscall documentation.

We're currently discussing integration of rseq thread registration into
glibc.

Once this is settled, I plan to provide a librseq which will contain headers
and documentation on how to use rseq without having to re-create the low-level
assembly every time.

Does this plan make sense to you ?

Thanks,

Mathieu

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