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From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Szabolcs Nagy <Szabolcs.Nagy@arm.com>
Cc: nd <nd@arm.com>, Peter Zijlstra <peterz@infradead.org>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	linux-api <linux-api@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Andy Lutomirski <luto@amacapital.net>,
	Dave Watson <davejwatson@fb.com>, Paul Turner <pjt@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Russell King <linux@arm.linux.org.uk>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Andi Kleen <andi@firstfloor.org>, Chris Lameter <cl@linux.com>,
	Ben Maurer <bmaurer@fb.com>, rostedt <rostedt@goodmis.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Catalin Marinas <Catalin.Marinas@arm.com>,
	Will Deacon <Will.Deacon@arm.com>,
	Michael Kerrisk <mtk.manpages@gmail.com>,
	Joel Fernandes <joelaf@google.com>, shuah <shuah@kernel.org>,
	carlos <carlos@redhat.com>, Florian Weimer <fweimer@redhat.com>,
	Joseph Myers <joseph@codesourcery.com>
Subject: Re: [RFC PATCH for 4.21 01/16] rseq/selftests: Add reference counter to coexist with glibc
Date: Thu, 11 Oct 2018 12:37:26 -0400 (EDT)
Message-ID: <1680616760.2469.1539275846360.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <3896e4f5-aab1-ae79-5360-088fd15ed380@arm.com>

----- On Oct 11, 2018, at 12:20 PM, Szabolcs Nagy Szabolcs.Nagy@arm.com wrote:

> On 11/10/18 16:13, Mathieu Desnoyers wrote:
>> ----- On Oct 11, 2018, at 6:37 AM, Szabolcs Nagy Szabolcs.Nagy@arm.com wrote:
>> 
>>> 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?
>> 
>> There is no refcount in struct rseq (the ABI between kernel and user-space).
>> The registration refcount was part of an earlier version of the rseq system
>> call,
>> but we decided against keeping it in the kernel.
>> 
>> So I'm adding one _after_ struct rseq, purely to allow interaction between
>> various user-space components (program/libraries).
> 
> then all those components must use the same
> 
>  rseq_register_current_thread
>  rseq_unregister_current_thread
> 
> functions and not call the syscall on their own.

Not quite. Each user (programs and shared objects) must handle the refcount in a
similar way if they wish to invoke the syscall by themselves. They can
alternately use the librseq APIs if they do not wish to have a local implementation
of the reference counting and syscall registration/unregistration.

> 
> in which case the refcount could be a static __thread variable.

Yes, but I want to limit the number of symbols we need to export
from glibc by appending the refcount field at the end of struct rseq.

> 
> but it's in a magic struct that's called "abi" which is confusing,
> the counter is not abi, it's in a hidden object.

No, it is really an ABI between user-space apps/libs. It's not meant to be
hidden. glibc implements its own register/unregister functions (it does not
link against librseq). librseq exposes register/unregister functions as public
APIs. Those also use the refcount. I also plan to have existing libraries, e.g.
liblttng-ust and possibly liburcu flavors, implement the
registration/unregistration and refcount handling on their own, so we don't
have to add a requirement on additional linking on librseq for pre-existing
libraries.

So that refcount is not an ABI between kernel and user-space, but it's a
user-space ABI nevertheless (between program and shared objects).

> 
>>> what prevents calling rseq_register_current_thread more than 4G times?
>> 
>> Nothing. It would indeed be cleaner to error out if we detect that refcount is
>> at
>> INT_MAX. Is that what you have in mind ?
> 
> yes

Allright, will fix.

> 
>>> why cant the kernel see that the same address is registered again and succeed?
>> 
>> It can, and it does. However, refcounting at user-level is needed to ensure
>> the registration "lifetime" for rseq covers its entire use. If we have two
>> libraries
>> using rseq, we end up with the following scenario:
>> 
>> Thread 1
>> 
>>   libA registers rseq
>>   libB registers rseq
>>   libB unregisters rseq
>>   libA uses rseq -> bug! it's been unregistered by libB.
>>   libA unregisters rseq -> unexpected, it's already been unregistered.
>>  
>> same applies if libA unregisters rseq before libB (and libB try to use rseq
>> after libA has unregistered).
>> 
>> The refcount in user-space fixes this.
> 
> i see.

Thanks for the feedback!

Mathieu

> 
>> Thoughts ?
>> 
>> Thanks,
>> 
>> Mathieu

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

  reply index

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-10 19:19 [RFC PATCH for 4.21 00/16] rseq updates, new cpu_opv system call Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 01/16] rseq/selftests: Add reference counter to coexist with glibc Mathieu Desnoyers
2018-10-11 10:37   ` Szabolcs Nagy
2018-10-11 15:13     ` Mathieu Desnoyers
2018-10-11 16:20       ` Szabolcs Nagy
2018-10-11 16:37         ` Mathieu Desnoyers [this message]
2018-10-11 17:04           ` Szabolcs Nagy
2018-10-11 19:42             ` Mathieu Desnoyers
2018-10-12  9:59               ` Szabolcs Nagy
2018-10-23 14:59                 ` Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 02/16] rseq/selftests: Adapt number of threads to the number of detected cpus Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 03/16] sched: Implement push_task_to_cpu (v2) Mathieu Desnoyers
2018-10-17  6:51   ` Srikar Dronamraju
2018-10-17 15:09     ` Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 04/16] mm: Introduce vm_map_user_ram, vm_unmap_user_ram Mathieu Desnoyers
2018-10-16 18:30   ` Steven Rostedt
2018-10-16 19:21     ` Mathieu Desnoyers
2018-10-16 19:40       ` Steven Rostedt
2018-10-17  0:27     ` Sergey Senozhatsky
2018-10-17 15:00       ` Mathieu Desnoyers
2018-10-17 15:04         ` Mathieu Desnoyers
2018-10-17 15:34           ` Sergey Senozhatsky
2018-10-10 19:19 ` [RFC PATCH for 4.21 05/16] mm: Provide is_vma_noncached Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 06/16] cpu_opv: Provide cpu_opv system call (v8) Mathieu Desnoyers
2018-10-16  8:10   ` Sergey Senozhatsky
2018-10-16 19:17     ` Mathieu Desnoyers
2018-10-17  1:46       ` Sergey Senozhatsky
2018-10-17  7:19   ` Srikar Dronamraju
2018-10-17 15:11     ` Mathieu Desnoyers
2018-10-17 16:09       ` Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 07/16] cpu_opv: limit amount of virtual address space used by cpu_opv Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 08/16] x86: Wire up cpu_opv system call Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 09/16] powerpc: " Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 10/16] arm: " Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 11/16] cpu-opv/selftests: Provide cpu-op library Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 12/16] cpu-opv/selftests: Provide basic test Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 13/16] cpu-opv/selftests: Provide percpu_op API Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 14/16] cpu-opv/selftests: Provide basic percpu ops test Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 15/16] cpu-opv/selftests: Provide parametrized tests Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 16/16] cpu-opv/selftests: Provide Makefile, scripts, gitignore Mathieu Desnoyers

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