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) [thread overview]
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
next prev parent reply other threads:[~2018-10-11 16:37 UTC|newest]
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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