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From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Szabolcs Nagy <Szabolcs.Nagy@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Boqun Feng <boqun.feng@gmail.com>, nd <nd@arm.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 11:13:28 -0400 (EDT)
Message-ID: <1917048565.2402.1539270808972.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <38596780-30f7-0763-0c17-7517dbf0bf59@arm.com>

----- 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).

> 
> 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 ?

> 
> 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.

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 [this message]
2018-10-11 16:20       ` Szabolcs Nagy
2018-10-11 16:37         ` Mathieu Desnoyers
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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