From: Manfred Spraul <manfred@colorfullife.com>
To: Davidlohr Bueso <dave@stgolabs.net>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>,
Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
"H. Peter Anvin" <hpa@zytor.com>,
Peter Zijlstra <peterz@infradead.org>,
Andrew Morton <akpm@linux-foundation.org>,
LKML <linux-kernel@vger.kernel.org>,
linux-next@vger.kernel.org, 1vier1@web.de,
felixh@informatik.uni-bremen.de, stable@vger.kernel.org
Subject: Re: [PATCH 1/2] ipc/sem.c: Fix complex_count vs. simple op race
Date: Thu, 30 Jun 2016 21:28:43 +0200 [thread overview]
Message-ID: <ac93292c-c995-9c05-9e97-06c19ea7a2bf@colorfullife.com> (raw)
In-Reply-To: <20160628052717.GA13793@linux-80c1.suse>
On 06/28/2016 07:27 AM, Davidlohr Bueso wrote:
> On Thu, 23 Jun 2016, Manfred Spraul wrote:
>
>> What I'm not sure yet is if smp_load_acquire() is sufficient:
>>
>> Thread A:
>>> if (!READ_ONCE(sma->complex_mode)) {
>> The code is test_and_test, no barrier requirements for first test
>
> Yeah, it would just make us take the big lock unnecessarily, nothing
> fatal
> and I agree its probably worth the optimization. It still might be worth
> commenting.
>
I'll extend the comment: "no locking and no memory barrier"
>>> /*
>>> * It appears that no complex operation is around.
>>> * Acquire the per-semaphore lock.
>>> */
>>> spin_lock(&sem->lock);
>>>
>>> if (!smp_load_acquire(&sma->complex_mode)) {
>>> /* fast path successful! */
>>> return sops->sem_num;
>>> }
>>> spin_unlock(&sem->lock);
>>> }
>>
>> Thread B:
>>> WRITE_ONCE(sma->complex_mode, true);
>>>
>>> /* We need a full barrier:
>>> * The write to complex_mode must be visible
>>> * before we read the first sem->lock spinlock state.
>>> */
>>> smp_mb();
>>>
>>> for (i = 0; i < sma->sem_nsems; i++) {
>>> sem = sma->sem_base + i;
>>> spin_unlock_wait(&sem->lock);
>>> }
>>
>> If thread A is allowed to issue read_spinlock;read complex_mode;write
>> spinlock, then thread B would not notice that thread A is in the
>> critical section
>
> Are you referring to the sem->lock word not being visibly locked
> before we
> read complex_mode (for the second time)? This issue was fixed in
> 2c610022711
> (locking/qspinlock: Fix spin_unlock_wait() some more). So
> smp_load_acquire
> should be enough afaict, or are you referring to something else?
>
You are right, I didn't read this patch fully.
If I understand it right, it means that spin_lock() is both an acquire
and a release - for qspinlocks.
It this valid for all spinlock implementations, for all architectures?
Otherwise: How can I detect in generic code if I can rely on a release
inside spin_lock()?
--
Manfred
next prev parent reply other threads:[~2016-06-30 19:28 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-06-15 5:23 linux-next: manual merge of the akpm-current tree with the tip tree Stephen Rothwell
2016-06-18 19:39 ` Manfred Spraul
2016-06-18 20:02 ` [PATCH 1/2] ipc/sem.c: Fix complex_count vs. simple op race Manfred Spraul
2016-06-18 20:02 ` [PATCH 2/2] ipc/sem: sem_lock with hysteresis Manfred Spraul
2016-06-21 20:29 ` Davidlohr Bueso
2016-06-25 17:37 ` Manfred Spraul
2016-06-28 17:54 ` Davidlohr Bueso
2016-06-20 23:04 ` [PATCH 1/2] ipc/sem.c: Fix complex_count vs. simple op race Andrew Morton
2016-06-23 18:55 ` Manfred Spraul
2016-06-21 0:30 ` Davidlohr Bueso
2016-06-23 19:22 ` Manfred Spraul
2016-06-28 5:27 ` Davidlohr Bueso
2016-06-30 19:28 ` Manfred Spraul [this message]
2016-07-01 16:52 ` Davidlohr Bueso
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