From: Davidlohr Bueso <dave@stgolabs.net>
To: Manfred Spraul <manfred@colorfullife.com>
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: Mon, 27 Jun 2016 22:27:17 -0700 [thread overview]
Message-ID: <20160628052717.GA13793@linux-80c1.suse> (raw)
In-Reply-To: <b5b4e032-8b1c-f102-ac23-3b331c2ed615@colorfullife.com>
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.
>> /*
>> * 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?
Thanks,
Davidlohr
next prev parent reply other threads:[~2016-06-28 5:27 UTC|newest]
Thread overview: 17+ 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 [this message]
2016-06-30 19:28 ` Manfred Spraul
2016-07-01 16:52 ` Davidlohr Bueso
2016-06-25 17:37 [PATCH 0/2] ipc/sem.c: sem_lock fixes Manfred Spraul
2016-06-25 17:37 ` [PATCH 1/2] ipc/sem.c: Fix complex_count vs. simple op race Manfred Spraul
2016-07-13 5:06 [PATCH 0/2] ipc/sem.c: sem_lock fixes Manfred Spraul
2016-07-13 5:06 ` [PATCH 1/2] ipc/sem.c: Fix complex_count vs. simple op race Manfred Spraul
2016-07-16 1:27 ` Davidlohr Bueso
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