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Date: Tue, 18 Feb 2014 14:39:31 -0500
From: Waiman Long <waiman.long@...com>
To: Peter Zijlstra <peterz@...radead.org>
CC: Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
linux-arch@...r.kernel.org, x86@...nel.org,
linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Michel Lespinasse <walken@...gle.com>,
Andi Kleen <andi@...stfloor.org>,
Rik van Riel <riel@...hat.com>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
George Spelvin <linux@...izon.com>,
Tim Chen <tim.c.chen@...ux.intel.com>,
Daniel J Blueman <daniel@...ascale.com>,
Alexander Fyodorov <halcy@...dex.ru>,
Aswin Chandramouleeswaran <aswin@...com>,
Scott J Norton <scott.norton@...com>,
Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@...com>
Subject: Re: [PATCH v4 1/3] qspinlock: Introducing a 4-byte queue spinlock
implementation
On 02/18/2014 02:39 AM, Peter Zijlstra wrote:
> On Mon, Feb 17, 2014 at 03:41:22PM -0500, Waiman Long wrote:
>> +void queue_spin_lock_slowpath(struct qspinlock *lock, int qsval)
>> +{
>> + unsigned int cpu_nr, qn_idx;
>> + struct qnode *node, *next;
>> + u32 prev_qcode, my_qcode;
>> +
>> +#ifdef queue_spin_trylock_quick
>> + /*
>> + * Try the quick spinning code path
>> + */
>> + if (queue_spin_trylock_quick(lock, qsval))
>> + return;
>> +#endif
> why oh why?
I could take this #ifdef away. I just need to add a default version that
always return 0.
>> + /*
>> + * Get the queue node
>> + */
>> + cpu_nr = smp_processor_id();
>> + node = get_qnode(&qn_idx);
>> +
>> + if (unlikely(!node)) {
>> + /*
>> + * This shouldn't happen, print a warning message
>> + *& busy spinning on the lock.
>> + */
>> + printk_sched(
>> + "qspinlock: queue node table exhausted at cpu %d!\n",
>> + cpu_nr);
>> + while (!queue_spin_trylock_unfair(lock))
>> + arch_mutex_cpu_relax();
>> + return;
>> + }
>> +
>> + /*
>> + * Set up the new cpu code to be exchanged
>> + */
>> + my_qcode = _SET_QCODE(cpu_nr, qn_idx);
>> +
>> + /*
>> + * Initialize the queue node
>> + */
>> + node->wait = true;
>> + node->next = NULL;
>> +
>> + /*
>> + * The lock may be available at this point, try again if no task was
>> + * waiting in the queue.
>> + */
>> + if (!(qsval>> _QCODE_OFFSET)&& queue_spin_trylock(lock)) {
>> + put_qnode();
>> + return;
>> + }
>> +
>> +#ifdef queue_code_xchg
>> + prev_qcode = queue_code_xchg(lock, my_qcode);
>> +#else
>> + /*
>> + * Exchange current copy of the queue node code
>> + */
>> + prev_qcode = atomic_xchg(&lock->qlcode, my_qcode);
>> + /*
>> + * It is possible that we may accidentally steal the lock. If this is
>> + * the case, we need to either release it if not the head of the queue
>> + * or get the lock and be done with it.
>> + */
>> + if (unlikely(!(prev_qcode& _QSPINLOCK_LOCKED))) {
>> + if (prev_qcode == 0) {
>> + /*
>> + * Got the lock since it is at the head of the queue
>> + * Now try to atomically clear the queue code.
>> + */
>> + if (atomic_cmpxchg(&lock->qlcode, my_qcode,
>> + _QSPINLOCK_LOCKED) == my_qcode)
>> + goto release_node;
>> + /*
>> + * The cmpxchg fails only if one or more tasks
>> + * are added to the queue. In this case, we need to
>> + * notify the next one to be the head of the queue.
>> + */
>> + goto notify_next;
>> + }
>> + /*
>> + * Accidentally steal the lock, release the lock and
>> + * let the queue head get it.
>> + */
>> + queue_spin_unlock(lock);
>> + } else
>> + prev_qcode&= ~_QSPINLOCK_LOCKED; /* Clear the lock bit */
>> + my_qcode&= ~_QSPINLOCK_LOCKED;
>> +#endif /* queue_code_xchg */
> WTF is this #ifdef for?
The #ifdef is harder to take away here. The point is that doing a 32-bit
exchange may accidentally steal the lock with the additional code to
handle that. Doing a 16-bit exchange, on the other hand, will never
steal the lock and so don't need the extra handling code. I could
construct a function with different return values to handle the
different cases if you think it will make the code easier to read.
>> + if (prev_qcode) {
>> + /*
>> + * Not at the queue head, get the address of the previous node
>> + * and set up the "next" fields of the that node.
>> + */
>> + struct qnode *prev = xlate_qcode(prev_qcode);
>> +
>> + ACCESS_ONCE(prev->next) = node;
>> + /*
>> + * Wait until the waiting flag is off
>> + */
>> + while (smp_load_acquire(&node->wait))
>> + arch_mutex_cpu_relax();
>> + }
>> +
>> + /*
>> + * At the head of the wait queue now
>> + */
>> + while (true) {
>> + u32 qcode;
>> + int retval;
>> +
>> + retval = queue_get_lock_qcode(lock,&qcode, my_qcode);
>> + if (retval> 0)
>> + ; /* Lock not available yet */
>> + else if (retval< 0)
>> + /* Lock taken, can release the node& return */
>> + goto release_node;
>> + else if (qcode != my_qcode) {
>> + /*
>> + * Just get the lock with other spinners waiting
>> + * in the queue.
>> + */
>> + if (queue_spin_trylock_unfair(lock))
>> + goto notify_next;
> Why is this an option at all?
>
>
Are you referring to the case (qcode != my_qcode)? This condition will
be true if more than one tasks have queued up.
-Longman
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