Re: [PATCH v15 09/15] pvqspinlock: Implement simple paravirt support for the qspinlock

From: Waiman Long
Date: Mon Apr 13 2015 - 11:51:49 EST

On 04/13/2015 11:08 AM, Peter Zijlstra wrote:
On Thu, Apr 09, 2015 at 05:41:44PM -0400, Waiman Long wrote:

+static void pv_wait_head(struct qspinlock *lock, struct mcs_spinlock *node)
+ struct __qspinlock *l = (void *)lock;
+ struct qspinlock **lp = NULL;
+ struct pv_node *pn = (struct pv_node *)node;
+ int slow_set = false;
+ int loop;
+ for (;;) {
+ for (loop = SPIN_THRESHOLD; loop; loop--) {
+ if (!READ_ONCE(l->locked))
+ return;
+ cpu_relax();
+ }
+ WRITE_ONCE(pn->state, vcpu_halted);
+ if (!lp)
+ lp = pv_hash(lock, pn);
+ /*
+ * lp must be set before setting _Q_SLOW_VAL
+ *
+ * [S] lp = lock [RmW] l = l->locked = 0
+ * MB MB
+ * [S] l->locked = _Q_SLOW_VAL [L] lp
+ *
+ * Matches the cmpxchg() in pv_queue_spin_unlock().
+ */
+ if (!slow_set&&
+ !cmpxchg(&l->locked, _Q_LOCKED_VAL, _Q_SLOW_VAL)) {
+ /*
+ * The lock is free and _Q_SLOW_VAL has never been
+ * set. Need to clear the hash bucket before getting
+ * the lock.
+ */
+ return;
+ } else if (slow_set&& !READ_ONCE(l->locked))
+ return;
+ slow_set = true;
I'm somewhat puzzled by the slow_set thing; what is wrong with the thing
I had, namely:

if (!lp) {
lp = pv_hash(lock, pn);

* comment
lv = cmpxchg(&l->locked, _Q_LOCKED_VAL, _Q_SLOW_VAL);
if (lv != _Q_LOCKED_VAL) {
/* we're woken, unhash and return */
+ pv_wait(&l->locked, _Q_SLOW_VAL);
If we get a spurious wakeup (due to device interrupts or random kick)
we'll loop around but ->locked will remain _Q_SLOW_VAL.
The purpose of the slow_set flag is not about the lock value. It is to make
sure that pv_hash_find() will always find a match. Consider the following

cpu1 cpu2 cpu3
---- ---- ----
spurious wakeup
loop l->locked

read _Q_SLOW_VAL

pv_hash()<- same entry

clear hash (?)

Here, the entry for cpu3 will be removed leading to panic when
pv_hash_find() can find the entry. So the hash entry can only be
removed if the other cpu has no chance to see _Q_SLOW_VAL.
Still confused. Afaict that cannot happen with my code. We only do the
cmpxchg(, _Q_SLOW_VAL) _once_.

Only on the first time around that loop will we hash the lock and set
the slow flag. And cpu3 cannot come in on the same entry because we've
not yet released the lock when we find and unhash.

Maybe I am not clear in my illustration. More than one locks can be hashed to the same value. So cpu3 is accessing a different lock which has the same hashed value as the lock used by cpu1 and cpu2.

Anyway, I remove the slow_set flag by unrolling the retry loop so that after pv_wait(), it goes into the 2nd loop instead of going back to the top. As a result, cmpxchg and pv_hash can only be called once.

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