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locking: Remove spin_unlock_wait() generic definitions

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There is no agreed-upon definition of spin_unlock_wait()'s semantics,
and it appears that all callers could do just as well with a lock/unlock
pair.  This commit therefore removes spin_unlock_wait() and related
definitions from core code.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Andrea Parri <parri.andrea@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Paul E. McKenney 2017-06-29 15:47:44 -07:00
parent a4f08141d0
commit d3a024abbc
4 changed files with 0 additions and 148 deletions
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117
kernel/locking/qspinlock.c
View file

@ -268,123 +268,6 @@ static __always_inline u32 __pv_wait_head_or_lock(struct qspinlock *lock,
#define queued_spin_lock_slowpath native_queued_spin_lock_slowpath
#endif
/*
* Various notes on spin_is_locked() and spin_unlock_wait(), which are
* 'interesting' functions:
*
* PROBLEM: some architectures have an interesting issue with atomic ACQUIRE
* operations in that the ACQUIRE applies to the LOAD _not_ the STORE (ARM64,
* PPC). Also qspinlock has a similar issue per construction, the setting of
* the locked byte can be unordered acquiring the lock proper.
*
* This gets to be 'interesting' in the following cases, where the /should/s
* end up false because of this issue.
*
*
* CASE 1:
*
* So the spin_is_locked() correctness issue comes from something like:
*
* CPU0 CPU1
*
* global_lock(); local_lock(i)
* spin_lock(&G) spin_lock(&L[i])
* for (i) if (!spin_is_locked(&G)) {
* spin_unlock_wait(&L[i]); smp_acquire__after_ctrl_dep();
* return;
* }
* // deal with fail
*
* Where it is important CPU1 sees G locked or CPU0 sees L[i] locked such
* that there is exclusion between the two critical sections.
*
* The load from spin_is_locked(&G) /should/ be constrained by the ACQUIRE from
* spin_lock(&L[i]), and similarly the load(s) from spin_unlock_wait(&L[i])
* /should/ be constrained by the ACQUIRE from spin_lock(&G).
*
* Similarly, later stuff is constrained by the ACQUIRE from CTRL+RMB.
*
*
* CASE 2:
*
* For spin_unlock_wait() there is a second correctness issue, namely:
*
* CPU0 CPU1
*
* flag = set;
* smp_mb(); spin_lock(&l)
* spin_unlock_wait(&l); if (!flag)
* // add to lockless list
* spin_unlock(&l);
* // iterate lockless list
*
* Which wants to ensure that CPU1 will stop adding bits to the list and CPU0
* will observe the last entry on the list (if spin_unlock_wait() had ACQUIRE
* semantics etc..)
*
* Where flag /should/ be ordered against the locked store of l.
*/
/*
* queued_spin_lock_slowpath() can (load-)ACQUIRE the lock before
* issuing an _unordered_ store to set _Q_LOCKED_VAL.
*
* This means that the store can be delayed, but no later than the
* store-release from the unlock. This means that simply observing
* _Q_LOCKED_VAL is not sufficient to determine if the lock is acquired.
*
* There are two paths that can issue the unordered store:
*
* (1) clear_pending_set_locked(): *,1,0 -> *,0,1
*
* (2) set_locked(): t,0,0 -> t,0,1 ; t != 0
* atomic_cmpxchg_relaxed(): t,0,0 -> 0,0,1
*
* However, in both cases we have other !0 state we've set before to queue
* ourseves:
*
* For (1) we have the atomic_cmpxchg_acquire() that set _Q_PENDING_VAL, our
* load is constrained by that ACQUIRE to not pass before that, and thus must
* observe the store.
*
* For (2) we have a more intersting scenario. We enqueue ourselves using
* xchg_tail(), which ends up being a RELEASE. This in itself is not
* sufficient, however that is followed by an smp_cond_acquire() on the same
* word, giving a RELEASE->ACQUIRE ordering. This again constrains our load and
* guarantees we must observe that store.
*
* Therefore both cases have other !0 state that is observable before the
* unordered locked byte store comes through. This means we can use that to
* wait for the lock store, and then wait for an unlock.
*/
#ifndef queued_spin_unlock_wait
void queued_spin_unlock_wait(struct qspinlock *lock)
{
u32 val;
for (;;) {
val = atomic_read(&lock->val);
if (!val) /* not locked, we're done */
goto done;
if (val & _Q_LOCKED_MASK) /* locked, go wait for unlock */
break;
/* not locked, but pending, wait until we observe the lock */
cpu_relax();
}
/* any unlock is good */
while (atomic_read(&lock->val) & _Q_LOCKED_MASK)
cpu_relax();
done:
smp_acquire__after_ctrl_dep();
}
EXPORT_SYMBOL(queued_spin_unlock_wait);
#endif
#endif /* _GEN_PV_LOCK_SLOWPATH */
/**