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Subject[PATCH 5/6] kernel/locking: Prevent slowpath writers getting held up by fastpath
Date
When a prospective writer takes the qrwlock locking slowpath due to the
lock being held, it attempts to cmpxchg the wmode field from 0 to
_QW_WAITING so that concurrent lockers also take the slowpath and queue
on the spinlock accordingly, allowing the lockers to drain.

Unfortunately, this isn't fair, because a fastpath writer that comes in
after the lock is made available but before the _QW_WAITING flag is set
can effectively jump the queue. If there is a steady stream of prospective
writers, then the waiter will be held off indefinitely.

This patch restores fairness by separating _QW_WAITING and _QW_LOCKED
into two bits in the wmode byte and having the waiter set _QW_WAITING
unconditionally. This then forces the slow-path for concurrent lockers,
but requires that a writer unlock operation performs an
atomic_sub_release instead of a store_release so that the waiting status
is preserved.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Waiman Long <longman@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
---
include/asm-generic/qrwlock.h | 4 ++--
kernel/locking/qrwlock.c | 20 +++++---------------
2 files changed, 7 insertions(+), 17 deletions(-)

diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
index 02c0a768e6b0..8b7edef500e5 100644
--- a/include/asm-generic/qrwlock.h
+++ b/include/asm-generic/qrwlock.h
@@ -41,7 +41,7 @@
* +----+----+----+----+
*/
#define _QW_WAITING 1 /* A writer is waiting */
-#define _QW_LOCKED 0xff /* A writer holds the lock */
+#define _QW_LOCKED 2 /* A writer holds the lock */
#define _QW_WMASK 0xff /* Writer mask */
#define _QR_SHIFT 8 /* Reader count shift */
#define _QR_BIAS (1U << _QR_SHIFT)
@@ -134,7 +134,7 @@ static inline void queued_read_unlock(struct qrwlock *lock)
*/
static inline void queued_write_unlock(struct qrwlock *lock)
{
- smp_store_release(&lock->wmode, 0);
+ (void)atomic_sub_return_release(_QW_LOCKED, &lock->cnts);
}

/*
diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index b7ea4647c74d..e940f2c2b4f2 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -40,8 +40,7 @@ void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
* so spin with ACQUIRE semantics until the lock is available
* without waiting in the queue.
*/
- atomic_cond_read_acquire(&lock->cnts, (VAL & _QW_WMASK)
- != _QW_LOCKED);
+ atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
return;
}
atomic_sub(_QR_BIAS, &lock->cnts);
@@ -57,7 +56,7 @@ void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
* that accesses can't leak upwards out of our subsequent critical
* section in the case that the lock is currently held for write.
*/
- atomic_cond_read_acquire(&lock->cnts, (VAL & _QW_WMASK) != _QW_LOCKED);
+ atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));

/*
* Signal the next one in queue to become queue head
@@ -80,19 +79,10 @@ void queued_write_lock_slowpath(struct qrwlock *lock)
(atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
goto unlock;

- /*
- * Set the waiting flag to notify readers that a writer is pending,
- * or wait for a previous writer to go away.
- */
- for (;;) {
- if (!READ_ONCE(lock->wmode) &&
- (cmpxchg_relaxed(&lock->wmode, 0, _QW_WAITING) == 0))
- break;
-
- cpu_relax();
- }
+ /* Set the waiting flag to notify readers that a writer is pending */
+ atomic_add(_QW_WAITING, &lock->cnts);

- /* When no more readers, set the locked flag */
+ /* When no more readers or writers, set the locked flag */
do {
atomic_cond_read_acquire(&lock->cnts, VAL == _QW_WAITING);
} while (atomic_cmpxchg_relaxed(&lock->cnts, _QW_WAITING,
--
2.1.4
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