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Date: Thu, 12 Oct 2017 13:20:51 +0100
From: Will Deacon <will.deacon@....com>
To: linux-kernel@...r.kernel.org
Cc: linux-arm-kernel@...ts.infradead.org, Jeremy.Linton@....com,
peterz@...radead.org, mingo@...hat.com, longman@...hat.com,
boqun.feng@...il.com, paulmck@...ux.vnet.ibm.com,
Will Deacon <will.deacon@....com>
Subject: [PATCH v3 5/5] kernel/locking: Prevent slowpath writers getting held up by fastpath
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 distinct fields: _QW_LOCKED continues to occupy the bottom byte
of the lockword so that it can be cleared unconditionally when unlocking,
but _QW_WAITING now occupies what used to be the bottom bit of the reader
count. This then forces the slow-path for concurrent lockers.
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Waiman Long <longman@...hat.com>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Tested-by: Waiman Long <longman@...hat.com>
Tested-by: Jeremy Linton <jeremy.linton@....com>
Tested-by: Adam Wallis <awallis@...eaurora.org>
Tested-by: Jan Glauber <jglauber@...ium.com>
Signed-off-by: Will Deacon <will.deacon@....com>
---
include/asm-generic/qrwlock.h | 23 +++++------------------
include/asm-generic/qrwlock_types.h | 8 ++++----
kernel/locking/qrwlock.c | 20 +++++---------------
3 files changed, 14 insertions(+), 37 deletions(-)
diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
index c716b02e8fd7..0f7062bd55e5 100644
--- a/include/asm-generic/qrwlock.h
+++ b/include/asm-generic/qrwlock.h
@@ -26,24 +26,11 @@
/*
* Writer states & reader shift and bias.
- *
- * | +0 | +1 | +2 | +3 |
- * ----+----+----+----+----+
- * LE | 78 | 56 | 34 | 12 | 0x12345678
- * ----+----+----+----+----+
- * | wr | rd |
- * +----+----+----+----+
- *
- * ----+----+----+----+----+
- * BE | 12 | 34 | 56 | 78 | 0x12345678
- * ----+----+----+----+----+
- * | rd | wr |
- * +----+----+----+----+
*/
-#define _QW_WAITING 1 /* A writer is waiting */
-#define _QW_LOCKED 0xff /* A writer holds the lock */
-#define _QW_WMASK 0xff /* Writer mask */
-#define _QR_SHIFT 8 /* Reader count shift */
+#define _QW_WAITING 0x100 /* A writer is waiting */
+#define _QW_LOCKED 0x0ff /* A writer holds the lock */
+#define _QW_WMASK 0x1ff /* Writer mask */
+#define _QR_SHIFT 9 /* Reader count shift */
#define _QR_BIAS (1U << _QR_SHIFT)
/*
@@ -134,7 +121,7 @@ static inline void queued_read_unlock(struct qrwlock *lock)
*/
static inline void queued_write_unlock(struct qrwlock *lock)
{
- smp_store_release(&lock->wmode, 0);
+ smp_store_release(&lock->wlocked, 0);
}
/*
diff --git a/include/asm-generic/qrwlock_types.h b/include/asm-generic/qrwlock_types.h
index 507f2dc51bba..8af752acbdc0 100644
--- a/include/asm-generic/qrwlock_types.h
+++ b/include/asm-generic/qrwlock_types.h
@@ -13,11 +13,11 @@ typedef struct qrwlock {
atomic_t cnts;
struct {
#ifdef __LITTLE_ENDIAN
- u8 wmode; /* Writer mode */
- u8 rcnts[3]; /* Reader counts */
+ u8 wlocked; /* Locked for write? */
+ u8 __lstate[3];
#else
- u8 rcnts[3]; /* Reader counts */
- u8 wmode; /* Writer mode */
+ u8 __lstate[3];
+ u8 wlocked; /* Locked for write? */
#endif
};
};
diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index 5825e0fc1a8e..c7471c3fb798 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -39,8 +39,7 @@ void queued_read_lock_slowpath(struct qrwlock *lock)
* 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);
@@ -56,7 +55,7 @@ void queued_read_lock_slowpath(struct qrwlock *lock)
* 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
@@ -79,19 +78,10 @@ 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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