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Message-ID: <tip-b519b56e378ee82caf9b079b04f5db87dedc3251@git.kernel.org>
Date: Wed, 25 Oct 2017 03:14:30 -0700
From: tip-bot for Will Deacon <tipbot@...or.com>
To: linux-tip-commits@...r.kernel.org
Cc: tglx@...utronix.de, longman@...hat.com, hpa@...or.com,
torvalds@...ux-foundation.org, mingo@...nel.org,
will.deacon@....com, linux-kernel@...r.kernel.org,
awallis@...eaurora.org, jeremy.linton@....com,
boqun.feng@...il.com, peterz@...radead.org,
paulmck@...ux.vnet.ibm.com, jglauber@...ium.com
Subject: [tip:locking/core] locking/qrwlock: Use atomic_cond_read_acquire()
when spinning in qrwlock
Commit-ID: b519b56e378ee82caf9b079b04f5db87dedc3251
Gitweb: https://git.kernel.org/tip/b519b56e378ee82caf9b079b04f5db87dedc3251
Author: Will Deacon <will.deacon@....com>
AuthorDate: Thu, 12 Oct 2017 13:20:49 +0100
Committer: Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 25 Oct 2017 10:57:24 +0200
locking/qrwlock: Use atomic_cond_read_acquire() when spinning in qrwlock
The qrwlock slowpaths involve spinning when either a prospective reader
is waiting for a concurrent writer to drain, or a prospective writer is
waiting for concurrent readers to drain. In both of these situations,
atomic_cond_read_acquire() can be used to avoid busy-waiting and make use
of any backoff functionality provided by the architecture.
This patch replaces the open-code loops and rspin_until_writer_unlock()
implementation with atomic_cond_read_acquire(). The write mode transition
zero to _QW_WAITING is left alone, since (a) this doesn't need acquire
semantics and (b) should be fast.
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>
Acked-by: Peter Zijlstra <peterz@...radead.org>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: Jeremy.Linton@....com
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: linux-arm-kernel@...ts.infradead.org
Link: http://lkml.kernel.org/r/1507810851-306-4-git-send-email-will.deacon@arm.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
include/asm-generic/qrwlock.h | 4 ++--
kernel/locking/qrwlock.c | 50 +++++++++++--------------------------------
2 files changed, 14 insertions(+), 40 deletions(-)
diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
index 02c0a76..c716b02 100644
--- a/include/asm-generic/qrwlock.h
+++ b/include/asm-generic/qrwlock.h
@@ -49,7 +49,7 @@
/*
* External function declarations
*/
-extern void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts);
+extern void queued_read_lock_slowpath(struct qrwlock *lock);
extern void queued_write_lock_slowpath(struct qrwlock *lock);
/**
@@ -100,7 +100,7 @@ static inline void queued_read_lock(struct qrwlock *lock)
return;
/* The slowpath will decrement the reader count, if necessary. */
- queued_read_lock_slowpath(lock, cnts);
+ queued_read_lock_slowpath(lock);
}
/**
diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index 1af791e..5825e0f 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -24,28 +24,10 @@
#include <asm/qrwlock.h>
/**
- * rspin_until_writer_unlock - inc reader count & spin until writer is gone
- * @lock : Pointer to queue rwlock structure
- * @writer: Current queue rwlock writer status byte
- *
- * In interrupt context or at the head of the queue, the reader will just
- * increment the reader count & wait until the writer releases the lock.
- */
-static __always_inline void
-rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts)
-{
- while ((cnts & _QW_WMASK) == _QW_LOCKED) {
- cpu_relax();
- cnts = atomic_read_acquire(&lock->cnts);
- }
-}
-
-/**
* queued_read_lock_slowpath - acquire read lock of a queue rwlock
* @lock: Pointer to queue rwlock structure
- * @cnts: Current qrwlock lock value
*/
-void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
+void queued_read_lock_slowpath(struct qrwlock *lock)
{
/*
* Readers come here when they cannot get the lock without waiting
@@ -53,13 +35,12 @@ void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
if (unlikely(in_interrupt())) {
/*
* Readers in interrupt context will get the lock immediately
- * if the writer is just waiting (not holding the lock yet).
- * The rspin_until_writer_unlock() function returns immediately
- * in this case. Otherwise, they will spin (with ACQUIRE
- * semantics) until the lock is available without waiting in
- * the queue.
+ * if the writer is just waiting (not holding the lock yet),
+ * so spin with ACQUIRE semantics until the lock is available
+ * without waiting in the queue.
*/
- rspin_until_writer_unlock(lock, cnts);
+ atomic_cond_read_acquire(&lock->cnts, (VAL & _QW_WMASK)
+ != _QW_LOCKED);
return;
}
atomic_sub(_QR_BIAS, &lock->cnts);
@@ -68,14 +49,14 @@ void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
* Put the reader into the wait queue
*/
arch_spin_lock(&lock->wait_lock);
+ atomic_add(_QR_BIAS, &lock->cnts);
/*
* The ACQUIRE semantics of the following spinning code ensure
* that accesses can't leak upwards out of our subsequent critical
* section in the case that the lock is currently held for write.
*/
- cnts = atomic_fetch_add_acquire(_QR_BIAS, &lock->cnts);
- rspin_until_writer_unlock(lock, cnts);
+ atomic_cond_read_acquire(&lock->cnts, (VAL & _QW_WMASK) != _QW_LOCKED);
/*
* Signal the next one in queue to become queue head
@@ -90,8 +71,6 @@ EXPORT_SYMBOL(queued_read_lock_slowpath);
*/
void queued_write_lock_slowpath(struct qrwlock *lock)
{
- u32 cnts;
-
/* Put the writer into the wait queue */
arch_spin_lock(&lock->wait_lock);
@@ -113,15 +92,10 @@ void queued_write_lock_slowpath(struct qrwlock *lock)
}
/* When no more readers, set the locked flag */
- for (;;) {
- cnts = atomic_read(&lock->cnts);
- if ((cnts == _QW_WAITING) &&
- (atomic_cmpxchg_acquire(&lock->cnts, _QW_WAITING,
- _QW_LOCKED) == _QW_WAITING))
- break;
-
- cpu_relax();
- }
+ do {
+ atomic_cond_read_acquire(&lock->cnts, VAL == _QW_WAITING);
+ } while (atomic_cmpxchg_relaxed(&lock->cnts, _QW_WAITING,
+ _QW_LOCKED) != _QW_WAITING);
unlock:
arch_spin_unlock(&lock->wait_lock);
}
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