diff options
author | Will Deacon <will.deacon@arm.com> | 2017-10-12 13:20:51 +0100 |
---|---|---|
committer | Ingo Molnar <mingo@kernel.org> | 2017-10-25 10:57:25 +0200 |
commit | d133166146333e1f13fc81c0e6c43c8d99290a8a (patch) | |
tree | 538bbeeec8c861d98478704af79ee42dfbb95dea /kernel/locking | |
parent | 087133ac90763cd339b6b67f2998f87dcc136c52 (diff) |
locking/qrwlock: 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.
Tested-by: Waiman Long <longman@redhat.com>
Tested-by: Jeremy Linton <jeremy.linton@arm.com>
Tested-by: Adam Wallis <awallis@codeaurora.org>
Tested-by: Jan Glauber <jglauber@cavium.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Jeremy.Linton@arm.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lkml.kernel.org/r/1507810851-306-6-git-send-email-will.deacon@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'kernel/locking')
-rw-r--r-- | kernel/locking/qrwlock.c | 20 |
1 files changed, 5 insertions, 15 deletions
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, |