linux-kernel - [PATCH v2 1/2] locking/qrwlock: Reduce reader/writer to reader lock transfer latency

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Thu,  9 Jul 2015 12:32:22 -0400
From:	Waiman Long <Waiman.Long@...com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Will Deacon <will.deacon@....com>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>,
	Waiman Long <Waiman.Long@...com>
Subject: [PATCH v2 1/2] locking/qrwlock: Reduce reader/writer to reader lock transfer latency

Currently, a reader will check first to make sure that the writer mode
byte is cleared before incrementing the reader count. That waiting is
not really necessary. It increases the latency in the reader/writer
to reader transition and reduces readers performance.

This patch eliminates that waiting. It also has the side effect
of reducing the chance of writer lock stealing and improving the
fairness of the lock. Using a locking microbenchmark, a 10-threads 5M
locking loop of mostly readers (RW ratio = 10,000:1) has the following
performance numbers in a Haswell-EX box:

        Kernel          Locking Rate (Kops/s)
        ------          ---------------------
        4.1.1               15,063,081
        Patched 4.1.1       17,241,552

Signed-off-by: Waiman Long <Waiman.Long@...com>
---
 kernel/locking/qrwlock.c |   12 ++++--------
 1 files changed, 4 insertions(+), 8 deletions(-)

diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index d9c36c5..6a7a3b8 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -88,15 +88,11 @@ void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
 	arch_spin_lock(&lock->lock);
 
 	/*
-	 * At the head of the wait queue now, wait until the writer state
-	 * goes to 0 and then try to increment the reader count and get
-	 * the lock. It is possible that an incoming writer may steal the
-	 * lock in the interim, so it is necessary to check the writer byte
-	 * to make sure that the write lock isn't taken.
+	 * At the head of the wait queue now, increment the reader count
+	 * and wait until the writer, if it has the lock, has gone away.
+	 * At ths stage, it is not possible for a writer to remain in the
+	 * waiting state (_QW_WAITING). So there won't be any deadlock.
 	 */
-	while (atomic_read(&lock->cnts) & _QW_WMASK)
-		cpu_relax_lowlatency();
-
 	cnts = atomic_add_return(_QR_BIAS, &lock->cnts) - _QR_BIAS;
 	rspin_until_writer_unlock(lock, cnts);
 
-- 
1.7.1

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/