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From: Davidlohr Bueso <dave@stgolabs.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Will Deacon <will@kernel.org>, Waiman Long <longman@redhat.com>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Mike Galbraith <efault@gmx.de>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [RFC] locking: rwbase: Take care of ordering guarantee for fastpath reader
Date: Wed, 8 Sep 2021 11:34:49 -0700	[thread overview]
Message-ID: <20210908183449.hidfjw4rm65eesww@offworld> (raw)
In-Reply-To: <YTjNcD7nyLiChTIJ@hirez.programming.kicks-ass.net>

On Wed, 08 Sep 2021, Peter Zijlstra wrote:
>Subject: lockin/rwbase: Take care of ordering guarantee for fastpath reader

locking

>From: Boqun Feng <boqun.feng@gmail.com>
>Date: Wed, 1 Sep 2021 23:06:27 +0800
>
>From: Boqun Feng <boqun.feng@gmail.com>
>
>Readers of rwbase can lock and unlock without taking any inner lock, if
>that happens, we need the ordering provided by atomic operations to
>satisfy the ordering semantics of lock/unlock. Without that, considering
>the follow case:
>
>	{ X = 0 initially }
>
>	CPU 0			CPU 1
>	=====			=====
>				rt_write_lock();
>				X = 1
>				rt_write_unlock():
>				  atomic_add(READER_BIAS - WRITER_BIAS, ->readers);
>				  // ->readers is READER_BIAS.
>	rt_read_lock():
>	  if ((r = atomic_read(->readers)) < 0) // True
>	    atomic_try_cmpxchg(->readers, r, r + 1); // succeed.
>	  <acquire the read lock via fast path>
>
>	r1 = X;	// r1 may be 0, because nothing prevent the reordering
>		// of "X=1" and atomic_add() on CPU 1.
>
>Therefore audit every usage of atomic operations that may happen in a
>fast path, and add necessary barriers.
>
>Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
>Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>Link: https://lkml.kernel.org/r/20210901150627.620830-1-boqun.feng@gmail.com

With a few comments below, feel free to add my:

Reviewed-by: Davidlohr Bueso <dbueso@suse.de>

>---
> kernel/locking/rwbase_rt.c |   41 ++++++++++++++++++++++++++++++++++++-----
> 1 file changed, 36 insertions(+), 5 deletions(-)
>
>--- a/kernel/locking/rwbase_rt.c
>+++ b/kernel/locking/rwbase_rt.c
>@@ -41,6 +41,12 @@
>  * The risk of writer starvation is there, but the pathological use cases
>  * which trigger it are not necessarily the typical RT workloads.
>  *
>+ * Fast-path orderings:
>+ * The lock/unlock of readers can run in fast paths: lock and unlock are only
>+ * atomic ops, and there is no inner lock to provide ACQUIRE and RELEASE
>+ * semantics of rwbase_rt. Atomic ops then should be stronger than _acquire()
>+ * and _release() to provide necessary ordering guarantee.

This last part reads funky. Guarantees must be acquire/release or stronger, not
necessarily stronger than.

...
>@@ -210,14 +224,23 @@ static int __sched rwbase_write_lock(str
>	atomic_sub(READER_BIAS, &rwb->readers);
>
>	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
>+
>+	/* The below set_*_state() thingy implies smp_mb() to provide ACQUIRE */
>+	readers = atomic_read(&rwb->readers);
>	/*
>	 * set_current_state() for rw_semaphore
>	 * current_save_and_set_rtlock_wait_state() for rwlock
>	 */
>	rwbase_set_and_save_current_state(state);
>
>-	/* Block until all readers have left the critical section. */
>-	for (; atomic_read(&rwb->readers);) {
>+	/*
>+	 * Block until all readers have left the critical section.
>+	 *
>+	 * _acqurie() is needed in case that the reader side runs in the fast
	   ^acquire

Thanks,
Davidlohr

  reply	other threads:[~2021-09-08 18:35 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-01 15:06 Boqun Feng
2021-09-01 18:53 ` Waiman Long
2021-09-01 20:22 ` Davidlohr Bueso
2021-09-02  5:02   ` Boqun Feng
2021-09-02 11:55 ` Peter Zijlstra
2021-09-03 14:50   ` Boqun Feng
2021-09-04 10:12     ` Peter Zijlstra
2021-09-04 10:14     ` Peter Zijlstra
2021-09-04 10:19       ` Boqun Feng
2021-09-08 11:51 ` Peter Zijlstra
2021-09-08 12:14   ` Peter Zijlstra
2021-09-08 13:00     ` Boqun Feng
2021-09-08 13:08   ` Boqun Feng
2021-09-08 14:41     ` Peter Zijlstra
2021-09-08 14:49       ` Peter Zijlstra
2021-09-08 18:34         ` Davidlohr Bueso [this message]
2021-09-08 13:27   ` Boqun Feng

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