Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 23 Apr 2019 16:17:14 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Waiman Long <llong@redhat.com>
Cc:     Waiman Long <longman@redhat.com>, Ingo Molnar <mingo@redhat.com>,
        Will Deacon <will.deacon@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        Davidlohr Bueso <dave@stgolabs.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        huang ying <huang.ying.caritas@gmail.com>
Subject: Re: [PATCH v4 14/16] locking/rwsem: Guard against making count
 negative
Message-ID: <20190423141714.GO11158@hirez.programming.kicks-ass.net>
References: <20190418135151.GB12232@hirez.programming.kicks-ass.net>
 <d931916b-41f5-f12d-8c38-0cccbc00a301@redhat.com>
 <20190418144036.GE12232@hirez.programming.kicks-ass.net>
 <4cbd3c18-c9c0-56eb-4e01-ee355a69057a@redhat.com>
 <20190419102647.GP7905@worktop.programming.kicks-ass.net>
 <20190419120207.GO4038@hirez.programming.kicks-ass.net>
 <20190419130304.GV14281@hirez.programming.kicks-ass.net>
 <20190419131522.GW14281@hirez.programming.kicks-ass.net>
 <57620139-92a3-4a21-56bd-5d6fff23214f@redhat.com>
 <7b1bfc26-6e90-bd65-ab46-08413acd80e9@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <7b1bfc26-6e90-bd65-ab46-08413acd80e9@redhat.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Sun, Apr 21, 2019 at 05:07:56PM -0400, Waiman Long wrote:

> How about the following chunks to disable preemption temporarily for the
> increment-check-decrement sequence?
> 
> diff --git a/include/linux/preempt.h b/include/linux/preempt.h
> index dd92b1a93919..4cc03ac66e13 100644
> --- a/include/linux/preempt.h
> +++ b/include/linux/preempt.h
> @@ -250,6 +250,8 @@ do { \
> ?#define preempt_enable_notrace()?????????????? barrier()
> ?#define preemptible()????????????????????????? 0
> ?
> +#define __preempt_disable_nop? /* preempt_disable() is nop */
> +
> ?#endif /* CONFIG_PREEMPT_COUNT */
> ?
> ?#ifdef MODULE
> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> index 043fd29b7534..54029e6af17b 100644
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -256,11 +256,64 @@ static inline struct task_struct
> *rwsem_get_owner(struct r
> ??????? return (struct task_struct *) (cowner
> ??????????????? ? cowner | (sowner & RWSEM_NONSPINNABLE) : sowner);
> ?}
> +
> +/*
> + * If __preempt_disable_nop is defined, calling preempt_disable() and
> + * preempt_enable() directly is the most efficient way. Otherwise, it may
> + * be more efficient to disable and enable interrupt instead for disabling
> + * preemption tempoarily.
> + */
> +#ifdef __preempt_disable_nop
> +#define disable_preemption()?? preempt_disable()
> +#define enable_preemption()??? preempt_enable()
> +#else
> +#define disable_preemption()?? local_irq_disable()
> +#define enable_preemption()??? local_irq_enable()
> +#endif

I'm not aware of an architecture where disabling interrupts is faster
than disabling preemption.

> +/*
> + * When the owner task structure pointer is merged into couunt, less bits
> + * will be available for readers. Therefore, there is a very slight chance
> + * that the reader count may overflow. We try to prevent that from
> happening
> + * by checking for the MS bit of the count and failing the trylock attempt
> + * if this bit is set.
> + *
> + * With preemption enabled, there is a remote possibility that preemption
> + * can happen in the narrow timing window between incrementing and
> + * decrementing the reader count and the task is put to sleep for a
> + * considerable amount of time. If sufficient number of such unfortunate
> + * sequence of events happen, we may still overflow the reader count.
> + * To avoid such possibility, we have to disable preemption for the
> + * whole increment-check-decrement sequence.
> + *
> + * The function returns true if there are too many readers and the count
> + * has already been properly decremented so the reader must go directly
> + * into the wait list.
> + */
> +static inline bool rwsem_read_trylock(struct rw_semaphore *sem, long *cnt)
> +{
> +?????? bool wait = false;????? /* Wait now flag */
> +
> +?????? disable_preemption();
> +?????? *cnt = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
> &sem->count);
> +?????? if (unlikely(*cnt < 0)) {
> +?????????????? atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
> +?????????????? wait = true;
> +?????? }
> +?????? enable_preemption();
> +?????? return wait;
> +}
> ?#else /* !CONFIG_RWSEM_OWNER_COUNT */

This also means you have to ensure CONFIG_NR_CPUS < 32K for
RWSEM_OWNER_COUNT.

> ?static inline struct task_struct *rwsem_get_owner(struct rw_semaphore *sem)
> ?{
> ??????? return READ_ONCE(sem->owner);
> ?}
> +
> +static inline bool rwsem_read_trylock(struct rw_semaphore *sem, long *cnt)
> +{
> +?????? *cnt = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
> &sem->count);
> +?????? return false;
> +}
> ?#endif /* CONFIG_RWSEM_OWNER_COUNT */
> ?
> ?/*
> @@ -981,32 +1034,18 @@ static inline void clear_wr_nonspinnable(struct
> rw_semaph
> ? * Wait for the read lock to be granted
> ? */
> ?static struct rw_semaphore __sched *
> -rwsem_down_read_slowpath(struct rw_semaphore *sem, int state, long count)
> +rwsem_down_read_slowpath(struct rw_semaphore *sem, int state, const
> bool wait)
> ?{
> -?????? long adjustment = -RWSEM_READER_BIAS;
> +?????? long count, adjustment = -RWSEM_READER_BIAS;
> ??????? bool wake = false;
> ??????? struct rwsem_waiter waiter;
> ??????? DEFINE_WAKE_Q(wake_q);
> ?
> -?????? if (unlikely(count < 0)) {
> +?????? if (unlikely(wait)) {
> ??????????????? /*
> -??????????????? * The sign bit has been set meaning that too many active
> -??????????????? * readers are present. We need to decrement reader count &
> -??????????????? * enter wait queue immediately to avoid overflowing the
> -??????????????? * reader count.
> -??????????????? *
> -??????????????? * As preemption is not disabled, there is a remote
> -??????????????? * possibility that preemption can happen in the narrow
> -??????????????? * timing window between incrementing and decrementing
> -??????????????? * the reader count and the task is put to sleep for a
> -??????????????? * considerable amount of time. If sufficient number
> -??????????????? * of such unfortunate sequence of events happen, we
> -??????????????? * may still overflow the reader count. It is extremely
> -??????????????? * unlikey, though. If this is a concern, we should consider
> -??????????????? * disable preemption during this timing window to make
> -??????????????? * sure that such unfortunate event will not happen.
> +??????????????? * The reader count has already been decremented and the
> +??????????????? * reader should go directly into the wait list now.
> ???????????????? */
> -?????????????? atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
> ??????????????? adjustment = 0;
> ??????????????? goto queue;
> ??????? }
> @@ -1358,11 +1397,12 @@ static struct rw_semaphore
> *rwsem_downgrade_wake(struct
> ? */
> ?inline void __down_read(struct rw_semaphore *sem)
> ?{
> -?????? long tmp = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
> -??????????????????????????????????????????????? &sem->count);
> +?????? long tmp;
> +?????? bool wait;
> ?
> +?????? wait = rwsem_read_trylock(sem, &tmp);
> ??????? if (unlikely(tmp & RWSEM_READ_FAILED_MASK)) {
> -?????????????? rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE, tmp);
> +?????????????? rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE, wait);
> ??????????????? DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
> ??????? } else {
> ??????????????? rwsem_set_reader_owned(sem);

I think I prefer that function returning/taking the bias/adjustment
value instead of a bool, if it is all the same.