Messages in this thread Patch in this message | | | Subject | Re: [PATCH v4 14/16] locking/rwsem: Guard against making count negative | From | Waiman Long <> | Date | Tue, 23 Apr 2019 10:31:42 -0400 |
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On 4/23/19 10:17 AM, Peter Zijlstra wrote: > 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.
I have actually done some performance test measuring the effects of disabling interrupt and preemption on readers (on x86-64 system).
Threads Before patch Disable irq Disable preemption ------- ------------ ----------- ------------------ 1 9,088 8,766 9,172 2 9,296 9,169 8,707 4 11,192 11,205 10,712 8 11,329 11,332 11,213
For uncontended case, disable interrupt is slower. The slowdown is gone once the rwsem becomes contended. So it may not be a good idea to disable interrupt as a proxy of disabling preemption.
BTW, preemption count is not enabled in typical distro production kernels like RHEL. So preempt_disable() is just a barrier. It is turned on in the debug kernel, though.
>> +/* >> + * 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.
Yes, that can be done.
> >> 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.
Sure, I can do that.
Cheers, Longman
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