Re: [PATCH] kernel: Introduce a write lock/unlock wrapper for tasklist_lock

["Aiqun Yu (Maria)" <quic_aiquny@quicinc.com>]

On 12/29/2023 6:20 AM, Matthew Wilcox wrote:
> On Wed, Dec 13, 2023 at 12:27:05PM -0600, Eric W. Biederman wrote:
>> Matthew Wilcox <willy@infradead.org> writes:
>>> I think the right way to fix this is to pass a boolean flag to
>>> queued_write_lock_slowpath() to let it know whether it can re-enable
>>> interrupts while checking whether _QW_WAITING is set.
>>
>> Yes.  It seems to make sense to distinguish between write_lock_irq and
>> write_lock_irqsave and fix this for all of write_lock_irq.
> 
> I wasn't planning on doing anything here, but Hillf kind of pushed me into
> it.  I think it needs to be something like this.  Compile tested only.
> If it ends up getting used,
Happy new year!
Thx Metthew for chiming into this. I think more thoughts will gain more 
perfect designs.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> 
> diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
> index 75b8f4601b28..1152e080c719 100644
> --- a/include/asm-generic/qrwlock.h
> +++ b/include/asm-generic/qrwlock.h
> @@ -33,8 +33,8 @@
>   /*
>    * External function declarations
>    */
> -extern void queued_read_lock_slowpath(struct qrwlock *lock);
> -extern void queued_write_lock_slowpath(struct qrwlock *lock);
> +void queued_read_lock_slowpath(struct qrwlock *lock);
> +void queued_write_lock_slowpath(struct qrwlock *lock, bool irq);
>   
>   /**
>    * queued_read_trylock - try to acquire read lock of a queued rwlock
> @@ -98,7 +98,21 @@ static inline void queued_write_lock(struct qrwlock *lock)
>   	if (likely(atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED)))
>   		return;
>   
> -	queued_write_lock_slowpath(lock);
> +	queued_write_lock_slowpath(lock, false);
> +}
> +
> +/**
> + * queued_write_lock_irq - acquire write lock of a queued rwlock
> + * @lock : Pointer to queued rwlock structure
> + */
> +static inline void queued_write_lock_irq(struct qrwlock *lock)
> +{
> +	int cnts = 0;
> +	/* Optimize for the unfair lock case where the fair flag is 0. */
> +	if (likely(atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED)))
> +		return;
> +
> +	queued_write_lock_slowpath(lock, true);
>   }
>   
>   /**
> @@ -138,6 +152,7 @@ static inline int queued_rwlock_is_contended(struct qrwlock *lock)
>    */
>   #define arch_read_lock(l)		queued_read_lock(l)
>   #define arch_write_lock(l)		queued_write_lock(l)
> +#define arch_write_lock_irq(l)		queued_write_lock_irq(l)
>   #define arch_read_trylock(l)		queued_read_trylock(l)
>   #define arch_write_trylock(l)		queued_write_trylock(l)
>   #define arch_read_unlock(l)		queued_read_unlock(l)
> diff --git a/include/linux/rwlock.h b/include/linux/rwlock.h
> index c0ef596f340b..897010b6ba0a 100644
> --- a/include/linux/rwlock.h
> +++ b/include/linux/rwlock.h
> @@ -33,6 +33,7 @@ do {								\
>    extern int do_raw_read_trylock(rwlock_t *lock);
>    extern void do_raw_read_unlock(rwlock_t *lock) __releases(lock);
>    extern void do_raw_write_lock(rwlock_t *lock) __acquires(lock);
> + extern void do_raw_write_lock_irq(rwlock_t *lock) __acquires(lock);
>    extern int do_raw_write_trylock(rwlock_t *lock);
>    extern void do_raw_write_unlock(rwlock_t *lock) __releases(lock);
>   #else
> @@ -40,6 +41,7 @@ do {								\
>   # define do_raw_read_trylock(rwlock)	arch_read_trylock(&(rwlock)->raw_lock)
>   # define do_raw_read_unlock(rwlock)	do {arch_read_unlock(&(rwlock)->raw_lock); __release(lock); } while (0)
>   # define do_raw_write_lock(rwlock)	do {__acquire(lock); arch_write_lock(&(rwlock)->raw_lock); } while (0)
> +# define do_raw_write_lock_irq(rwlock)	do {__acquire(lock); arch_write_lock_irq(&(rwlock)->raw_lock); } while (0)
>   # define do_raw_write_trylock(rwlock)	arch_write_trylock(&(rwlock)->raw_lock)
>   # define do_raw_write_unlock(rwlock)	do {arch_write_unlock(&(rwlock)->raw_lock); __release(lock); } while (0)
>   #endif
> diff --git a/include/linux/rwlock_api_smp.h b/include/linux/rwlock_api_smp.h
> index dceb0a59b692..6257976dfb72 100644
> --- a/include/linux/rwlock_api_smp.h
> +++ b/include/linux/rwlock_api_smp.h
> @@ -193,7 +193,7 @@ static inline void __raw_write_lock_irq(rwlock_t *lock)
>   	local_irq_disable();
>   	preempt_disable();
>   	rwlock_acquire(&lock->dep_map, 0, 0, _RET_IP_);
> -	LOCK_CONTENDED(lock, do_raw_write_trylock, do_raw_write_lock);
> +	LOCK_CONTENDED(lock, do_raw_write_trylock, do_raw_write_lock_irq);
>   }
>   
>   static inline void __raw_write_lock_bh(rwlock_t *lock)
> diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
> index d2ef312a8611..6c644a71b01d 100644
> --- a/kernel/locking/qrwlock.c
> +++ b/kernel/locking/qrwlock.c
> @@ -61,9 +61,10 @@ EXPORT_SYMBOL(queued_read_lock_slowpath);
>   
>   /**
>    * queued_write_lock_slowpath - acquire write lock of a queued rwlock
> - * @lock : Pointer to queued rwlock structure
> + * @lock: Pointer to queued rwlock structure
> + * @irq: True if we can enable interrupts while spinning
>    */
> -void __lockfunc queued_write_lock_slowpath(struct qrwlock *lock)
> +void __lockfunc queued_write_lock_slowpath(struct qrwlock *lock, bool irq)
>   {
>   	int cnts;
>   
> @@ -82,7 +83,11 @@ void __lockfunc queued_write_lock_slowpath(struct qrwlock *lock)
>   
Also a new state showed up after the current design:
1. locked flag with _QW_WAITING, while irq enabled.
2. And this state will be only in interrupt context.
3. lock->wait_lock is hold by the write waiter.
So per my understanding, a different behavior also needed to be done in 
queued_write_lock_slowpath:
   when (unlikely(in_interrupt())) , get the lock directly.
So needed to be done in release path. This is to address Hillf's concern 
on possibility of deadlock.

Add Hillf here to merge thread. I am going to have a tested patch V2 
accordingly.
Feel free to let me know your thoughts prior on that.
>   	/* When no more readers or writers, set the locked flag */
>   	do {
> +		if (irq)
> +			local_irq_enable();
I think write_lock_irqsave also needs to be take account. So 
loal_irq_save(flags) should be take into account here.
>   		cnts = atomic_cond_read_relaxed(&lock->cnts, VAL == _QW_WAITING);
> +		if (irq)
> +			local_irq_disable();
ditto.
>   	} while (!atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED));
>   unlock:
>   	arch_spin_unlock(&lock->wait_lock);
> diff --git a/kernel/locking/spinlock_debug.c b/kernel/locking/spinlock_debug.c
> index 87b03d2e41db..bf94551d7435 100644
> --- a/kernel/locking/spinlock_debug.c
> +++ b/kernel/locking/spinlock_debug.c
> @@ -212,6 +212,13 @@ void do_raw_write_lock(rwlock_t *lock)
>   	debug_write_lock_after(lock);
>   }
>   
> +void do_raw_write_lock_irq(rwlock_t *lock)
> +{
> +	debug_write_lock_before(lock);
> +	arch_write_lock_irq(&lock->raw_lock);
> +	debug_write_lock_after(lock);
> +}
> +
>   int do_raw_write_trylock(rwlock_t *lock)
>   {
>   	int ret = arch_write_trylock(&lock->raw_lock);

-- 
Thx and BRs,
Aiqun(Maria) Yu