Re: [PATCH] locking/rwlocks: do not starve writers

[Peter Zijlstra]

On Fri, Jun 17, 2022 at 02:10:39AM -0700, Eric Dumazet wrote:
> --- a/kernel/locking/qrwlock.c
> +++ b/kernel/locking/qrwlock.c
> @@ -23,16 +23,6 @@ void queued_read_lock_slowpath(struct qrwlock *lock)
>  	/*
>  	 * Readers come here when they cannot get the lock without waiting
>  	 */
> -	if (unlikely(in_interrupt())) {
> -		/*
> -		 * Readers in interrupt context will get the lock immediately
> -		 * if the writer is just waiting (not holding the lock yet),
> -		 * so spin with ACQUIRE semantics until the lock is available
> -		 * without waiting in the queue.
> -		 */
> -		atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
> -		return;
> -	}
>  	atomic_sub(_QR_BIAS, &lock->cnts);
>  
>  	trace_contention_begin(lock, LCB_F_SPIN | LCB_F_READ);

This is known to break tasklist_lock.