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

From: Peter Zijlstra <peterz@infradead.org>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Eric Dumazet <edumazet@google.com>,
	Ingo Molnar <mingo@redhat.com>, Waiman Long <longman@redhat.com>,
	Boqun Feng <boqun.feng@gmail.com>, Will Deacon <will@kernel.org>,
	Roman Penyaev <rpenyaev@suse.de>,
	Shakeel Butt <shakeelb@google.com>
Subject: Re: [PATCH] locking/rwlocks: do not starve writers
Date: Fri, 17 Jun 2022 14:07:27 +0200	[thread overview]
Message-ID: <YqxufxqsnHjVfQOs@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <20220617091039.2257083-1-eric.dumazet@gmail.com>

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.