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

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Date:   Fri, 17 Jun 2022 14:07:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will@...nel.org>,
        Roman Penyaev <rpenyaev@...e.de>,
        Shakeel Butt <shakeelb@...gle.com>
Subject: Re: [PATCH] locking/rwlocks: do not starve writers

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.