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Message-ID: <YqxufxqsnHjVfQOs@worktop.programming.kicks-ass.net>
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.
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