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Date: Tue, 26 Dec 2023 20:49:38 +0000
From: Matthew Wilcox <willy@infradead.org>
To: Hillf Danton <hdanton@sina.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
	Maria Yu <quic_aiquny@quicinc.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] kernel: Introduce a write lock/unlock wrapper for
 tasklist_lock
Message-ID: <ZYs8Yp/7TovnVMLC@casper.infradead.org>
References: <20231213101745.4526-1-quic_aiquny@quicinc.com>
 <ZXnaNSrtaWbS2ivU@casper.infradead.org>
 <20231226104652.1491-1-hdanton@sina.com>
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In-Reply-To: <20231226104652.1491-1-hdanton@sina.com>

On Tue, Dec 26, 2023 at 06:46:52PM +0800, Hillf Danton wrote:
> On Wed, 13 Dec 2023 12:27:05 -0600 Eric W. Biederman <ebiederm@xmission.com>
> > Matthew Wilcox <willy@infradead.org> writes:
> > > On Wed, Dec 13, 2023 at 06:17:45PM +0800, Maria Yu wrote:
> > >> +static inline void write_lock_tasklist_lock(void)
> > >> +{
> > >> +	while (1) {
> > >> +		local_irq_disable();
> > >> +		if (write_trylock(&tasklist_lock))
> > >> +			break;
> > >> +		local_irq_enable();
> > >> +		cpu_relax();
> > >
> > > This is a bad implementation though.  You don't set the _QW_WAITING flag
> > > so readers don't know that there's a pending writer.  Also, I've seen
> > > cpu_relax() pessimise CPU behaviour; putting it into a low-power mode
> > > that takes a while to wake up from.
> > >
> > > 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.
> 
> 	lock(&lock->wait_lock)
> 	enable irq
> 	int
> 	lock(&lock->wait_lock)
> 
> You are adding chance for recursive locking.

Did you bother to read queued_read_lock_slowpath() before writing this
email?

        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;