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Message-ID: <ZYv5IhyCVeYhbj8b@casper.infradead.org> Date: Wed, 27 Dec 2023 10:14:58 +0000 From: Matthew Wilcox <willy@...radead.org> To: "Aiqun Yu (Maria)" <quic_aiquny@...cinc.com> Cc: Hillf Danton <hdanton@...a.com>, "Eric W. Biederman" <ebiederm@...ssion.com>, linux-kernel@...r.kernel.org Subject: Re: [PATCH] kernel: Introduce a write lock/unlock wrapper for tasklist_lock On Wed, Dec 27, 2023 at 09:41:29AM +0800, Aiqun Yu (Maria) wrote: > On 12/26/2023 6:46 PM, Hillf Danton wrote: > > On Wed, 13 Dec 2023 12:27:05 -0600 Eric W. Biederman <ebiederm@...ssion.com> > > > Matthew Wilcox <willy@...radead.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. > > Thx for the comments for discuss of the deadlock possibility. While I think > deadlock can be differentiate with below 2 scenarios: > 1. queued_write_lock_slowpath being triggered in interrupt context. > tasklist_lock don't have write_lock_irq(save) in interrupt context. > while for common rw lock, maybe write_lock_irq(save) usage in interrupt > context is a possible. > so may introduce a state when lock->wait_lock is released and left the > _QW_WAITING flag. > Welcome others to suggest on designs and comments. Hm? I am confused. You're talking about the scenario where: - CPU B holds the lock for read - CPU A attempts to get the lock for write in user context, fails, sets the _QW_WAITING flag - CPU A re-enables interrupts - CPU A executes an interrupt handler which calls queued_write_lock() - If CPU B has dropped the read lock in the meantime, atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED) succeeds - CPU A calls queued_write_unlock() which stores 0 to the lock and we _lose_ the _QW_WAITING flag for the userspace waiter. How do we end up with CPU A leaving the _QW_WAITING flag set?
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