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Message-ID: <6e762e8e-b031-4e37-97c1-56390c9b8076@quicinc.com> Date: Wed, 27 Dec 2023 09:41:29 +0800 From: "Aiqun Yu (Maria)" <quic_aiquny@...cinc.com> To: Hillf Danton <hdanton@...a.com>, Matthew Wilcox <willy@...radead.org> CC: "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 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. 2.queued_read_lock_slowpath can be triggered in interrupt context. And it already have the handle to avoid possible deadlock. In the queued_read_lock_slowpath, there is check whether current context is in interrupt or not, and get the lock directly of only write lock waiting. Pls reference[1]: /* * 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; } [1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/locking/qrwlock.c >> >> Yes. It seems to make sense to distinguish between write_lock_irq and >> write_lock_irqsave and fix this for all of write_lock_irq. >> >> Either that or someone can put in the work to start making the >> tasklist_lock go away. >> >> Eric -- Thx and BRs, Aiqun(Maria) Yu
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