Re: [PATCH 7/9] irq: remove needless lock in takedown_cpu()

From: Thomas Gleixner <tglx@linutronix.de>
To: Pingfan Liu <kernelfans@gmail.com>
Cc: linux-kernel@vger.kernel.org,
	Valentin Schneider <valentin.schneider@arm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Frederic Weisbecker <frederic@kernel.org>,
	Baokun Li <libaokun1@huawei.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Steven Price <steven.price@arm.com>,
	"Jason A. Donenfeld" <Jason@zx2c4.com>,
	Yuan ZhaoXiong <yuanzhaoxiong@baidu.com>,
	YueHaibing <yuehaibing@huawei.com>,
	Randy Dunlap <rdunlap@infradead.org>
Subject: Re: [PATCH 7/9] irq: remove needless lock in takedown_cpu()
Date: Mon, 25 Apr 2022 11:43:03 +0200	[thread overview]
Message-ID: <87czh533dk.ffs@tglx> (raw)
In-Reply-To: <YmYOAfz3Oh1bYiVi@piliu.users.ipa.redhat.com>

On Mon, Apr 25 2022 at 10:57, Pingfan Liu wrote:
> On Thu, Apr 21, 2022 at 06:11:56PM +0200, Thomas Gleixner wrote:
>> > -	irq_lock_sparse();
>> 
>> Not everything is about RCU here. You really need to look at all moving
>> parts:
>> 
>> irq_migrate_all_off_this_cpu() relies on the allocated_irqs bitmap and
>> the sparse tree to be in consistent state, which is only guaranteed when
>> the sparse lock is held.
>> 
>
> For the irq which transfer from active to inactive(disappearing) after
> fetching, desc->lock can serve the sync purpose. In this case,
> irq_lock_sparse() is not needed. For a emergeing irq, I am not sure
> about it.

No, it's required for the free case. The alloc case is
uninteresting. Care to look into the code?

irq_free_descs()
   lock(sparse);
   free_descs();
   bitmap_clear(allocated_irqs, from, cnt);
   unlock_sparse);

As free_descs() sets the sparse tree entry to NULL, up to the point
where bitmap_clear() finishes the state is inconsistent.

Now look at irq_migrate_all_off_this_cpu() and figure out what happens
when stop_machine() hits into the inconsistent state.

This can be fixed, but not by making mysterious claims about RCU and
desc->lock.

Thanks,

        tglx