linux-kernel - Re: 0087-rcu-Use-a-raw_spinlock

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Message-ID: <20200415133423.7mohynwqanoiqzkx@linutronix.de>
Date:   Wed, 15 Apr 2020 15:34:23 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Mike Galbraith <efault@....de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: 0087-rcu-Use-a-raw_spinlock_t-for-kfree-batching.patch

On 2020-04-05 06:53:43 [+0200], Mike Galbraith wrote:
> Hi Sebastian,
Hi Mike,

> Just a heads up wrt $subject...
> 
> I had done the same in my 5.6 tree, but then 5.7 came along, and I had
> to revisit due to 34c881745549e adding an allocation under that lock
> with irqs disabled.  The same commit that added the lock added this.
> 
> 	local_irq_save(flags);  // For safely calling this_cpu_ptr().
> 
> Whacking that instead of converting the lock cures 5.6 and 5.7 woes.

So I think you are telling me that I made the lock raw and then they
added in v5.7 a __get_free_page() call in
kfree_call_rcu_add_ptr_to_bulk() which is invoked from the free path.

This is indeed not something that made me happy. Now let me think about
this for a moment…

> 	-Mike

Sebastian