From: Jason Gunthorpe <jgg@nvidia.com>
To: Niklas Schnelle <schnelle@linux.ibm.com>
Cc: Matthew Rosato <mjrosato@linux.ibm.com>,
iommu@lists.linux.dev, Joerg Roedel <joro@8bytes.org>,
Will Deacon <will@kernel.org>,
Robin Murphy <robin.murphy@arm.com>,
Gerd Bayer <gbayer@linux.ibm.com>,
Pierre Morel <pmorel@linux.ibm.com>,
linux-s390@vger.kernel.org, borntraeger@linux.ibm.com,
hca@linux.ibm.com, gor@linux.ibm.com,
gerald.schaefer@linux.ibm.com, agordeev@linux.ibm.com,
svens@linux.ibm.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/5] iommu/s390: Use RCU to allow concurrent domain_list iteration
Date: Fri, 28 Oct 2022 08:28:14 -0300 [thread overview]
Message-ID: <Y1u8zrKUh2S/J6LM@nvidia.com> (raw)
In-Reply-To: <1e10c0605e65f43acf6d2b5e71c7c74d6ff980b5.camel@linux.ibm.com>
On Fri, Oct 28, 2022 at 11:29:00AM +0200, Niklas Schnelle wrote:
> > rcu_head = kzalloc(rcu_head, GFP_NOWAIT, GFP_NOWARN)
> > if (!rcu_head)
> > synchronize_rcu()
> > else
> > call_rcu(rcu_head)
> >
> > And then call kmem_cache_free() from the rcu callback
>
> Hmm, maybe a stupid question but why can't I just put the rcu_head in
> struct s390_domain and then do a call_rcu() on that with a callback
> that does:
>
> dma_cleanup_tables(s390_domain->dma_table);
> kfree(s390_domain);
>
> I.e. the rest of the current s390_domain_free().
> Then I don't have to worry about failing to allocate the rcu_head and
> it's simple enough. Basically just do the actual freeing of the
> s390_domain via call_rcu().
Oh, if you never reallocate the dma_table then yes that is a good idea
> Or do you mean to use a kref that is taken by RCU readers together with
> rcu_read_lock() and dropped at rcu_read_unlock() such that during the
> RCU read critical sections the refcount can't fall below 1 and the
> domain is actually freed once we have a) put the initial reference
> during s390_domain_free() and b) put all temporary references on
> exiting the RCU read critical sections?
Yes, this is a common pattern. Usually you want to optimize away the
global lock that protects, say, a linked list and then accept a local
lock/refcount inside the object
Jason
next prev parent reply other threads:[~2022-10-28 11:28 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-18 14:51 [PATCH 0/5] iommu/s390: Further improvements Niklas Schnelle
2022-10-18 14:51 ` [PATCH 1/5] iommu/s390: Make attach succeed even if the device is in error state Niklas Schnelle
2022-10-28 15:55 ` Matthew Rosato
2022-10-18 14:51 ` [PATCH 2/5] iommu/s390: Add I/O TLB ops Niklas Schnelle
2022-10-28 16:03 ` Matthew Rosato
2022-10-31 16:11 ` Robin Murphy
2022-11-02 10:51 ` Niklas Schnelle
2022-10-18 14:51 ` [PATCH 3/5] iommu/s390: Use RCU to allow concurrent domain_list iteration Niklas Schnelle
2022-10-18 15:18 ` Jason Gunthorpe
2022-10-19 8:31 ` Niklas Schnelle
2022-10-19 11:53 ` Jason Gunthorpe
2022-10-20 8:51 ` Niklas Schnelle
2022-10-20 11:05 ` Jason Gunthorpe
2022-10-21 12:08 ` Niklas Schnelle
2022-10-21 13:36 ` Jason Gunthorpe
2022-10-21 15:01 ` Niklas Schnelle
2022-10-21 15:04 ` Jason Gunthorpe
2022-10-24 15:22 ` Niklas Schnelle
2022-10-24 16:26 ` Jason Gunthorpe
2022-10-27 12:44 ` Niklas Schnelle
2022-10-27 12:56 ` Jason Gunthorpe
2022-10-27 13:35 ` Niklas Schnelle
2022-10-27 14:03 ` Jason Gunthorpe
2022-10-28 9:29 ` Niklas Schnelle
2022-10-28 11:28 ` Jason Gunthorpe [this message]
2022-10-21 15:05 ` Niklas Schnelle
2022-10-18 14:51 ` [PATCH 4/5] iommu/s390: Optimize IOMMU table walking Niklas Schnelle
2022-10-18 14:51 ` [PATCH 5/5] s390/pci: use lock-free I/O translation updates Niklas Schnelle
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