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From: "Ahmed S. Darwish" <a.darwish@linutronix.de>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: linux-kernel@vger.kernel.org, Peter Xu <peterx@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
	Christoph Hellwig <hch@lst.de>, Hugh Dickins <hughd@google.com>,
	Jan Kara <jack@suse.cz>, Jann Horn <jannh@google.com>,
	John Hubbard <jhubbard@nvidia.com>,
	Kirill Shutemov <kirill@shutemov.name>,
	Kirill Tkhai <ktkhai@virtuozzo.com>,
	Leon Romanovsky <leonro@nvidia.com>,
	Linux-MM <linux-mm@kvack.org>, Michal Hocko <mhocko@suse.com>,
	Oleg Nesterov <oleg@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Sebastian Siewior <bigeasy@linutronix.de>
Subject: Re: [PATCH v2 0/2] Add a seqcount between gup_fast and copy_page_range()
Date: Mon, 2 Nov 2020 23:19:45 +0100
Message-ID: <20201102221945.GA48454@lx-t490> (raw)
In-Reply-To: <0-v2-dfe9ecdb6c74+2066-gup_fork_jgg@nvidia.com>

Hello Jason,

Thanks for keeping me in the loop on this.

I've also added the locking maintainers in Cc. IMHO there are some
seqlock.h API violations in this series, and they should have the final
say on this.

On Fri, Oct 30, 2020 at 11:46:19AM -0300, Jason Gunthorpe wrote:
>
> As discussed and suggested by Linus use a seqcount to close the small race
> between gup_fast and copy_page_range().
>
> Unfortunately the good suggestion to just use write_seqcount_begin() blows
> up lockdep immediately due to the (new?) requirement that the write side
> of seqcount be in a preempt disabled region.

Disabling preemption for seqcount_t write-side critical sections was
never a new requirement. It has always been this way, for the reasons
explained at Documentation/locking/seqlock.rst, "Introduction" section.

The recent seqcount_t changes did not mandate any new rules. This was
done explicitly, and on-purpose, not to break any of the *large* set of
existing seqcount_t call sites. It added multiple lockdep asserts
though, to catch a number of (already) buggy users, and they were fixed
beforehand.

It seems you have a special case here, so I'll continue discussing this
at patch #2 where the code resides. Just wanted to answer the "(new?)"
part above.

>                                               For this application it does
> not seem like a good idea, nor is it necessary as we don't spin on retry.
> This is solved by being the first place to use raw_write_seqcount_t_begin()
>

Regardless of this series write side preemptibility requirements, the
"_write_seqcount_*t*_()" interfaces are internal to seqlock.h and should
_never_ be used outside of it.

For plain seqcount_t, raw_write_seqcount_begin() is equivalent to
raw_write_seqcount_*t*_begin() anyway, and should already satisfy your
needs.

/me jumps to patch #2 now...

Thanks,

--
Ahmed S. Darwish
Linutronix GmbH

  parent reply index

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <0-v2-dfe9ecdb6c74+2066-gup_fork_jgg@nvidia.com>
2020-10-30 14:46 ` [PATCH v2 1/2] mm: reorganize internal_get_user_pages_fast() Jason Gunthorpe
2020-10-30 16:29   ` Jan Kara
2020-10-30 21:31   ` John Hubbard
2020-10-30 22:36   ` Peter Xu
2020-11-02 22:19 ` Ahmed S. Darwish [this message]
2020-11-02 22:39   ` [PATCH v2 0/2] Add a seqcount between gup_fast and copy_page_range() Linus Torvalds
2020-11-02 23:18     ` Ahmed S. Darwish
     [not found] ` <2-v2-dfe9ecdb6c74+2066-gup_fork_jgg@nvidia.com>
2020-10-30 16:51   ` [PATCH v2 2/2] mm: prevent gup_fast from racing with COW during fork Jan Kara
     [not found]     ` <20201030170226.GF2620339@nvidia.com>
2020-11-02  8:31       ` Jan Kara
2020-10-30 22:52   ` Peter Xu
     [not found]     ` <20201030235121.GQ2620339@nvidia.com>
2020-10-31 15:26       ` Peter Xu
2020-11-03  0:33         ` Ahmed S. Darwish
2020-11-03  0:17       ` Ahmed S. Darwish
     [not found]         ` <20201103002532.GL2620339@nvidia.com>
2020-11-03  0:41           ` Ahmed S. Darwish
2020-11-03  2:20             ` John Hubbard
2020-11-03  6:52               ` Ahmed S. Darwish
2020-11-03 17:40                 ` Linus Torvalds
2020-11-04  1:32                   ` Ahmed S. Darwish
2020-11-04  2:01                     ` John Hubbard
2020-11-04  3:17                       ` Ahmed S. Darwish
2020-11-04 18:38                         ` Linus Torvalds
2020-11-04 19:54                           ` Ahmed S. Darwish
2020-11-10 11:53                         ` Peter Zijlstra
2020-11-03 17:03         ` Peter Xu
2020-11-02 23:58   ` Ahmed S. Darwish

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