From: "Ahmed S. Darwish" <a.darwish@linutronix.de> To: John Hubbard <jhubbard@nvidia.com> Cc: Jason Gunthorpe <jgg@nvidia.com>, Peter Xu <peterx@redhat.com>, linux-kernel@vger.kernel.org, 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>, 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 2/2] mm: prevent gup_fast from racing with COW during fork Date: Tue, 3 Nov 2020 07:52:25 +0100 Message-ID: <20201103065225.GA63301@lx-t490> (raw) In-Reply-To: <e03dba4e-fd1d-a32c-c99c-fc3fa51419c4@nvidia.com> On Mon, Nov 02, 2020 at 06:20:45PM -0800, John Hubbard wrote: > On 11/2/20 4:41 PM, Ahmed S. Darwish wrote: > > On Mon, Nov 02, 2020 at 08:25:32PM -0400, Jason Gunthorpe wrote: > > > On Tue, Nov 03, 2020 at 01:17:12AM +0100, Ahmed S. Darwish wrote: > > > > > > > Please stick with the official exported API: raw_write_seqcount_begin(). > > > > > > How did you know this was 'offical exported API' ?? > > > > > > > All the official exported seqlock.h APIs are marked with verbose > > kernel-doc annotations on top. The rest are internal... > > > > OK, but no one here was able to deduce that, probably because there is not > enough consistency throughout the kernel to be able to assume such things--even > though your seqlock project is internally consistent. It's just not *quite* > enough communication. > > I think if we added the following it would be very nice: > The problem is, I've already documented seqlock.h to death.... There are more comments than code in there, and there is "seqlock.rst" under Documentation/ to further describe the big picture. There comes a point where you decide what level of documentation to add, and what level to skip. Because in the end, you don't want to confuse "Joe, the general driver developer" with too much details that's not relevant to their task at hand. (I work in the Embedded domain, and I've seen so much ugly code from embedded drivers/SoC developers already, sorry) See for example my reply to Linus, where any talk about the lockdep-free and barrier-free parts of the API was explicitly not mentioned in seqlock.rst. This was done on purpose: 1) you want to keep the generic case simple, but the special case do-able, 2) you want to encourage people to use the standard entry/exit points as much as possible. > a) Short comments to the "unofficial and internal" routines, identifying them as > such, and > > b) Short comments to the "official API for general use", also identifying > those as such. > I really think the already added kernel-doc is sufficient... See for example __read_seqcount_begin() and __read_seqcount_retry(). They begin with "__", but they are semi-external seqlock.h API that are used by VFS to avoid barriers. And these APIs are then polymorphised according to the write serialization lock type, and so on. So the most consistent way for seqlock.h was to use kernel-doc as *the* marker for exported functions. This is not unique to seqlock.h by the way. The same pattern is heavily used by the DRM folks. Yes, of course, we can add even more comments to seqlock.h, but then, I honestly think it would be too much that maybe people will just skip reading the whole thing altogether... > c) A comment about what makes "raw" actually raw, for seqlock. > That's already documented. What more can really be written than what's in seqlock.h below?? /** * raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep /** * raw_seqcount_begin() - begin a seqcount_t read critical section w/o * lockdep and w/o counter stabilization /** * raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep /** * raw_write_seqcount_end() - end a seqcount_t write section w/o lockdep > > Since I'm proposing new work, I'll also offer to help, perhaps by putting together > a small patch to get it kicked off, if you approve of the idea. > Patches are always welcome of course, and please put me in Cc. I don't approve or deny anything though, that's the locking maintainers job :) Kind regards, > John Hubbard > NVIDIA -- Ahmed S. Darwish Linutronix GmbH
next prev parent reply index Thread overview: 28+ 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 ` [PATCH v2 0/2] Add a seqcount between gup_fast and copy_page_range() Ahmed S. Darwish 2020-11-02 22:39 ` 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 [this message] 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 2020-12-09 18:38 ` [tip: locking/core] seqlock: Prefix internal seqcount_t-only macros with a "do_" tip-bot2 for Ahmed S. Darwish 2020-12-09 18:38 ` [tip: locking/core] seqlock: kernel-doc: Specify when preemption is automatically altered tip-bot2 for Ahmed S. Darwish 2020-11-10 11:53 ` [PATCH v2 2/2] mm: prevent gup_fast from racing with COW during fork Peter Zijlstra 2020-12-03 10:35 ` [tip: locking/core] seqlock: Rename __seqprop() users tip-bot2 for Peter Zijlstra 2020-11-03 17:03 ` [PATCH v2 2/2] mm: prevent gup_fast from racing with COW during fork Peter Xu 2020-11-02 23:58 ` Ahmed S. Darwish
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