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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Soheil Hassas Yeganeh <soheil@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Eric Dumazet <edumazet@google.com>,
	Guantao Liu <guantaol@google.com>,
	Khazhismel Kumykov <khazhy@google.com>,
	Linux-MM <linux-mm@kvack.org>,
	mm-commits@vger.kernel.org, Al Viro <viro@zeniv.linux.org.uk>,
	Willem de Bruijn <willemb@google.com>
Subject: Re: [patch 13/15] epoll: check ep_events_available() upon timeout
Date: Mon, 2 Nov 2020 12:12:37 -0800	[thread overview]
Message-ID: <CAHk-=wh826P9cz0g_FGRCbz1ZE4pfhCHuQBQ-J_BBxeo5x94vQ@mail.gmail.com> (raw)
In-Reply-To: <CACSApvZiWLAzSpkkBSo9=U_Fpx85_xCryaDh+sFsr7Uw8-RjUg@mail.gmail.com>

On Mon, Nov 2, 2020 at 11:55 AM Soheil Hassas Yeganeh <soheil@google.com> wrote:
>
> Given that you're calling ep_events_available() under lock, I think
> this should address the inefficiency for the non-racy timeout case,  I
> mentioned above. The remaining races are preexisting and all result in
> spurious events, which should be fine.

Well, they might result in not seeing events that are there, but for
that case we end up going the logn way around and doing the "wait for
events" code, and that should take care of it, I guess.

So it's racy, but it gets fixed up later regardless of which way it
races, I think.

It might be worth a comment or two.

And that "goto fetch_events" thing might be worth writing as an actual
loop, rather than the pointless one I removed.

I suspect that old pointless loop might have been historical (some
kind of "loop waiting for event"), since that's how those things are
usually written. I didn't go and look at the history.

Anyway, I've dropped the original 13/15 patch, buit I'll apply the
test-case one (14/15) even if it might now fail in the current state.

I hope that you will end up submitting some cleaned-up (and tested!)
version of that patch I cobbled together - I'll archive this thread
for now on the assumption that the resolution of all this will come
back to me later...

                     Linus

  reply	other threads:[~2020-11-02 20:14 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-02  1:06 incoming Andrew Morton
2020-11-02  1:07 ` [patch 01/15] mm/mremap_pages: fix static key devmap_managed_key updates Andrew Morton
2020-11-02  1:07 ` [patch 02/15] hugetlb_cgroup: fix reservation accounting Andrew Morton
2020-11-02  1:07 ` [patch 03/15] mm: memcontrol: correct the NR_ANON_THPS counter of hierarchical memcg Andrew Morton
2020-11-02  1:07 ` [patch 04/15] mm: memcg: link page counters to root if use_hierarchy is false Andrew Morton
2020-11-02  1:07   ` [LTP] " Andrew Morton
2020-11-02  1:07 ` [patch 05/15] kasan: adopt KUNIT tests to SW_TAGS mode Andrew Morton
2020-11-02  1:07 ` [patch 06/15] mm: mempolicy: fix potential pte_unmap_unlock pte error Andrew Morton
2020-11-02  1:07 ` [patch 07/15] ptrace: fix task_join_group_stop() for the case when current is traced Andrew Morton
2020-11-02  1:07 ` [patch 08/15] lib/crc32test: remove extra local_irq_disable/enable Andrew Morton
2020-11-02  1:07 ` [patch 09/15] mm/truncate.c: make __invalidate_mapping_pages() static Andrew Morton
2020-11-02  1:07 ` [patch 10/15] kthread_worker: prevent queuing delayed work from timer_fn when it is being canceled Andrew Morton
2020-11-02  1:07 ` [patch 11/15] mm, oom: keep oom_adj under or at upper limit when printing Andrew Morton
2020-11-02  1:08 ` [patch 12/15] mm: always have io_remap_pfn_range() set pgprot_decrypted() Andrew Morton
2020-11-02  1:08 ` [patch 13/15] epoll: check ep_events_available() upon timeout Andrew Morton
2020-11-02 17:08   ` Linus Torvalds
2020-11-02 17:08     ` Linus Torvalds
2020-11-02 17:48     ` Soheil Hassas Yeganeh
2020-11-02 17:48       ` Soheil Hassas Yeganeh
2020-11-02 18:51       ` Linus Torvalds
2020-11-02 18:51         ` Linus Torvalds
2020-11-02 19:38         ` Linus Torvalds
2020-11-02 19:38           ` Linus Torvalds
2020-11-02 19:54         ` Soheil Hassas Yeganeh
2020-11-02 19:54           ` Soheil Hassas Yeganeh
2020-11-02 20:12           ` Linus Torvalds [this message]
2020-11-02 20:12             ` Linus Torvalds
2020-11-02  1:08 ` [patch 14/15] epoll: add a selftest for epoll timeout race Andrew Morton
2020-11-02  1:08 ` [patch 15/15] kernel/hung_task.c: make type annotations consistent Andrew Morton

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