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Biederman) To: Alexey Gladkov References: <116c7669744404364651e3b380db2d82bb23f983.1610722473.git.gladkov.alexey@gmail.com> <20210118194551.h2hrwof7b3q5vgoi@example.org> <20210118205629.zro2qkd3ut42bpyq@example.org> <87eeig74kv.fsf@x220.int.ebiederm.org> <20210121120427.iiggfmw3tpsmyzeb@example.org> Date: Thu, 21 Jan 2021 09:50:34 -0600 In-Reply-To: <20210121120427.iiggfmw3tpsmyzeb@example.org> (Alexey Gladkov's message of "Thu, 21 Jan 2021 13:04:27 +0100") Message-ID: <87ft2u2ss5.fsf@x220.int.ebiederm.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 X-XM-SPF: eid=1l2cFN-0029Ec-Pa; ; ; mid=<87ft2u2ss5.fsf@x220.int.ebiederm.org>; ; ; hst=in02.mta.xmission.com; ; ; ip=68.227.160.95; ; ; frm=ebiederm@xmission.com; ; ; spf=neutral X-XM-AID: U2FsdGVkX1+qagvrCbWceuXumPpyz27FGwz07GA2gjk= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [RFC PATCH v3 1/8] Use refcount_t for ucounts reference counting X-SA-Exim-Version: 4.2.1 (built Sat, 08 Feb 2020 21:53:50 +0000) X-SA-Exim-Scanned: Yes (on in02.mta.xmission.com) Cc: Jens Axboe , Kees Cook , Kernel Hardening , Linux Containers , Jann Horn , LKML , Oleg Nesterov , Linux-MM , Andrew Morton , Linus Torvalds , io-uring X-BeenThere: containers@lists.linux-foundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Linux Containers List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: containers-bounces@lists.linux-foundation.org Sender: "Containers" Alexey Gladkov writes: > On Tue, Jan 19, 2021 at 07:57:36PM -0600, Eric W. Biederman wrote: >> Alexey Gladkov writes: >> >> > On Mon, Jan 18, 2021 at 12:34:29PM -0800, Linus Torvalds wrote: >> >> On Mon, Jan 18, 2021 at 11:46 AM Alexey Gladkov >> >> wrote: >> >> > >> >> > Sorry about that. I thought that this code is not needed when switching >> >> > from int to refcount_t. I was wrong. >> >> >> >> Well, you _may_ be right. I personally didn't check how the return >> >> value is used. >> >> >> >> I only reacted to "it certainly _may_ be used, and there is absolutely >> >> no comment anywhere about why it wouldn't matter". >> > >> > I have not found examples where checked the overflow after calling >> > refcount_inc/refcount_add. >> > >> > For example in kernel/fork.c:2298 : >> > >> > current->signal->nr_threads++; >> > atomic_inc(¤t->signal->live); >> > refcount_inc(¤t->signal->sigcnt); >> > >> > $ semind search signal_struct.sigcnt >> > def include/linux/sched/signal.h:83 refcount_t sigcnt; >> > m-- kernel/fork.c:723 put_signal_struct if (refcount_dec_and_test(&sig->sigcnt)) >> > m-- kernel/fork.c:1571 copy_signal refcount_set(&sig->sigcnt, 1); >> > m-- kernel/fork.c:2298 copy_process refcount_inc(¤t->signal->sigcnt); >> > >> > It seems to me that the only way is to use __refcount_inc and then compare >> > the old value with REFCOUNT_MAX >> > >> > Since I have not seen examples of such checks, I thought that this is >> > acceptable. Sorry once again. I have not tried to hide these changes. >> >> The current ucount code does check for overflow and fails the increment >> in every case. >> >> So arguably it will be a regression and inferior error handling behavior >> if the code switches to the ``better'' refcount_t data structure. >> >> I originally didn't use refcount_t because silently saturating and not >> bothering to handle the error makes me uncomfortable. >> >> Not having to acquire the ucounts_lock every time seems nice. Perhaps >> the path forward would be to start with stupid/correct code that always >> takes the ucounts_lock for every increment of ucounts->count, that is >> later replaced with something more optimal. >> >> Not impacting performance in the non-namespace cases and having good >> performance in the other cases is a fundamental requirement of merging >> code like this. > > Did I understand your suggestion correctly that you suggest to use > spin_lock for atomic_read and atomic_inc ? > > If so, then we are already incrementing the counter under ucounts_lock. > > ... > if (atomic_read(&ucounts->count) == INT_MAX) > ucounts = NULL; > else > atomic_inc(&ucounts->count); > spin_unlock_irq(&ucounts_lock); > return ucounts; > > something like this ? Yes. But without atomics. Something a bit more like: > ... > if (ucounts->count == INT_MAX) > ucounts = NULL; > else > ucounts->count++; > spin_unlock_irq(&ucounts_lock); > return ucounts; I do believe at some point we will want to say using the spin_lock for ucounts->count is cumbersome, and suboptimal and we want to change it to get a better performing implementation. Just for getting the semantics correct we should be able to use just ucounts_lock for locking. Then when everything is working we can profile and optimize the code. I just don't want figuring out what is needed to get hung up over little details that we can change later. Eric _______________________________________________ Containers mailing list Containers@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/containers