linux-kernel - Re: [PATCH] ipc: optimize semget/shmget/msgget for lots of keys

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Date:   Thu, 3 Aug 2017 19:14:29 +0200
From:   Guillaume Knispel <guillaume.knispel@...ersonicimagine.com>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Manfred Spraul <manfred@...orfullife.com>,
        Kees Cook <keescook@...omium.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Serge Hallyn <serge@...lyn.com>,
        Andrey Vagin <avagin@...nvz.org>,
        Marc Pardo <marc.pardo@...ersonicimagine.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ipc: optimize semget/shmget/msgget for lots of keys

On Wed, Aug 02, 2017 at 01:06:44PM -0700, Davidlohr Bueso wrote:
> On Mon, 31 Jul 2017, Guillaume Knispel wrote:
> >static int __init ipc_init(void)
> >{
> >-	sem_init();
> >-	msg_init();
> >+	int err_sem, err_msg;
> >+
> >+	err_sem = sem_init();
> >+	WARN(err_sem, "ipc: sysV sem_init failed: %d\n", err_sem);
> >+	err_msg = msg_init();
> >+	WARN(err_msg, "ipc: sysV msg_init failed: %d\n", err_msg);
> >	shm_init();
> 
> This shows the ugliness of the underlying ipc init asymmetry. Specifically,
> 140d0b2108f (Do 'shm_init_ns()' in an early pure_initcall) was the final
> nail in the coffin to fix an exit_shm() race.
> 
> While normally we could just initialize the ipc_ids fields statically and
> be over with initcall dependencies, your patch will require inits be done
> dynamically for the rhashtable_init(). Oh well.
> 
> Also, why do you do this?
> 
> >-pure_initcall(ipc_ns_init);
> >+core_initcall(ipc_ns_init);

In linux/init.h I saw that a pure_initcall is reserved to only
initialize variables and must have no dependency on anything else;
I interpreted that, + "pure" in the name, thinking we should not e.g.
allocate in a pure_initcall, however I see that net_ns_init() calls
kmem_cache_create() and others, so maybe we can keep ipc_ns_init() as
a pure_initcall?

Guillaume