linux-kernel - Re: [PATCH 2/3] static_call: Align static_call_is

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Message-ID: <34ceee59-4276-90b5-871b-cda303901ce3@rasmusvillemoes.dk>
Date:   Fri, 19 Mar 2021 16:44:36 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, jpoimboe@...hat.com, jbaron@...mai.com,
        rostedt@...dmis.org, ardb@...nel.org, linux-kernel@...r.kernel.org,
        sumit.garg@...aro.org, oliver.sang@...el.com, jarkko@...nel.org
Subject: Re: [PATCH 2/3] static_call: Align static_call_is_init() patching
 condition

On 19/03/2021 15.40, Rasmus Villemoes wrote:
> On 19/03/2021 15.13, Peter Zijlstra wrote:
> 
>>> Dunno, probably overkill, but perhaps we could have an atomic_t (or
>>> refcount, whatever) init_ref inited to 1, with init_ref_get() doing an
>>> inc_unless_zero, and iff you get a ref, you're free to call (/patch)
>>> __init functions and access __initdata, but must do init_ref_put(), with
>>> PID1 dropping its initial ref and waiting for it to drop to 0 before
>>> doing the *free_initmem() calls.
>>
>> I'd as soon simply add another SYSTEM state. That way we don't have to
>> worry about who else looks at RUNNING for what etc..
> 
> I don't understand. How would that solve the
> 
> PID1                           PIDX
>                                ok = system_state < INIT_MEM_GONE;
> system_state = INIT_MEM_GONE;
> free_initmem();
> system_state = RUNNING;
>                                if (ok)
>                                    poke init mem
> 
> race? I really don't see any way arbitrary threads can reliably check
> how far PID1 has progressed at one point in time and use that
> information (a few lines) later to access init memory without
> synchronizing with PID1.
> 
> AFAICT, having an atomic_t object representing the init memory 

something like

--- a/init/main.c
+++ b/init/main.c
@@ -1417,6 +1417,18 @@ void __weak free_initmem(void)
        free_initmem_default(POISON_FREE_INITMEM);
 }

+static atomic_t init_mem_ref = ATOMIC_INIT(1);
+static DECLARE_COMPLETION(init_mem_may_go);
+bool init_mem_get(void)
+{
+       return atomic_inc_not_zero(&init_mem_ref);
+}
+void init_mem_put(void)
+{
+       if (atomic_dec_and_test(&init_mem_ref))
+               complete(&init_mem_may_go);
+}
+
 static int __ref kernel_init(void *unused)
 {
        int ret;
@@ -1424,6 +1436,8 @@ static int __ref kernel_init(void *unused)
        kernel_init_freeable();
        /* need to finish all async __init code before freeing the memory */
        async_synchronize_full();
+       init_mem_put();
+       wait_for_completion(&init_mem_may_go);
        kprobe_free_init_mem();
        ftrace_free_init_mem();
        kgdb_free_init_mem();