From: Rasmus Villemoes <linux@rasmusvillemoes.dk>
To: Peter Zijlstra <peterz@infradead.org>
Cc: x86@kernel.org, jpoimboe@redhat.com, jbaron@akamai.com,
rostedt@goodmis.org, ardb@kernel.org,
linux-kernel@vger.kernel.org, sumit.garg@linaro.org,
oliver.sang@intel.com, jarkko@kernel.org
Subject: Re: [PATCH 2/3] static_call: Align static_call_is_init() patching condition
Date: Fri, 19 Mar 2021 16:44:36 +0100 [thread overview]
Message-ID: <34ceee59-4276-90b5-871b-cda303901ce3@rasmusvillemoes.dk> (raw)
In-Reply-To: <c4e9dfdf-c83a-3314-8c55-5b2371a56ec8@rasmusvillemoes.dk>
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();
next prev parent reply other threads:[~2021-03-19 15:45 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-18 11:31 [PATCH 0/3] static_call() vs __exit fixes Peter Zijlstra
2021-03-18 11:31 ` [PATCH 1/3] static_call: Fix static_call_set_init() Peter Zijlstra
2021-03-19 12:25 ` [tip: locking/urgent] " tip-bot2 for Peter Zijlstra
2021-03-18 11:31 ` [PATCH 2/3] static_call: Align static_call_is_init() patching condition Peter Zijlstra
2021-03-19 12:25 ` [tip: locking/urgent] " tip-bot2 for Peter Zijlstra
2021-03-19 13:31 ` [PATCH 2/3] " Rasmus Villemoes
2021-03-19 14:13 ` Peter Zijlstra
2021-03-19 14:40 ` Rasmus Villemoes
2021-03-19 15:44 ` Rasmus Villemoes [this message]
2021-03-22 16:59 ` Peter Zijlstra
2021-03-18 11:31 ` [PATCH 3/3] static_call: Fix static_call_update() sanity check Peter Zijlstra
2021-03-18 11:42 ` Peter Zijlstra
2021-03-18 16:13 ` Josh Poimboeuf
2021-03-18 16:58 ` Peter Zijlstra
2021-03-19 12:57 ` Peter Zijlstra
2021-03-19 18:00 ` Steven Rostedt
2021-03-19 19:34 ` Peter Zijlstra
2021-03-19 20:57 ` Steven Rostedt
2021-03-22 14:50 ` Jessica Yu
2021-03-22 16:54 ` Peter Zijlstra
2021-03-22 17:36 ` Jessica Yu
2021-03-22 13:07 ` Jessica Yu
2021-03-22 14:06 ` Peter Zijlstra
2021-03-19 12:25 ` [tip: locking/urgent] " tip-bot2 for Peter Zijlstra
2021-03-18 12:15 ` [PATCH 0/3] static_call() vs __exit fixes Sumit Garg
2021-03-18 19:38 ` Jarkko Sakkinen
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