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Message-ID: <5ad7dffa-204e-4d37-acf6-0206d7a87f37@I-love.SAKURA.ne.jp>
Date: Thu, 25 Jul 2024 13:24:18 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: LKML <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] orphaned patches for 6.11
On 2024/07/25 4:34, Linus Torvalds wrote:
> But honestly, the minimal fix would seem to be this two-liner:
>
> --- a/kernel/ksysfs.c
> +++ b/kernel/ksysfs.c
> @@ -92,7 +92,9 @@ static ssize_t profiling_store(struct kobject *kobj,
> const char *buf, size_t count)
> {
> int ret;
> + static DEFINE_MUTEX(prof_store_mutex);
>
> + guard(mutex)(&prof_store_mutex);
> if (prof_on)
> return -EEXIST;
> /*
>
> which I have admittedly not tested at all, but seems trivial.
If guard() is already backported to older kernels, that would work for
the kernel/ksysfs.c part. But I prefer killable version, for a large
memory allocation which may cause current thread waiting on this lock
might be chosen as an OOM victim is performed from profile_init().
I wish if we had a macro that only does unlock upon function exit.
Then, we can use explicit killable lock and automatic unlock (which is
something like golang's "defer" statement).
> And once that "no more multiple concurrent profile initialization" bug
> is fixed, everything else is fine. The assignment to "prof_buffer"
> will now be the last thing that is done, and when it's done the
> profiling should all be good.
Unfortunately, there is a race where KMSAN would complain even if
profile initialization is serialized.
profile_init() {
(...snipped...)
profile_tick(int type) {
struct pt_regs *regs = get_irq_regs();
if (!alloc_cpumask_var(&prof_cpu_mask, GFP_KERNEL))
return -ENOMEM;
if (!user_mode(regs) && cpumask_available(prof_cpu_mask) &&
// cpumask_available(prof_cpu_mask) returns true after
// alloc_cpumask_var(&prof_cpu_mask) completes, but
cpumask_test_cpu(smp_processor_id(), prof_cpu_mask))
// KMSAN complains about uninit-value here, for
profile_hit(type, (void *)profile_pc(regs));
}
// due to use of GFP_KERNEL, prof_cpu_mask remains
// uninitialzed until cpumask_copy() completes.
cpumask_copy(prof_cpu_mask, cpu_possible_mask);
prof_buffer = kzalloc(buffer_bytes, GFP_KERNEL|__GFP_NOWARN);
(...snipped...)
}
Edward Adam Davis and I did s/alloc_cpumask_var/zalloc_cpumask_var/ .
But I also removed cpumask_copy() unless CONFIG_SMP=n, for
cpuhp_setup_state(CPUHP_AP_ONLINE_DYN) in create_proc_profile() calls
cpumask_set_cpu() as needed. That is, currently cpumask_copy() is called
needlessly and inappropriately.
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