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Message-ID: <7f31f938-cd4f-bb1b-d44d-57adabf62c51@intel.com>
Date: Thu, 3 Aug 2023 14:15:13 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
<linux-kernel@...r.kernel.org>, <x86@...nel.org>
CC: "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
"Dave Hansen" <dave.hansen@...ux.intel.com>,
Ingo Molnar <mingo@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86/microcode: Remove microcode_mutex.
Nit: The full stop at the end is not needed.
On 8/3/2023 1:32 AM, Sebastian Andrzej Siewior wrote:
> microcode_mutex is only used by reload_store(). It has a comment saying
> "to synchronize with each other". This probably means the sysfs
> interface vs the legacy interface which was removed in commit
> 181b6f40e9ea8 ("x86/microcode: Rip out the OLD_INTERFACE").
>
There is also commit b6f86689d5b7 ("x86/microcode: Rip out the subsys
interface gunk") which last year removed another usage of microcode_mutex.
> The sysfs interface does not need additional synchronisation vs itself
> because it is provided as kernfs_ops::mutex which is acquired in
> kernfs_fop_write_iter().
>
> Remove superfluous microcode_mutex.
I agree, the current usage does look unnecessary.
Maybe reword the commit message to say that after these two Rip outs
there are no of *other* usages of microcode_mutex to synchronize with?
>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
> This poped up as "defined but not used" on RT builds without
> CONFIG_MICROCODE_LATE_LOADING enabled.
This issue has been raised a couple of times recently but the
justification has been deemed insufficient since it can't be reproduced
with a .config file.
See:
https://lore.kernel.org/lkml/20230324114720.1756466-1-john.ogness@linutronix.de/
and
https://lore.kernel.org/lkml/20230522062713.427998-1-christian.gmeiner@gmail.com/
However, your current justification of not needing the mutex itself
seems reasonable to me.
>
> arch/x86/kernel/cpu/microcode/core.c | 6 ------
> 1 file changed, 6 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
> index 3afcf3de0dd49..2f9d35744bc41 100644
> --- a/arch/x86/kernel/cpu/microcode/core.c
> +++ b/arch/x86/kernel/cpu/microcode/core.c
> @@ -54,15 +54,12 @@ LIST_HEAD(microcode_cache);
> *
> * All non cpu-hotplug-callback call sites use:
> *
> - * - microcode_mutex to synchronize with each other;
> * - cpus_read_lock/unlock() to synchronize with
> * the cpu-hotplug-callback call sites.
> *
> * We guarantee that only a single cpu is being
> * updated at any particular moment of time.
> */
> -static DEFINE_MUTEX(microcode_mutex);
> -
> struct ucode_cpu_info ucode_cpu_info[NR_CPUS];
>
> struct cpu_info_ctx {
> @@ -488,10 +485,7 @@ static ssize_t reload_store(struct device *dev,
> if (tmp_ret != UCODE_NEW)
> goto put;
>
> - mutex_lock(µcode_mutex);
> ret = microcode_reload_late();
> - mutex_unlock(µcode_mutex);
> -
> put:
> cpus_read_unlock();
>
The code changes look fine to me.
You can also add below to the patch.
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 52683fddafaf..777340724ec3 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -2276,8 +2276,7 @@ void store_cpu_caps(struct cpuinfo_x86 *curr_info)
> * @prev_info: CPU capabilities stored before an update.
> *
> * The microcode loader calls this upon late microcode load to recheck features,
> - * only when microcode has been updated. Caller holds microcode_mutex and CPU
> - * hotplug lock.
> + * only when microcode has been updated. Caller holds CPU hotplug lock.
> *
> * Return: None
> */
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