From: Julien Grall <julien.grall@arm.com>
To: Will Deacon <will.deacon@arm.com>
Cc: linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org,
linux-arm-kernel@lists.infradead.org, tglx@linutronix.de,
rostedt@goodmis.org, bigeasy@linutronix.de,
suzuki.poulose@arm.com, catalin.marinas@arm.com,
dave.martin@arm.com
Subject: Re: [PATCH] arm64/cpufeature: Convert hook_lock to raw_spin_lock_t in cpu_enable_ssbs()
Date: Thu, 30 May 2019 14:55:05 +0100 [thread overview]
Message-ID: <13c15c54-c17f-78bc-ccf7-791e9e901b7b@arm.com> (raw)
In-Reply-To: <20190530120129.GD13751@fuggles.cambridge.arm.com>
Hi Will,
On 5/30/19 1:01 PM, Will Deacon wrote:
> On Thu, May 30, 2019 at 12:30:58PM +0100, Julien Grall wrote:
>> cpu_enable_ssbs() is called via stop_machine() as part of the cpu_enable
>> callback. A spin lock is used to ensure the hook is registered before
>> the rest of the callback is executed.
>>
>> On -RT spin_lock() may sleep. However, all the callees in stop_machine()
>> are expected to not sleep. Therefore a raw_spin_lock() is required here.
>>
>> Given this is already done under stop_machine() and the work done under
>> the lock is quite small, the latency should not increase too much.
>>
>> Signed-off-by: Julien Grall <julien.grall@arm.com>
>>
>> ---
>>
>> It was noticed when looking at the current use of spin_lock in
>> arch/arm64. I don't have a platform calling that callback, so I have
>> hacked the code to reproduce the error and check it is now fixed.
>> ---
>> arch/arm64/kernel/cpufeature.c | 6 +++---
>> 1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
>> index ca27e08e3d8a..2a7159fda3ce 100644
>> --- a/arch/arm64/kernel/cpufeature.c
>> +++ b/arch/arm64/kernel/cpufeature.c
>> @@ -1194,14 +1194,14 @@ static struct undef_hook ssbs_emulation_hook = {
>> static void cpu_enable_ssbs(const struct arm64_cpu_capabilities *__unused)
>> {
>> static bool undef_hook_registered = false;
>> - static DEFINE_SPINLOCK(hook_lock);
>> + static DEFINE_RAW_SPINLOCK(hook_lock);
>>
>> - spin_lock(&hook_lock);
>> + raw_spin_lock(&hook_lock);
>> if (!undef_hook_registered) {
>> register_undef_hook(&ssbs_emulation_hook);
>> undef_hook_registered = true;
>> }
>> - spin_unlock(&hook_lock);
>> + raw_spin_unlock(&hook_lock);
>
> Makes sense to me. We could probably avoid the lock entirely if we wanted
> to (via atomic_dec_if_positive), but I'm not sure it's really worth it.
I would prefer to remove the lock if it is possible. However, I was
under the impression the lock is necessary so the hook is registered
before any CPU attempt to configure the PSTATE.
Cheers,
--
Julien Grall
next prev parent reply other threads:[~2019-05-30 13:55 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-30 11:30 [PATCH] arm64/cpufeature: Convert hook_lock to raw_spin_lock_t in cpu_enable_ssbs() Julien Grall
2019-05-30 12:01 ` Will Deacon
2019-05-30 13:55 ` Julien Grall [this message]
2019-06-04 13:49 ` Catalin Marinas
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