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 <[email protected]>
---
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);
if (arm64_get_ssbd_state() == ARM64_SSBD_FORCE_DISABLE) {
sysreg_clear_set(sctlr_el1, 0, SCTLR_ELx_DSSBS);
--
2.11.0
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 <[email protected]>
>
> ---
>
> 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.
Will
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 <[email protected]>
>>
>> ---
>>
>> 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
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 <[email protected]>
Queued for 5.3. Thanks.
--
Catalin