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.gr...@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
