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@xxxxxxx> --- 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
