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. Will