On Wed, May 07, 2025, Yosry Ahmed wrote: > On Tue, May 06, 2025 at 08:57:36AM -0700, Sean Christopherson wrote: > > Sleepable spinlocks aside, the lockdep_assert_irqs_enabled() in > > smp_call_function_many_cond() already provides sufficient of coverage for that > > case. And if code is using some other form of IPI communication *and* taking raw > > spinlocks, then I think it goes without saying that developers would need to be > > very, very careful. > > I think we are not talking about the same thing, or I am > misunderstanding you. The lockdep_assert_irqs_enabled() assertion in > smp_call_function_many_cond() does not protect against this case AFAICT. > > Basically imagine that a new code path is added that does: > > spin_lock_irq(&srso_lock); > /* Some trivial logic, no IPI sending */ > spin_unlock_irq(&srso_lock); > > I believe spin_lock_irq() will disable IRQs (at least on some setups) > then spin on the lock. Yes, because the most common use case for spin_lock_irq() is to prevent deadlock due to the lock being taken in IRQ context. > Now imagine svm_srso_vm_destroy() is already holding the lock and sends > the IPI from CPU 1, while CPU 2 is executing the above code with IRQs > already disabled and spinning on the lock. > > This is the deadlock scenario I am talking about. The lockdep assertion > in smp_call_function_many_cond() doesn't help because IRQs are enabled > on CPU 1, the problem is that they are disabled on CPU 2. > > Lockdep can detect this by keeping track of the fact that some code > paths acquire the lock with IRQs off while some code paths acquire the > lock and send IPIs, I think. I understand the scenario, I just don't see any meaningful risk in this case, which in turn means I don't see any reason to use an inferior lock type (for this particular case) to protect the count. spin_lock_irq() isn't a tool that's used willy-nilly, and the usage of srso_lock is extremely limited. If we manage to merge code that does spin_lock_irq(&srso_lock), you have my full permission to mock my ineptitude :-)
