On Thu, May 21, 2020 at 11:39:39AM +0200, Peter Zijlstra wrote: > On Thu, May 21, 2020 at 02:40:36AM +0200, Frederic Weisbecker wrote:
> This: > > > smp_call_function_single_async() { > > smp_call_function_single_async() { > > // verified csd->flags != CSD_LOCK // verified > > csd->flags != CSD_LOCK > > csd->flags = CSD_LOCK csd->flags = > > CSD_LOCK > > concurrent smp_call_function_single_async() using the same csd is what > I'm looking at as well. So something like this ought to cure the fundamental problem and make smp_call_function_single_async() more user friendly, but also more expensive. The problem is that while the ILB case is easy to fix, I can't seem to find an equally nice solution for the ttwu_remote_queue() case; that would basically require sticking the wake_csd in task_struct, I'll also post that. So it's either this: --- kernel/smp.c | 21 ++++++++++++++++----- 1 file changed, 16 insertions(+), 5 deletions(-) diff --git a/kernel/smp.c b/kernel/smp.c index 84303197caf9..d1ca2a2d1cc7 100644 --- a/kernel/smp.c +++ b/kernel/smp.c @@ -109,6 +109,12 @@ static __always_inline void csd_lock_wait(call_single_data_t *csd) smp_cond_load_acquire(&csd->flags, !(VAL & CSD_FLAG_LOCK)); } +/* + * csd_lock() can use non-atomic operations to set CSD_FLAG_LOCK because it's + * users are careful to only use CPU-local data. IOW, there is no cross-cpu + * lock usage. Also, you're not allowed to use smp_call_function*() from IRQs, + * and must be extra careful from SoftIRQ. + */ static __always_inline void csd_lock(call_single_data_t *csd) { csd_lock_wait(csd); @@ -318,7 +324,7 @@ EXPORT_SYMBOL(smp_call_function_single); /** * smp_call_function_single_async(): Run an asynchronous function on a - * specific CPU. + * specific CPU. * @cpu: The CPU to run on. * @csd: Pre-allocated and setup data structure * @@ -339,18 +345,23 @@ EXPORT_SYMBOL(smp_call_function_single); */ int smp_call_function_single_async(int cpu, call_single_data_t *csd) { + unsigned int csd_flags; int err = 0; preempt_disable(); - if (csd->flags & CSD_FLAG_LOCK) { + /* + * Unlike the regular smp_call_function*() APIs, this one is actually + * usable from IRQ context, also the -EBUSY return value suggests + * it is safe to share csd's. + */ + csd_flags = READ_ONCE(csd->flags); + if (csd_flags & CSD_FLAG_LOCK || + cmpxchg(&csd->flags, csd_flags, csd_flags | CSD_FLAG_LOCK) != csd_flags) { err = -EBUSY; goto out; } - csd->flags = CSD_FLAG_LOCK; - smp_wmb(); - err = generic_exec_single(cpu, csd, csd->func, csd->info); out: