On Wed, 18 Mar 2020, Thomas Gleixner wrote:
From: Thomas Gleixner <tglx@xxxxxxxxxxxxx> completion uses a wait_queue_head_t to enqueue waiters. wait_queue_head_t contains a spinlock_t to protect the list of waiters which excludes it from being used in truly atomic context on a PREEMPT_RT enabled kernel. The spinlock in the wait queue head cannot be replaced by a raw_spinlock because: - wait queues can have custom wakeup callbacks, which acquire other spinlock_t locks and have potentially long execution times - wake_up() walks an unbounded number of list entries during the wake up and may wake an unbounded number of waiters. For simplicity and performance reasons complete() should be usable on PREEMPT_RT enabled kernels. completions do not use custom wakeup callbacks and are usually single waiter, except for a few corner cases. Replace the wait queue in the completion with a simple wait queue (swait), which uses a raw_spinlock_t for protecting the waiter list and therefore is safe to use inside truly atomic regions on PREEMPT_RT. There is no semantical or functional change: - completions use the exclusive wait mode which is what swait provides - complete() wakes one exclusive waiter - complete_all() wakes all waiters while holding the lock which protects the wait queue against newly incoming waiters. The conversion to swait preserves this behaviour. complete_all() might cause unbound latencies with a large number of waiters being woken at once, but most complete_all() usage sites are either in testing or initialization code or have only a really small number of concurrent waiters which for now does not cause a latency problem. Keep it simple for now. The fixup of the warning check in the USB gadget driver is just a straight forward conversion of the lockless waiter check from one waitqueue type to the other. Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Arnd Bergmann <arnd@xxxxxxxx>
Reviewed-by: Davidlohr Bueso <dbueso@xxxxxxx>
