Messages in this thread | | | Date | Fri, 20 Mar 2020 02:01:06 -0700 | From | Davidlohr Bueso <> | Subject | Re: [patch V2 11/15] completion: Use simple wait queues |
| |
On Wed, 18 Mar 2020, Thomas Gleixner wrote:
>From: Thomas Gleixner <tglx@linutronix.de> > >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@linutronix.de> >Cc: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Davidlohr Bueso <dbueso@suse.de>
| |