On 2023-01-19 11:02:20 [+0000], Mel Gorman wrote:
> > - Once the writer removes READER_BIAS, it forces the reader into the
> > slowpath.
>
> Removed in __rwbase_write_trylock IIUC
And added back in case try trylock failed via __rwbase_write_unlock().
The RTmutex is unlocked and the READER_BIAS is "returned".
> > At that time the writer does not own the wait_lock meaning
> > the reader _could_ check the timeout before writer had a chance to set
> > it. The worst thing is probably that if jiffies does not have the
> > highest bit set then it will always disable the reader bias here.
> > The easiest thing is probably to check timeout vs 0 and ensure on the
> > writer side that the lowest bit is always set (in the unlikely case it
> > will end up as zero).
> >
>
> I am missing something important. On the read side, we have
>
Look at this side by side:
writer reader
| static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
| unsigned int state)
| {
| /* Force readers into slow path */
| atomic_sub(READER_BIAS, &rwb->readers);
| static int __sched __rwbase_read_lock(struct rwbase_rt *rwb,
| unsigned int state)
| {
| struct rt_mutex_base *rtm = &rwb->rtmutex;
| int ret;
|
| raw_spin_lock_irq(&rtm->wait_lock);
Reader has the lock, writer will wait.
| /*
| * Allow readers, as long as the writer has not completely
| * acquired the semaphore for write.
| */
| if (atomic_read(&rwb->readers) != WRITER_BIAS) {
here, the timeout value is not yet populated by the writer so the reader
compares vs 0.
| atomic_inc(&rwb->readers);
| raw_spin_unlock_irq(&rtm->wait_lock);
| return 0;
| }
|
| raw_spin_lock_irqsave(&rtm->wait_lock, flags);
| if (__rwbase_write_trylock(rwb))
| goto out_unlock;
|
Hope this makes it easier.
Sebastian
