Re: [PATCH RT 1/2] rwsem-rt: Do not allow readers to nest

From: Sebastian Andrzej Siewior
Date: Wed Feb 18 2015 - 14:57:20 EST

* Steven Rostedt | 2014-04-08 22:47:01 [-0400]:

>From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>
>The readers of mainline rwsems are not allowed to nest, the rwsems in the
>PREEMPT_RT kernel should not nest either.

I applied this and this is the reason why cpufreq isn't working. What I
see in cpufreq is:
| [004] ....... 61.416288: store: down_read_try
| [004] ....... 61.416296: cpufreq_cpu_get: down_read_try
| [004] ....... 61.416301: cpufreq_cpu_put.part.6: up_read
| [004] ....... 61.416332: store: up_read

as you see, one code path takes the read path of rw_sema twice.

Looking at the generic implementation, we have:
|#define RWSEM_UNLOCKED_VALUE 0x00000000L
|#define RWSEM_ACTIVE_BIAS 0x00000001L

| static inline int __down_read_trylock(struct rw_semaphore *sem)
| {
| long tmp;
| while ((tmp = sem->count) >= 0) {
| if (tmp == cmpxchg(&sem->count, tmp,
| return 1;
| }
| }
| return 0;
| }

While sem->count is >= 0 we loop and take the semaphore. So we can have
five readers at once. The first writer would set count to a negative
value resulting in trylock failure.

|static inline void __down_read(struct rw_semaphore *sem)
| if (unlikely(atomic_long_inc_return((atomic_long_t*)&sem->count) <= 0))
| rwsem_down_read_failed(sem);

Here the same thing but without cmpxchg(). _If_ after an increment the
value is negative then we take slowpath. Otherwise we have the lock.

I think I'm going to revert this patch. Where is it written that
multiple readers of a RW-semaphore can not nest? According to the code
we can even have multiple readers without nesting (two+ processes may
take a reader lock).

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