On Wed, 18 Feb 2015 20:57:10 +0100
Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> wrote:
> * 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:
> | test.sh-788 [004] ....... 61.416288: store: down_read_try
> | test.sh-788 [004] ....... 61.416296: cpufreq_cpu_get: down_read_try
> | test.sh-788 [004] ....... 61.416301: cpufreq_cpu_put.part.6: up_read
> | test.sh-788 [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
> |#define RWSEM_WAITING_BIAS (-RWSEM_ACTIVE_MASK-1)
>
> | static inline int __down_read_trylock(struct rw_semaphore *sem)
> | {
> | long tmp;
> |
> | while ((tmp = sem->count) >= 0) {
> | if (tmp == cmpxchg(&sem->count, tmp,
> | tmp + RWSEM_ACTIVE_READ_BIAS)) {
> | 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.
OK, so I need to make it so it can nest with trylock. I have to look at
the patch again because it has been a while.
>
> 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).
An RW sem must not do two down_read()s on the same lock (it's fine for
a trylock if it has a fail safe for it). The reason is, the second
down_read() will block if there's a writer waiting. Thus you are
guaranteed a deadlock if you have the lock for read, a write comes in
and waits, and you grab the RW sem again, because it will block, and
the writer is waiting for the reader to release. Thus you have a
deadlock.
I'll have to revisit this. I also need to revisit the multi readers
(although Thomas hates it, but he even admitted there's a better way to
do it. Now only if I could remember what that was ;-)
Thanks,
-- Steve
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