On Mon, May 14, 2018 at 2:51 PM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> On 05/14, Amir Goldstein wrote:
>>
>> On Mon, May 14, 2018 at 12:50 PM, Jan Kara <jack@xxxxxxx> wrote:
>> > On Sun 13-05-18 18:04:04, Oleg Nesterov wrote:
>> >> On 05/13, Amir Goldstein wrote:
>> >> >
>> >> > Since kernel v4.17-rc1 and DEBUG_RWSEMS, I see the
>> >> > warning below after filesystem freeze/thaw.
>> >> >
>> >> > This is a case where one process acquires a bunch of rwsem
>> >> > and another process releases them.
>> >> >
>> >> > To convey this use case to lockdep, freeze_super() calls
>> >> > lockdep_sb_freeze_release() on exit and thaw_super()
>> >> > calls lockdep_sb_freeze_acquire() on entry.
>> >>
>> >> This was already discussed, but I forgot the result...
>> >>
>> >> So once again, why we can't simply update percpu_rwsem_acquire() ?
>> >> Or we can check CONFIG_RWSEM_SPIN_ON_OWNER to match percpu_rwsem_release(),
>> >> but CONFIG_DEBUG_RWSEMS explains the purpose better.
>> >
>> > Yeah, what you suggests seems reasonable to me. So feel free to add:
>> >
>> > Acked-by: Jan Kara <jack@xxxxxxx>
>> >
>>
>> How about this version? A bit more prudent and also addresses the
>> TODO in commit 55cc156505f2 ("percpu-rwsem: introduce
>> percpu_rwsem_release() and percpu_rwsem_acquire()")
>
> Well, to me this looks like unnecessary overcomplication...
>
> In any case the patch looks technically wrong, afaics
> rwsem_can_spin_on_owner() will crash if it hits owner == RWSEM_USER_OWNED.
>
Fair enough. I can drop RWSEM_USER_OWNED.
Do you think I should keep percpu_rwsem_{set,clear}_owner()
helpers?
Thanks,
Amir.
> Oleg.
>
>> diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
>> index b1f37a89e368..323d5ba6a60d 100644
>> --- a/include/linux/percpu-rwsem.h
>> +++ b/include/linux/percpu-rwsem.h
>> @@ -127,13 +127,16 @@ extern void percpu_free_rwsem(struct
>> percpu_rw_semaphore *);
>> #define percpu_rwsem_assert_held(sem) \
>> lockdep_assert_held(&(sem)->rw_sem)
>>
>> +extern void percpu_rwsem_set_user_owned(struct percpu_rw_semaphore *sem);
>> +extern void percpu_rwsem_set_owner(struct percpu_rw_semaphore *sem);
>> +
>> static inline void percpu_rwsem_release(struct percpu_rw_semaphore *sem,
>> bool read, unsigned long ip)
>> {
>> lock_release(&sem->rw_sem.dep_map, 1, ip);
>> #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
>> if (!read)
>> - sem->rw_sem.owner = NULL;
>> + percpu_rwsem_set_user_owned(sem);
>> #endif
>> }
>>
>> @@ -141,6 +144,10 @@ static inline void percpu_rwsem_acquire(struct
>> percpu_rw_semaphore *sem,
>> bool read, unsigned long ip)
>> {
>> lock_acquire(&sem->rw_sem.dep_map, 0, 1, read, 1, NULL, ip);
>> +#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
>> + if (!read)
>> + percpu_rwsem_set_owner(sem);
>> +#endif
>> }
>>
>> #endif
>> diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
>> index 883cf1b92d90..afa65915541f 100644
>> --- a/kernel/locking/percpu-rwsem.c
>> +++ b/kernel/locking/percpu-rwsem.c
>> @@ -7,6 +7,8 @@
>> #include <linux/sched.h>
>> #include <linux/errno.h>
>>
>> +#include "rwsem.h"
>> +
>> int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
>> const char *name, struct lock_class_key *rwsem_key)
>> {
>> @@ -190,3 +192,17 @@ void percpu_up_write(struct percpu_rw_semaphore *sem)
>> rcu_sync_exit(&sem->rss);
>> }
>> EXPORT_SYMBOL_GPL(percpu_up_write);
>> +
>> +void percpu_rwsem_set_user_owned(struct percpu_rw_semaphore *sem)
>> +{
>> + DEBUG_RWSEMS_WARN_ON(sem->rw_sem.owner != current);
>> + sem->rw_sem.owner = RWSEM_USER_OWNED;
>> +}
>> +EXPORT_SYMBOL_GPL(percpu_rwsem_set_user_owned);
>> +
>> +void percpu_rwsem_set_owner(struct percpu_rw_semaphore *sem)
>> +{
>> + DEBUG_RWSEMS_WARN_ON(sem->rw_sem.owner != RWSEM_USER_OWNED);
>> + sem->rw_sem.owner = current;
>> +}
>> +EXPORT_SYMBOL_GPL(percpu_rwsem_set_user_owned);
>> diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
>> index a17cba8d94bb..f686596ec033 100644
>> --- a/kernel/locking/rwsem.h
>> +++ b/kernel/locking/rwsem.h
>> @@ -11,10 +11,14 @@
>> * 2) RWSEM_READER_OWNED
>> * - lock is currently or previously owned by readers (lock is free
>> * or not set by owner yet)
>> - * 3) Other non-zero value
>> + * 3) RWSEM_USER_OWNED
>> + * - lock is currently owned by userspace (previously owned by writer
>> + * and should be handed over to a new writer before being freed)
>> + * 4) Other non-zero value
>> * - a writer owns the lock
>> */
>> #define RWSEM_READER_OWNED ((struct task_struct *)1UL)
>> +#define RWSEM_USER_OWNED ((struct task_struct *)2UL)
>>
>> #ifdef CONFIG_DEBUG_RWSEMS
>> # define DEBUG_RWSEMS_WARN_ON(c) DEBUG_LOCKS_WARN_ON(c)
>
