linux-kernel - Re: DEBUG_RWSEMS warning from thaw

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Message-ID: <CAOQ4uxiKQDB-5iFztNz8wPECi5fKMPxEi5cxaKM_zRc9yeticQ@mail.gmail.com>
Date:   Mon, 14 May 2018 14:32:08 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Oleg Nesterov <oleg@...hat.com>, Waiman Long <longman@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: DEBUG_RWSEMS warning from thaw_super()

On Mon, May 14, 2018 at 12:50 PM, Jan Kara <jack@...e.cz> 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@...e.cz>
>

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()")

Sorry for spaces instead of tabs, I'll re-post properly if this is acked.

Thanks,
Amir.

-----
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)