linux-kernel - Re: DEBUG_RWSEMS warning from thaw

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Date:   Sun, 13 May 2018 18:04:04 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     Waiman Long <longman@...hat.com>, Ingo Molnar <mingo@...nel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>, Al Viro <viro@...iv.linux.org.uk>
Subject: Re: DEBUG_RWSEMS warning from thaw_super()

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.

Oleg.

--- x/include/linux/percpu-rwsem.h
+++ x/include/linux/percpu-rwsem.h
@@ -141,6 +141,10 @@ static inline void percpu_rwsem_acquire(
 					bool read, unsigned long ip)
 {
 	lock_acquire(&sem->rw_sem.dep_map, 0, 1, read, 1, NULL, ip);
+#ifdef CONFIG_DEBUG_RWSEMS
+	if (!read)
+		sem->rw_sem.owner = current;
+#endif
 }
 
 #endif