Date:   Wed, 16 May 2018 08:37:24 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v4 2/2] locking/percpu-rwsem: Annotate rwsem ownership
 transfer by setting RWSEM_OWNER_UNKNOWN

On Wed, May 16, 2018 at 12:49 AM, Waiman Long <longman@...hat.com> wrote:
> The filesystem freezing code needs to transfer ownership of a rwsem
> embedded in a percpu-rwsem from the task that does the freezing to
> another one that does the thawing by calling percpu_rwsem_release()
> after freezing and percpu_rwsem_acquire() before thawing.
>
> However, the new rwsem debug code runs afoul with this scheme by warning
> that the task that releases the rwsem isn't the one that acquires it.
>
> [   20.302978] ------------[ cut here ]------------
> [   20.305016] DEBUG_LOCKS_WARN_ON(sem->owner != get_current())
> [   20.305029] WARNING: CPU: 1 PID: 1401 at
> /home/amir/build/src/linux/kernel/locking/rwsem.c:133 up_write+0x59/0x79
> [   20.311252] CPU: 1 PID: 1401 Comm: fsfreeze Not tainted 4.17.0-rc3-xfstests-00049-g39e47bf59eb3 #3276
> [   20.314808] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
> BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
> [   20.318403] RIP: 0010:up_write+0x59/0x79
> [   20.320928] RSP: 0018:ffffc90000717e48 EFLAGS: 00010286
> [   20.322955] RAX: 0000000000000030 RBX: ffff880078f1c680 RCX: ffff880078e42200
> [   20.325665] RDX: ffffffff810cc9c1 RSI: 0000000000000001 RDI: 0000000000000202
> [   20.328844] RBP: ffffc90000717e80 R08: 0000000000000001 R09: 0000000000000001
> [   20.332340] R10: ffffc90000717c58 R11: ffffffff836807ad R12: ffff880078f1c388
> [   20.335095] R13: ffff880078a8b980 R14: 0000000000000000 R15: 00000000fffffff7
> [   20.338009] FS:  00007fb61ca42700(0000) GS:ffff88007f400000(0000) knlGS:0000000000000000
> [   20.341423] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [   20.343772] CR2: 00007fb61c559b30 CR3: 0000000078da6000 CR4: 00000000000006e0
> [   20.346463] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [   20.349201] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [   20.351960] Call Trace:
> [   20.352911]  percpu_up_write+0x1f/0x28
> [   20.354344]  thaw_super_locked+0xdf/0x120
> [   20.355944]  do_vfs_ioctl+0x270/0x5f1
> [   20.357390]  ? __se_sys_newfstat+0x2e/0x39
> [   20.358969]  ? entry_SYSCALL_64_after_hwframe+0x59/0xbe
> [   20.360991]  ksys_ioctl+0x52/0x71
> [   20.362384]  __x64_sys_ioctl+0x16/0x19
> [   20.363702]  do_syscall_64+0x5d/0x167
> [   20.365099]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> To work properly with the rwsem debug code, we need to annotate that the
> rwsem ownership is unknown during the tranfer period until a brave soul
> comes forward to acquire the ownership. During that period, optimistic
> spinning will be disabled.
>
> Signed-off-by: Waiman Long <longman@...hat.com>

Looks good and tested

Thanks,
Amir.

> ---
>  include/linux/percpu-rwsem.h | 6 +++++-
>  include/linux/rwsem.h        | 6 ++++++
>  kernel/locking/rwsem-xadd.c  | 2 ++
>  3 files changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
> index b1f37a8..79b99d6 100644
> --- a/include/linux/percpu-rwsem.h
> +++ b/include/linux/percpu-rwsem.h
> @@ -133,7 +133,7 @@ static inline void percpu_rwsem_release(struct percpu_rw_semaphore *sem,
>         lock_release(&sem->rw_sem.dep_map, 1, ip);
>  #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
>         if (!read)
> -               sem->rw_sem.owner = NULL;
> +               sem->rw_sem.owner = RWSEM_OWNER_UNKNOWN;
>  #endif
>  }
>
> @@ -141,6 +141,10 @@ 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)
> +               sem->rw_sem.owner = current;
> +#endif
>  }
>
>  #endif
> diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
> index 56707d5..ab93b6e 100644
> --- a/include/linux/rwsem.h
> +++ b/include/linux/rwsem.h
> @@ -44,6 +44,12 @@ struct rw_semaphore {
>  #endif
>  };
>
> +/*
> + * Setting bit 0 of the owner field with other non-zero bits will indicate
> + * that the rwsem is writer-owned with an unknown owner.
> + */
> +#define RWSEM_OWNER_UNKNOWN    ((struct task_struct *)-1L)
> +
>  extern struct rw_semaphore *rwsem_down_read_failed(struct rw_semaphore *sem);
>  extern struct rw_semaphore *rwsem_down_read_failed_killable(struct rw_semaphore *sem);
>  extern struct rw_semaphore *rwsem_down_write_failed(struct rw_semaphore *sem);
> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index 604d247..a903367 100644
> --- a/kernel/locking/rwsem-xadd.c
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -352,6 +352,8 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
>         struct task_struct *owner;
>         bool ret = true;
>
> +       BUILD_BUG_ON(!rwsem_has_anonymous_owner(RWSEM_OWNER_UNKNOWN));
> +
>         if (need_resched())
>                 return false;
>
> --
> 1.8.3.1
>

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