Re: [PATCH v4 2/2] locking/percpu-rwsem: Annotate rwsem ownership transfer by setting RWSEM_OWNER_UNKNOWN

From: Waiman Long <longman@redhat.com>
To: Amir Goldstein <amir73il@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Davidlohr Bueso <dave@stgolabs.net>,
	"Theodore Y. Ts'o" <tytso@mit.edu>,
	Oleg Nesterov <oleg@redhat.com>, Jan Kara <jack@suse.cz>,
	Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v4 2/2] locking/percpu-rwsem: Annotate rwsem ownership transfer by setting RWSEM_OWNER_UNKNOWN
Date: Wed, 16 May 2018 09:17:39 -0400	[thread overview]
Message-ID: <3bbdbfa8-0a18-e4e9-eaa9-085022e4e11a@redhat.com> (raw)
In-Reply-To: <CAOQ4uxgtsj=u8uVHJJHHuYEVWbf2BFg4YH9emMZrO4ZG+vf_4Q@mail.gmail.com>

On 05/16/2018 01:37 AM, Amir Goldstein wrote:
> On Wed, May 16, 2018 at 12:49 AM, Waiman Long <longman@redhat.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@redhat.com>
> Looks good and tested
>
> Thanks,
> Amir.
>
Thanks for the testing.

Cheers,
Longman