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Subject[PATCH v3 2/2] locking/percpu-rwsem: Annotate rwsem ownership transfer by setting RWSEM_OWNER_UNKNOWN
Date
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>
---
include/linux/percpu-rwsem.h | 6 +++++-
include/linux/rwsem.h | 5 +++++
2 files changed, 10 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..fc2ed27 100644
--- a/include/linux/rwsem.h
+++ b/include/linux/rwsem.h
@@ -44,6 +44,11 @@ struct rw_semaphore {
#endif
};

+/*
+ * Owner value to indicate the rwsem's owner is not currently known.
+ */
+#define RWSEM_OWNER_UNKNOWN ((struct task_struct *)-1)
+
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);
--
1.8.3.1
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