Re: [PATCH] xen/xenbus: Avoid a lockdep warning when adding a watch

[Juergen Gross <jgross@xxxxxxxx>]

On 07.06.23 14:36, Petr Pavlu wrote:
> The following lockdep warning appears during boot on a Xen dom0 system:
>
> [   96.388794] ======================================================
> [   96.388797] WARNING: possible circular locking dependency detected
> [   96.388799] 6.4.0-rc5-default+ #8 Tainted: G            EL
> [   96.388803] ------------------------------------------------------
> [   96.388804] xenconsoled/1330 is trying to acquire lock:
> [   96.388808] ffffffff82acdd10 (xs_watch_rwsem){++++}-{3:3}, at: 
> register_xenbus_watch+0x45/0x140
> [   96.388847]
>                 but task is already holding lock:
> [   96.388849] ffff888100c92068 (&u->msgbuffer_mutex){+.+.}-{3:3}, at: 
> xenbus_file_write+0x2c/0x600
> [   96.388862]
>                 which lock already depends on the new lock.
>
> [   96.388864]
>                 the existing dependency chain (in reverse order) is:
> [   96.388866]
>                 -> #2 (&u->msgbuffer_mutex){+.+.}-{3:3}:
> [   96.388874]        __mutex_lock+0x85/0xb30
> [   96.388885]        xenbus_dev_queue_reply+0x48/0x2b0
> [   96.388890]        xenbus_thread+0x1d7/0x950
> [   96.388897]        kthread+0xe7/0x120
> [   96.388905]        ret_from_fork+0x2c/0x50
> [   96.388914]
>                 -> #1 (xs_response_mutex){+.+.}-{3:3}:
> [   96.388923]        __mutex_lock+0x85/0xb30
> [   96.388930]        xenbus_backend_ioctl+0x56/0x1c0
> [   96.388935]        __x64_sys_ioctl+0x90/0xd0
> [   96.388942]        do_syscall_64+0x5c/0x90
> [   96.388950]        entry_SYSCALL_64_after_hwframe+0x72/0xdc
> [   96.388957]
>                 -> #0 (xs_watch_rwsem){++++}-{3:3}:
> [   96.388965]        __lock_acquire+0x1538/0x2260
> [   96.388972]        lock_acquire+0xc6/0x2b0
> [   96.388976]        down_read+0x2d/0x160
> [   96.388983]        register_xenbus_watch+0x45/0x140
> [   96.388990]        xenbus_file_write+0x53d/0x600
> [   96.388994]        vfs_write+0xe4/0x490
> [   96.389003]        ksys_write+0xb8/0xf0
> [   96.389011]        do_syscall_64+0x5c/0x90
> [   96.389017]        entry_SYSCALL_64_after_hwframe+0x72/0xdc
> [   96.389023]
>                 other info that might help us debug this:
>
> [   96.389025] Chain exists of:
>                   xs_watch_rwsem --> xs_response_mutex --> &u->msgbuffer_mutex
>
> [   96.413429]  Possible unsafe locking scenario:
>
> [   96.413430]        CPU0                    CPU1
> [   96.413430]        ----                    ----
> [   96.413431]   lock(&u->msgbuffer_mutex);
> [   96.413432]                                lock(xs_response_mutex);
> [   96.413433]                                lock(&u->msgbuffer_mutex);
> [   96.413434]   rlock(xs_watch_rwsem);
> [   96.413436]
>                  *** DEADLOCK ***
>
> [   96.413436] 1 lock held by xenconsoled/1330:
> [   96.413438]  #0: ffff888100c92068 (&u->msgbuffer_mutex){+.+.}-{3:3}, at: 
> xenbus_file_write+0x2c/0x600
> [   96.413446]
>
> An ioctl call IOCTL_XENBUS_BACKEND_SETUP (record #1 in the report)
> results in calling xenbus_alloc() -> xs_suspend() which introduces
> ordering xs_watch_rwsem --> xs_response_mutex. The xenbus_thread()
> operation (record #2) creates xs_response_mutex --> &u->msgbuffer_mutex.
> An XS_WATCH write to the xenbus file then results in a complain about
> the opposite lock order &u->msgbuffer_mutex --> xs_watch_rwsem.
>
> The dependency xs_watch_rwsem --> xs_response_mutex is spurious. Avoid
> it and the warning by changing the ordering in xs_suspend(), first
> acquire xs_response_mutex and then xs_watch_rwsem. Reverse also the
> unlocking order in xs_suspend_cancel() for consistency, but keep
> xs_resume() as is because it needs to have xs_watch_rwsem unlocked only
> after exiting xs suspend and re-adding all watches.
>
> Signed-off-by: Petr Pavlu <petr.pavlu@xxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>

Sorry it took so long,


Juergen

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