• Juergen Gross's avatar
    xen: avoid deadlock in xenbus driver · 618ebd55
    Juergen Gross authored
    commit 1a3fc2c4 upstream.
    
    There has been a report about a deadlock in the xenbus driver:
    
    [  247.979498] ======================================================
    [  247.985688] WARNING: possible circular locking dependency detected
    [  247.991882] 4.12.0-rc4-00022-gc4b25c0 #575 Not tainted
    [  247.997040] ------------------------------------------------------
    [  248.003232] xenbus/91 is trying to acquire lock:
    [  248.007875]  (&u->msgbuffer_mutex){+.+.+.}, at: [<ffff00000863e904>]
    xenbus_dev_queue_reply+0x3c/0x230
    [  248.017163]
    [  248.017163] but task is already holding lock:
    [  248.023096]  (xb_write_mutex){+.+...}, at: [<ffff00000863a940>]
    xenbus_thread+0x5f0/0x798
    [  248.031267]
    [  248.031267] which lock already depends on the new lock.
    [  248.031267]
    [  248.039615]
    [  248.039615] the existing dependency chain (in reverse order) is:
    [  248.047176]
    [  248.047176] -> #1 (xb_write_mutex){+.+...}:
    [  248.052943]        __lock_acquire+0x1728/0x1778
    [  248.057498]        lock_acquire+0xc4/0x288
    [  248.061630]        __mutex_lock+0x84/0x868
    [  248.065755]        mutex_lock_nested+0x3c/0x50
    [  248.070227]        xs_send+0x164/0x1f8
    [  248.074015]        xenbus_dev_request_and_reply+0x6c/0x88
    [  248.079427]        xenbus_file_write+0x260/0x420
    [  248.084073]        __vfs_write+0x48/0x138
    [  248.088113]        vfs_write+0xa8/0x1b8
    [  248.091983]        SyS_write+0x54/0xb0
    [  248.095768]        el0_svc_naked+0x24/0x28
    [  248.099897]
    [  248.099897] -> #0 (&u->msgbuffer_mutex){+.+.+.}:
    [  248.106088]        print_circular_bug+0x80/0x2e0
    [  248.110730]        __lock_acquire+0x1768/0x1778
    [  248.115288]        lock_acquire+0xc4/0x288
    [  248.119417]        __mutex_lock+0x84/0x868
    [  248.123545]        mutex_lock_nested+0x3c/0x50
    [  248.128016]        xenbus_dev_queue_reply+0x3c/0x230
    [  248.133005]        xenbus_thread+0x788/0x798
    [  248.137306]        kthread+0x110/0x140
    [  248.141087]        ret_from_fork+0x10/0x40
    
    It is rather easy to avoid by dropping xb_write_mutex before calling
    xenbus_dev_queue_reply().
    
    Fixes: fd8aa909 ("xen: optimize xenbus
    driver for multiple concurrent xenstore accesses").
    Reported-by: default avatarAndre Przywara <andre.przywara@arm.com>
    Signed-off-by: default avatarJuergen Gross <jgross@suse.com>
    Tested-by: default avatarAndre Przywara <andre.przywara@arm.com>
    Signed-off-by: default avatarJuergen Gross <jgross@suse.com>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    618ebd55
xenbus_comms.c 11.2 KB