• Isaac J. Manjarres's avatar
    stop_machine: Disable preemption when waking two stopper threads · 9fb8d5dc
    Isaac J. Manjarres authored
    When cpu_stop_queue_two_works() begins to wake the stopper threads, it does
    so without preemption disabled, which leads to the following race
    condition:
    
    The source CPU calls cpu_stop_queue_two_works(), with cpu1 as the source
    CPU, and cpu2 as the destination CPU. When adding the stopper threads to
    the wake queue used in this function, the source CPU stopper thread is
    added first, and the destination CPU stopper thread is added last.
    
    When wake_up_q() is invoked to wake the stopper threads, the threads are
    woken up in the order that they are queued in, so the source CPU's stopper
    thread is woken up first, and it preempts the thread running on the source
    CPU.
    
    The stopper thread will then execute on the source CPU, disable preemption,
    and begin executing multi_cpu_stop(), and wait for an ack from the
    destination CPU's stopper thread, with preemption still disabled. Since the
    worker thread that woke up the stopper thread on the source CPU is affine
    to the source CPU, and preemption is disabled on the source CPU, that
    thread will never run to dequeue the destination CPU's stopper thread from
    the wake queue, and thus, the destination CPU's stopper thread will never
    run, causing the source CPU's stopper thread to wait forever, and stall.
    
    Disable preemption when waking the stopper threads in
    cpu_stop_queue_two_works().
    
    Fixes: 0b26351b ("stop_machine, sched: Fix migrate_swap() vs. active_balance() deadlock")
    Co-Developed-by: default avatarPrasad Sodagudi <psodagud@codeaurora.org>
    Signed-off-by: default avatarPrasad Sodagudi <psodagud@codeaurora.org>
    Co-Developed-by: default avatarPavankumar Kondeti <pkondeti@codeaurora.org>
    Signed-off-by: default avatarPavankumar Kondeti <pkondeti@codeaurora.org>
    Signed-off-by: default avatarIsaac J. Manjarres <isaacm@codeaurora.org>
    Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
    Cc: peterz@infradead.org
    Cc: matt@codeblueprint.co.uk
    Cc: bigeasy@linutronix.de
    Cc: gregkh@linuxfoundation.org
    Cc: stable@vger.kernel.org
    Link: https://lkml.kernel.org/r/1530655334-4601-1-git-send-email-isaacm@codeaurora.org
    9fb8d5dc
stop_machine.c 17.4 KB