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    Subject[PATCH 4.14 49/50] xen: dont reschedule in preemption off sections
    Date
    From: Juergen Gross <jgross@suse.com>

    For support of long running hypercalls xen_maybe_preempt_hcall() is
    calling cond_resched() in case a hypercall marked as preemptible has
    been interrupted.

    Normally this is no problem, as only hypercalls done via some ioctl()s
    are marked to be preemptible. In rare cases when during such a
    preemptible hypercall an interrupt occurs and any softirq action is
    started from irq_exit(), a further hypercall issued by the softirq
    handler will be regarded to be preemptible, too. This might lead to
    rescheduling in spite of the softirq handler potentially having set
    preempt_disable(), leading to splats like:

    BUG: sleeping function called from invalid context at drivers/xen/preempt.c:37
    in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 20775, name: xl
    INFO: lockdep is turned off.
    CPU: 1 PID: 20775 Comm: xl Tainted: G D W 5.4.46-1_prgmr_debug.el7.x86_64 #1
    Call Trace:
    <IRQ>
    dump_stack+0x8f/0xd0
    ___might_sleep.cold.76+0xb2/0x103
    xen_maybe_preempt_hcall+0x48/0x70
    xen_do_hypervisor_callback+0x37/0x40
    RIP: e030:xen_hypercall_xen_version+0xa/0x20
    Code: ...
    RSP: e02b:ffffc900400dcc30 EFLAGS: 00000246
    RAX: 000000000004000d RBX: 0000000000000200 RCX: ffffffff8100122a
    RDX: ffff88812e788000 RSI: 0000000000000000 RDI: 0000000000000000
    RBP: ffffffff83ee3ad0 R08: 0000000000000001 R09: 0000000000000001
    R10: 0000000000000000 R11: 0000000000000246 R12: ffff8881824aa0b0
    R13: 0000000865496000 R14: 0000000865496000 R15: ffff88815d040000
    ? xen_hypercall_xen_version+0xa/0x20
    ? xen_force_evtchn_callback+0x9/0x10
    ? check_events+0x12/0x20
    ? xen_restore_fl_direct+0x1f/0x20
    ? _raw_spin_unlock_irqrestore+0x53/0x60
    ? debug_dma_sync_single_for_cpu+0x91/0xc0
    ? _raw_spin_unlock_irqrestore+0x53/0x60
    ? xen_swiotlb_sync_single_for_cpu+0x3d/0x140
    ? mlx4_en_process_rx_cq+0x6b6/0x1110 [mlx4_en]
    ? mlx4_en_poll_rx_cq+0x64/0x100 [mlx4_en]
    ? net_rx_action+0x151/0x4a0
    ? __do_softirq+0xed/0x55b
    ? irq_exit+0xea/0x100
    ? xen_evtchn_do_upcall+0x2c/0x40
    ? xen_do_hypervisor_callback+0x29/0x40
    </IRQ>
    ? xen_hypercall_domctl+0xa/0x20
    ? xen_hypercall_domctl+0x8/0x20
    ? privcmd_ioctl+0x221/0x990 [xen_privcmd]
    ? do_vfs_ioctl+0xa5/0x6f0
    ? ksys_ioctl+0x60/0x90
    ? trace_hardirqs_off_thunk+0x1a/0x20
    ? __x64_sys_ioctl+0x16/0x20
    ? do_syscall_64+0x62/0x250
    ? entry_SYSCALL_64_after_hwframe+0x49/0xbe

    Fix that by testing preempt_count() before calling cond_resched().

    In kernel 5.8 this can't happen any more due to the entry code rework
    (more than 100 patches, so not a candidate for backporting).

    The issue was introduced in kernel 4.3, so this patch should go into
    all stable kernels in [4.3 ... 5.7].

    Reported-by: Sarah Newman <srn@prgmr.com>
    Fixes: 0fa2f5cb2b0ecd8 ("sched/preempt, xen: Use need_resched() instead of should_resched()")
    Cc: Sarah Newman <srn@prgmr.com>
    Cc: stable@vger.kernel.org
    Signed-off-by: Juergen Gross <jgross@suse.com>
    Tested-by: Chris Brannon <cmb@prgmr.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    drivers/xen/preempt.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/drivers/xen/preempt.c
    +++ b/drivers/xen/preempt.c
    @@ -31,7 +31,7 @@ EXPORT_SYMBOL_GPL(xen_in_preemptible_hca
    asmlinkage __visible void xen_maybe_preempt_hcall(void)
    {
    if (unlikely(__this_cpu_read(xen_in_preemptible_hcall)
    - && need_resched())) {
    + && need_resched() && !preempt_count())) {
    /*
    * Clear flag as we may be rescheduled on a different
    * cpu.

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