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 --- drivers/xen/preempt.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/xen/preempt.c b/drivers/xen/preempt.c index 17240c5325a3..6ad87b5c95ed 100644 --- a/drivers/xen/preempt.c +++ b/drivers/xen/preempt.c @@ -27,7 +27,7 @@ EXPORT_SYMBOL_GPL(xen_in_preemptible_hcall); 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.