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Re: [PATCH] xen: don't reschedule in preemption off sections

On 10.07.20 13:55, Jan Beulich wrote:
On 10.07.2020 12:50, Jürgen Groß wrote:
On 10.07.20 11:49, Jan Beulich wrote:
On 10.07.2020 09:50, Juergen Gross wrote:
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:
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
? 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.

Reported-by: Sarah Newman <srn@xxxxxxxxx>
Fixes: 0fa2f5cb2b0ecd8 ("sched/preempt, xen: Use need_resched() instead of 
Cc: Sarah Newman <srn@xxxxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
   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())) {

Doesn't this have the at least latent risk of hiding issues in
other call trees (by no longer triggering logging like the one
that has propmted this change)? Wouldn't it be better to save,
clear, and restore the flag in one of xen_evtchn_do_upcall() or

First regarding "risk of hiding issues": it seems as if lots of kernels
aren't even configured to trigger this logging. It would need
CONFIG_DEBUG_ATOMIC_SLEEP to be enabled and at least SUSE kernels don't
seem to have it on. I suspect the occasional xen_mc_flush() failures we
have seen are related to this problem.

And in theory saving, clearing and restoring the flag would be fine, but
it can't be done in a single function with the code flow as up to 5.7.
What would need to be done is to save and clear the flag in e.g.
__xen_evtchn_do_upcall() and to pass it to xen_maybe_preempt_hcall() as
a parameter. In xen_maybe_preempt_hcall() the passed flag value would
need to be used for the decision whether to call cond_resched() and then
the flag could be restored (after the cond_resched() call).

I'm afraid I don't follow: If __xen_evtchn_do_upcall() cleared the flag,
xen_maybe_preempt_hcall() would amount to a no-op (up and until the
flag's prior value would get restored), wouldn't it? No need to pass
anything into there.

The problem is after __xen_evtchn_do_upcall() restoring the flag.
As soon as irq_exit() is being called (either by xen_evtchn_do_upcall()
or by the caller of xen_hvm_evtchn_do_upcall()) softirq handling might
be executed resulting in another hypercall, which might be preempted
afterwards. And this is the case which happened in the original
report by Sarah.




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