On Tue, 10 Nov 2020 at 14:53, Marco Elver <elver@xxxxxxxxxx> wrote: > > To toggle the allocation gates, we set up a delayed work that calls > toggle_allocation_gate(). Here we use wait_event() to await an > allocation and subsequently disable the static branch again. However, if > the kernel has stopped doing allocations entirely, we'd wait > indefinitely, and stall the worker task. This may also result in the > appropriate warnings if CONFIG_DETECT_HUNG_TASK=y. > > Therefore, introduce a 1 second timeout and use wait_event_timeout(). If > the timeout is reached, the static branch is disabled and a new delayed > work is scheduled to try setting up an allocation at a later time. > > Note that, this scenario is very unlikely during normal workloads once > the kernel has booted and user space tasks are running. It can, however, > happen during early boot after KFENCE has been enabled, when e.g. > running tests that do not result in any allocations. > > Link: https://lkml.kernel.org/r/CADYN=9J0DQhizAGB0-jz4HOBBh+05kMBXb4c0cXMS7Qi5NAJiw@xxxxxxxxxxxxxx > Reported-by: Anders Roxell <anders.roxell@xxxxxxxxxx> > Signed-off-by: Marco Elver <elver@xxxxxxxxxx> > --- > mm/kfence/core.c | 6 +++++- > 1 file changed, 5 insertions(+), 1 deletion(-) > > diff --git a/mm/kfence/core.c b/mm/kfence/core.c > index 9358f42a9a9e..933b197b8634 100644 > --- a/mm/kfence/core.c > +++ b/mm/kfence/core.c > @@ -592,7 +592,11 @@ static void toggle_allocation_gate(struct work_struct *work) > /* Enable static key, and await allocation to happen. */ > atomic_set(&allocation_gate, 0); > static_branch_enable(&kfence_allocation_key); > - wait_event(allocation_wait, atomic_read(&allocation_gate) != 0); > + /* > + * Await an allocation. Timeout after 1 second, in case the kernel stops > + * doing allocations, to avoid stalling this worker task for too long. > + */ > + wait_event_timeout(allocation_wait, atomic_read(&allocation_gate) != 0, HZ); > > /* Disable static key and reset timer. */ > static_branch_disable(&kfence_allocation_key); > -- > 2.29.2.222.g5d2a92d10f8-goog > I gave them a spin on next-20201105 [1] and on next-20201110 [2]. I eventually got to a prompt on next-20201105. However, I got to this kernel panic on the next-20201110: [...] [ 1514.089966][ T1] Testing event system initcall: OK [ 1514.806232][ T1] Running tests on all trace events: [ 1514.857835][ T1] Testing all events: [ 1525.503262][ C0] hrtimer: interrupt took 10902600 ns [ 1623.861452][ C0] BUG: workqueue lockup - pool cpus=0 node=0 flags=0x0 nice=0 stuck for 65s! [...] [ 7823.104349][ T28] Tainted: G W 5.10.0-rc3-next-20201110-00008-g8dc06700529d #3 [ 7833.206491][ T28] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [ 7840.750700][ T28] task:kworker/0:1 state:D stack:26640 pid: 1872 ppid: 2 flags:0x00000428 [ 7875.642531][ T28] Workqueue: events toggle_allocation_gate [ 7889.178334][ T28] Call trace: [ 7897.066649][ T28] __switch_to+0x1cc/0x1e0 [ 7905.326856][ T28] 0xffff00000f7077b0 [ 7928.354644][ T28] INFO: lockdep is turned off. [ 7934.022572][ T28] Kernel panic - not syncing: hung_task: blocked tasks [ 7934.032039][ T28] CPU: 0 PID: 28 Comm: khungtaskd Tainted: G W 5.10.0-rc3-next-20201110-00008-g8dc06700529d #3 [ 7934.045586][ T28] Hardware name: linux,dummy-virt (DT) [ 7934.053677][ T28] Call trace: [ 7934.060276][ T28] dump_backtrace+0x0/0x420 [ 7934.067635][ T28] show_stack+0x38/0xa0 [ 7934.091277][ T28] dump_stack+0x1d4/0x278 [ 7934.098878][ T28] panic+0x304/0x5d8 [ 7934.114923][ T28] check_hung_uninterruptible_tasks+0x5e4/0x640 [ 7934.123823][ T28] watchdog+0x138/0x160 [ 7934.131561][ T28] kthread+0x23c/0x260 [ 7934.138590][ T28] ret_from_fork+0x10/0x18 [ 7934.146631][ T28] Kernel Offset: disabled [ 7934.153749][ T28] CPU features: 0x0240002,20002004 [ 7934.161476][ T28] Memory Limit: none [ 7934.171272][ T28] ---[ end Kernel panic - not syncing: hung_task: blocked tasks ]--- Cheers, Anders [1] https://people.linaro.org/~anders.roxell/output-next-20201105-test.log [2] https://people.linaro.org/~anders.roxell/output-next-20201110-test.log
