Messages in this thread | | | Date | Thu, 23 Nov 2023 16:01:19 +0100 | From | Peter Zijlstra <> | Subject | Re: CONFIG_PROVE_RAW_LOCK_NESTING false positive? |
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On Thu, Nov 23, 2023 at 09:00:41AM +0000, David Woodhouse wrote: > Is this telling me that I'm no longer allowed to take a read_lock() in > a callback for an HRTIMER_MODE_ABS_HARD timer? Is that intentional? > > If I must, I can probably cope with this by using read_trylock() > instead. The object being locked is a cache, and we opportunistically > try to use it from the fast path but fall back to a slow path in > process context which will revalidate and try again. So if someone > *has* taken the write lock, it's a fairly safe bet that the cache is > going to be invalidated and we were going to take the slow path anyway. > > [ 62.336965] ============================= > [ 62.336973] [ BUG: Invalid wait context ] > [ 62.336992] 6.7.0-rc1+ #1437 Tainted: G I > [ 62.337001] ----------------------------- > [ 62.337008] qemu-system-x86/1935 is trying to lock: > [ 62.337017] ffffc900018fecc0 (&gpc->lock){....}-{3:3}, at: kvm_xen_set_evtchn_fast+0xe7/0x460 [kvm] > [ 62.337133] other info that might help us debug this: > [ 62.337142] context-{2:2} > [ 62.337148] 2 locks held by qemu-system-x86/1935: > [ 62.337156] #0: ffff888108f780b0 (&vcpu->mutex){+.+.}-{4:4}, at: kvm_vcpu_ioctl+0x7f/0x730 [kvm] > [ 62.337239] #1: ffffc900018ff2d8 (&kvm->srcu){.?.+}-{0:0}, at: kvm_xen_set_evtchn_fast+0xcd/0x460 [kvm] > [ 62.337339] stack backtrace: > [ 62.337346] CPU: 7 PID: 1935 Comm: qemu-system-x86 Tainted: G I 6.7.0-rc1+ #1437 > [ 62.337370] Hardware name: Intel Corporation S2600CW/S2600CW, BIOS SE5C610.86B.01.01.0008.021120151325 02/11/2015 > [ 62.337384] Call Trace: > [ 62.337390] <IRQ> > [ 62.337395] dump_stack_lvl+0x57/0x90 > [ 62.337407] __lock_acquire+0x7bb/0xbb0 > [ 62.337416] ? __lock_acquire+0x4f0/0xbb0 > [ 62.337425] lock_acquire.part.0+0xad/0x240 > [ 62.337433] ? kvm_xen_set_evtchn_fast+0xe7/0x460 [kvm] > [ 62.337512] ? rcu_is_watching+0xd/0x40 > [ 62.337520] ? lock_acquire+0xf2/0x110 > [ 62.337529] __raw_read_lock_irqsave+0x4e/0xa0 > [ 62.337538] ? kvm_xen_set_evtchn_fast+0xe7/0x460 [kvm] > [ 62.337604] kvm_xen_set_evtchn_fast+0xe7/0x460 [kvm] > [ 62.337669] ? kvm_xen_set_evtchn_fast+0xcd/0x460 [kvm] > [ 62.337734] xen_timer_callback+0x86/0xc0 [kvm]
xen_timer_callback is HRTIMER_MODE_ABS_HARD, which means it will still run in IRQ context for PREEMPT_RT.
OTOH read_lock_irqsave() is not a raw spinlock and will be turned into a blocking lock.
This then gives scheduling from IRQ context, which is somewhat frowned upon.
Warning is real and valid.
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