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SubjectRe: [PATCH v2] PCI/MSI: Avoid torn updates to MSI pairs
On Tue, Jan 28, 2020 at 6:38 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Evan,
>
> Thomas Gleixner <tglx@linutronix.de> writes:
> > It's worthwhile, but that needs some deep thoughts about locking and
> > ordering plus the inevitable race conditions this creates. If it would
> > be trivial, I surely wouldn't have hacked up the retrigger mess.
>
> So after staring at it for a while, I came up with the patch below.
>
> Your idea of going through some well defined transition vector is just
> not feasible due to locking and life-time issues.
>
> I'm taking a similar but easier to handle approach.
>
> 1) Move the interrupt to the new vector on the old (local) CPU
>
> 2) Move it to the new CPU
>
> 3) Check if the new vector is pending on the local CPU. If yes
> retrigger it on the new CPU.
>
> That might give a spurious interrupt if the new vector on the local CPU
> is in use. But as I said before this is nothing to worry about. If the
> affected device driver fails to handle that spurious interrupt then it
> is broken anyway.
>
> In theory we could teach the vector allocation logic to search for an
> unused pair of vectors on both CPUs, but the required code for that is
> hardly worth the trouble. In the end the situation that no pair is found
> has to be handled anyway. So rather than making this the corner case
> which is never tested and then leads to hard to debug issues, I prefer
> to make it more likely to happen.
>
> The patch is only lightly tested, but so far it survived.
>

Hi Thomas,
Thanks for the patch, I gave it a try. I get the following splat, then a hang:

[ 62.173778] ============================================
[ 62.179723] WARNING: possible recursive locking detected
[ 62.185657] 4.19.96 #2 Not tainted
[ 62.189453] --------------------------------------------
[ 62.195388] migration/1/17 is trying to acquire lock:
[ 62.201031] 000000006885da2d (vector_lock){-.-.}, at:
apic_retrigger_irq+0x31/0x63
[ 62.209508]
[ 62.209508] but task is already holding lock:
[ 62.216026] 000000006885da2d (vector_lock){-.-.}, at:
msi_set_affinity+0x13c/0x27b
[ 62.224498]
[ 62.224498] other info that might help us debug this:
[ 62.231791] Possible unsafe locking scenario:
[ 62.231791]
[ 62.238406] CPU0
[ 62.241135] ----
[ 62.243863] lock(vector_lock);
[ 62.247467] lock(vector_lock);
[ 62.251071]
[ 62.251071] *** DEADLOCK ***
[ 62.251071]
[ 62.257687] May be due to missing lock nesting notation
[ 62.257687]
[ 62.265274] 2 locks held by migration/1/17:
[ 62.269946] #0: 00000000cfa9d8c3 (&irq_desc_lock_class){-.-.}, at:
irq_migrate_all_off_this_cpu+0x44/0x28f
[ 62.280846] #1: 000000006885da2d (vector_lock){-.-.}, at:
msi_set_affinity+0x13c/0x27b
[ 62.289801]
[ 62.289801] stack backtrace:
[ 62.294669] CPU: 1 PID: 17 Comm: migration/1 Not tainted 4.19.96 #2
[ 62.310713] Call Trace:
[ 62.313446] dump_stack+0xac/0x11e
[ 62.317255] __lock_acquire+0x64f/0x19bc
[ 62.321646] ? find_held_lock+0x3d/0xb8
[ 62.325936] ? pci_conf1_write+0x4f/0xdf
[ 62.330320] lock_acquire+0x1b2/0x1fa
[ 62.334413] ? apic_retrigger_irq+0x31/0x63
[ 62.339097] _raw_spin_lock_irqsave+0x51/0x7d
[ 62.343972] ? apic_retrigger_irq+0x31/0x63
[ 62.348646] apic_retrigger_irq+0x31/0x63
[ 62.353124] msi_set_affinity+0x25a/0x27b
[ 62.357606] irq_do_set_affinity+0x37/0xaa
[ 62.362191] irq_migrate_all_off_this_cpu+0x1c1/0x28f
[ 62.367841] fixup_irqs+0x15/0xd2
[ 62.371544] cpu_disable_common+0x20a/0x217
[ 62.376217] native_cpu_disable+0x1f/0x24
[ 62.380696] take_cpu_down+0x41/0x95
[ 62.384691] multi_cpu_stop+0xbd/0x14b
[ 62.388878] ? _raw_spin_unlock_irq+0x2c/0x40
[ 62.393746] ? stop_two_cpus+0x2c5/0x2c5
[ 62.398127] cpu_stopper_thread+0x84/0x100
[ 62.402705] smpboot_thread_fn+0x1a9/0x25f
[ 62.407281] ? cpu_report_death+0x81/0x81
[ 62.411760] kthread+0x146/0x14e
[ 62.415364] ? cpu_report_death+0x81/0x81
[ 62.419846] ? kthread_blkcg+0x31/0x31
[ 62.424042] ret_from_fork+0x24/0x50

-Evan

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