Jan Kiszka wrote: > Philippe Gerum wrote: >> On Wed, 2009-04-22 at 14:26 +0200, Jan Kiszka wrote: >>> Hi all, >>> >>> issuing rtdm_irq_request and, thus, xnintr_attach can trigger a >>> "I-pipe: Detected stalled topmost domain, probably caused by a bug." if >>> the interrupt type is MSI: >>> >>> [<ffffffff80273cce>] ipipe_check_context+0xe7/0xe9 >>> [<ffffffff8049dae9>] _spin_lock_irqsave+0x18/0x54 >>> [<ffffffff8037dcc2>] pci_bus_read_config_dword+0x3c/0x87 >>> [<ffffffff80387c1d>] read_msi_msg+0x61/0xe1 >>> [<ffffffff8021c5b8>] ? assign_irq_vector+0x3e/0x49 >>> [<ffffffff8021d7b2>] set_msi_irq_affinity+0x6d/0xc8 >>> [<ffffffff8021fa5d>] __ipipe_set_irq_affinity+0x6c/0x77 >>> [<ffffffff80274231>] ipipe_set_irq_affinity+0x34/0x3d >>> [<ffffffff8027c572>] xnintr_attach+0xaa/0x11e >>> >>> Two option to fix this, but I'm currently undecided which one to go: >>> - harden pci_lock (drivers/pci/access.c) - didn't we applied such a >>> MSI-related workaround before? >> This did not work as expected: pathological latency spots. This said, >> the vanilla code has evolved since I tried this quick hack months ago, >> so it may be worth to look at this option once again. >> >>> - move xnarch_set_irq_affinity out of intrlock (but couldn't we face >>> even more pci_lock related issues?) >>> >> Since upstream decided to use PCI config reads even inside hot paths >> when processing MSI interrupts, the only sane way would be to make the >> locking used there Adeos-aware, likely virtualizing the interrupt mask. >> The way upstream generally deals with MSI is currently a problem for us. > > Hmm, guess this needs a closer look again. But I vaguely recall upstream > had removed the config reading at least from the hot paths due to > complaints about performance.
I went through the critical MSI code paths again. Its irqchip comes with ack, mask/unmask and set_affinity which may be used by Xenomai. The ack is most important as it runs during early dispatching, but it maps to ack_apic_edge, thus is clean. The rest is contaminated with PCI config access. At least pci_lock is involved for this, depending on the PCI access method, also pci_config_lock. And then there are other code paths that call wake_up_all while holding pci_lock. This locks more or less hopeless. On the other hand, we shouldn't depend on mask/unmask or set_affinity while in critical Xenomai/I-pipe code paths. The MSI interrupt flow is the same as for edge-triggered (except that there is even no EIO). That means we do not mask deferred interrupts. And that means we should be safe once the affinity setting is fixed. Jan
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