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    [IA64] Revert "prevent ia64 from invoking irq handlers on offline CPUs" · 66db2e63
    Alex Chiang authored
    This reverts commit e7b14036.
    Commit e7b14036 removes the targetted disabled CPU from the
    cpu_online_map after calls to migrate_platform_irqs and fixup_irqs.
    Paul McKenney states that the reasoning behind the patch was to
    prevent irq handlers from running on CPUs marked offline because:
    	RCU happily ignores CPUs that don't have their bits set in
    	cpu_online_map, so if there are RCU read-side critical sections
    	in the irq handlers being run, RCU will ignore them.  If the
    	other CPUs were running, they might sequence through the RCU
    	state machine, which could result in data structures being
    	yanked out from under those irq handlers, which in turn could
    	result in oopses or worse.
    Unfortunately, both ia64 functions above look at cpu_online_map to find
    a new CPU to migrate interrupts onto. This means we can potentially
    migrate an interrupt off ourself back to... ourself. Uh oh.
    This causes an oops when we finally try to process pending interrupts on
    the CPU we want to disable. The oops results from calling __do_IRQ with
    a NULL pt_regs:
    Unable to handle kernel NULL pointer dereference (address 0000000000000040)
    Call Trace:
     [<a000000100016930>] show_stack+0x50/0xa0
                                    sp=e0000009c922fa00 bsp=e0000009c92214d0
     [<a0000001000171a0>] show_regs+0x820/0x860
                                    sp=e0000009c922fbd0 bsp=e0000009c9221478
     [<a00000010003c700>] die+0x1a0/0x2e0
                                    sp=e0000009c922fbd0 bsp=e0000009c9221438
     [<a0000001006e92f0>] ia64_do_page_fault+0x950/0xa80
                                    sp=e0000009c922fbd0 bsp=e0000009c92213d8
     [<a00000010000c7a0>] ia64_native_leave_kernel+0x0/0x270
                                    sp=e0000009c922fc60 bsp=e0000009c92213d8
     [<a0000001000ecdb0>] profile_tick+0xd0/0x1c0
                                    sp=e0000009c922fe30 bsp=e0000009c9221398
     [<a00000010003bb90>] timer_interrupt+0x170/0x3e0
                                    sp=e0000009c922fe30 bsp=e0000009c9221330
     [<a00000010013a800>] handle_IRQ_event+0x80/0x120
                                    sp=e0000009c922fe30 bsp=e0000009c92212f8
     [<a00000010013aa00>] __do_IRQ+0x160/0x4a0
                                    sp=e0000009c922fe30 bsp=e0000009c9221290
     [<a000000100012290>] ia64_process_pending_intr+0x2b0/0x360
                                    sp=e0000009c922fe30 bsp=e0000009c9221208
     [<a0000001000112d0>] fixup_irqs+0xf0/0x2a0
                                    sp=e0000009c922fe30 bsp=e0000009c92211a8
     [<a00000010005bd80>] __cpu_disable+0x140/0x240
                                    sp=e0000009c922fe30 bsp=e0000009c9221168
     [<a0000001006c5870>] take_cpu_down+0x50/0xa0
                                    sp=e0000009c922fe30 bsp=e0000009c9221148
     [<a000000100122610>] stop_cpu+0xd0/0x200
                                    sp=e0000009c922fe30 bsp=e0000009c92210f0
     [<a0000001000e0440>] kthread+0xc0/0x140
                                    sp=e0000009c922fe30 bsp=e0000009c92210c8
     [<a000000100014ab0>] kernel_thread_helper+0xd0/0x100
                                    sp=e0000009c922fe30 bsp=e0000009c92210a0
     [<a00000010000a4c0>] start_kernel_thread+0x20/0x40
                                    sp=e0000009c922fe30 bsp=e0000009c92210a0
    I don't like this revert because it is fragile. ia64 is getting lucky
    because we seem to only ever process timer interrupts in this path, but
    if we ever race with an IPI here, we definitely use RCU and have the
    potential of hitting an oops that Paul describes above.
    Patching ia64's timer_interrupt() to check for NULL pt_regs is
    insufficient though, as we still hit the above oops.
    As a short term solution, I do think that this revert is the right
    answer. The revert hold up under repeated testing (24+ hour test runs)
    with this setup:
    	- 8-way rx6600
    	- randomly toggling CPU online/offline state every 2 seconds
    	- running CPU exercisers, memory hog, disk exercisers, and
    	  network stressors
    	- average system load around ~160
    In the long term, we really need to figure out why we set pt_regs = NULL
    in ia64_process_pending_intr(). If it turns out that it is unnecessary
    to do so, then we could safely re-introduce e7b14036
     (along with some
    other logic to be smarter about migrating interrupts).
    One final note: x86 also removes the disabled CPU from cpu_online_map
    and then re-enables interrupts for 1ms, presumably to handle any pending
    arch/x86/kernel/irq_32.c (and irq_64.c):
    	[remove cpu from cpu_online_map]
    			[break CPU affinities]
    So they are doing implicitly what ia64 is doing explicitly.
    Signed-off-by: default avatarAlex Chiang <>
    Signed-off-by: default avatarTony Luck <aegl@agluck-desktop.(none)>