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[PATCH] genirq core: fix handle_level_irq()

while porting the -rt tree to 2.6.18-rc7 i noticed the following
screaming-IRQ scenario on an SMP system:

2274 0Dn.:1 0.001ms: do_IRQ+0xc/0x103 <= (ret_from_intr+0x0/0xf)
2274 0Dn.:1 0.010ms: do_IRQ+0xc/0x103 <= (ret_from_intr+0x0/0xf)
2274 0Dn.:1 0.020ms: do_IRQ+0xc/0x103 <= (ret_from_intr+0x0/0xf)
2274 0Dn.:1 0.029ms: do_IRQ+0xc/0x103 <= (ret_from_intr+0x0/0xf)
2274 0Dn.:1 0.039ms: do_IRQ+0xc/0x103 <= (ret_from_intr+0x0/0xf)
2274 0Dn.:1 0.048ms: do_IRQ+0xc/0x103 <= (ret_from_intr+0x0/0xf)
2274 0Dn.:1 0.058ms: do_IRQ+0xc/0x103 <= (ret_from_intr+0x0/0xf)
2274 0Dn.:1 0.068ms: do_IRQ+0xc/0x103 <= (ret_from_intr+0x0/0xf)
2274 0Dn.:1 0.077ms: do_IRQ+0xc/0x103 <= (ret_from_intr+0x0/0xf)
2274 0Dn.:1 0.087ms: do_IRQ+0xc/0x103 <= (ret_from_intr+0x0/0xf)
2274 0Dn.:1 0.097ms: do_IRQ+0xc/0x103 <= (ret_from_intr+0x0/0xf)

as it turns out, the bug is caused by handle_level_irq(), which if it
races with another CPU already handling this IRQ, it _unmasks_ the IRQ
line on the way out. This is not how 2.6.17 works, and we introduced
this bug in one of the early genirq cleanups right before it went into
-mm. (the bug was not in the genirq patchset for a long time, and we
didnt notice the bug due to the lack of -rt rebase to the new genirq
code. -rt, and hardirq-preemption in particular opens up such races much
wider than anything else.)

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

authored by

Ingo Molnar and committed by
Linus Torvalds 86998aa6 47a5c6fa

+3 -3
+3 -3
kernel/irq/chip.c
··· 252 252 mask_ack_irq(desc, irq); 253 253 254 254 if (unlikely(desc->status & IRQ_INPROGRESS)) 255 - goto out; 255 + goto out_unlock; 256 256 desc->status &= ~(IRQ_REPLAY | IRQ_WAITING); 257 257 kstat_cpu(cpu).irqs[irq]++; 258 258 ··· 263 263 action = desc->action; 264 264 if (unlikely(!action || (desc->status & IRQ_DISABLED))) { 265 265 desc->status |= IRQ_PENDING; 266 - goto out; 266 + goto out_unlock; 267 267 } 268 268 269 269 desc->status |= IRQ_INPROGRESS; ··· 276 276 277 277 spin_lock(&desc->lock); 278 278 desc->status &= ~IRQ_INPROGRESS; 279 - out: 280 279 if (!(desc->status & IRQ_DISABLED) && desc->chip->unmask) 281 280 desc->chip->unmask(irq); 281 + out_unlock: 282 282 spin_unlock(&desc->lock); 283 283 } 284 284