Messages in this thread | | | Date | Thu, 17 Feb 2011 18:18:29 -0500 | From | Chris Metcalf <> | Subject | Re: IGMP and rwlock: Dead ocurred again on TILEPro |
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On 2/17/2011 6:11 PM, David Miller wrote: > From: Chris Metcalf <cmetcalf@tilera.com> > Date: Thu, 17 Feb 2011 18:04:13 -0500 > >> On 2/17/2011 5:53 PM, David Miller wrote: >>> From: Chris Metcalf <cmetcalf@tilera.com> >>> Date: Thu, 17 Feb 2011 17:49:46 -0500 >>> >>>> The fix is to disable interrupts for the arch_read_lock family of methods. >>> How does that help handle the race when it happens between different >>> cpus, instead of between IRQ and non-IRQ context on the same CPU? >> There's no race in that case, since the lock code properly backs off and >> retries until the other cpu frees it. The distinction here is that the >> non-IRQ context is "wedged" by the IRQ context. >> >>> Why don't you just use the generic spinlock based rwlock code on Tile, >>> since that is all that your atomic instructions can handle >>> sufficiently? >> The tile-specific code encodes reader/writer information in the same 32-bit >> word that the test-and-set instruction manipulates, so it's more efficient >> both in space and time. This may not really matter for rwlocks, since no >> one cares much about them any more, but that was the motivation. > Ok, but IRQ disabling is going to be very expensive.
The interrupt architecture on Tile allows a write to a special-purpose register to put you into a "critical section" where no interrupts or faults are delivered. So we just need to bracket the read_lock operations with two SPR writes; each takes six machine cycles, so we're only adding 12 cycles to the total cost of taking or releasing a read lock on an rwlock.
-- Chris Metcalf, Tilera Corp. http://www.tilera.com
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