Messages in this thread | | | Date | Mon, 22 Nov 2010 08:35:41 -0500 | From | Chris Metcalf <> | Subject | Re: [PATCH] arch/tile: fix rwlock so would-be write lockers don't block new readers |
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On 11/22/2010 12:39 AM, Cypher Wu wrote: > 2010/11/15 Chris Metcalf <cmetcalf@tilera.com>: >> This avoids a deadlock in the IGMP code where one core gets a read >> lock, another core starts trying to get a write lock (thus blocking >> new readers), and then the first core tries to recursively re-acquire >> the read lock. >> >> We still try to preserve some degree of balance by giving priority >> to additional write lockers that come along while the lock is held >> for write, so they can all complete quickly and return the lock to >> the readers. >> >> Signed-off-by: Chris Metcalf <cmetcalf@tilera.com> >> --- >> This should apply relatively cleanly to 2.6.26.7 source code too. >> [...] > > I've finished my business trip and tested that patch for more than an > hour and it works. The test is still running now. > > But it seems there still has a potential problem: we used ticket lock > for write_lock(), and if there are so many write_lock() occurred, is > 256 ticket enough for 64 or even more cores to avoiding overflow? > Since is we try to write_unlock() and there's already write_lock() > waiting we'll only adding current ticket.
This is OK, since each core can issue at most one (blocking) write_lock(), and we have only 64 cores. Future >256 core machines will be based on TILE-Gx anyway, which doesn't have the 256-core limit since it doesn't use the spinlock_32.c implementation.
-- Chris Metcalf, Tilera Corp. http://www.tilera.com
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