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    SubjectRe: [PATCH] arch/tile: fix rwlock so would-be write lockers don't block new readers
    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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