Messages in this thread | | | Subject | Re: [PATCH] arch/tile: fix rwlock so would-be write lockers don't block new readers | From | Eric Dumazet <> | Date | Mon, 15 Nov 2010 15:52:10 +0100 |
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Le lundi 15 novembre 2010 à 09:18 -0500, Chris Metcalf a écrit : > 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. > > arch/tile/lib/spinlock_32.c | 29 ++++++++++++++++++----------- > 1 files changed, 18 insertions(+), 11 deletions(-) > > diff --git a/arch/tile/lib/spinlock_32.c b/arch/tile/lib/spinlock_32.c > index 485e24d..5cd1c40 100644 > --- a/arch/tile/lib/spinlock_32.c > +++ b/arch/tile/lib/spinlock_32.c > @@ -167,23 +167,30 @@ void arch_write_lock_slow(arch_rwlock_t *rwlock, u32 val) > * when we compare them. > */ > u32 my_ticket_; > + u32 iterations = 0; > > - /* Take out the next ticket; this will also stop would-be readers. */ > - if (val & 1) > - val = get_rwlock(rwlock); > - rwlock->lock = __insn_addb(val, 1 << WR_NEXT_SHIFT); > + /* > + * Wait until there are no readers, then bump up the next > + * field and capture the ticket value. > + */ > + for (;;) { > + if (!(val & 1)) { > + if ((val >> RD_COUNT_SHIFT) == 0) > + break; > + rwlock->lock = val; > + } > + delay_backoff(iterations++);
Are you sure a writer should have a growing delay_backoff() ?
It seems to me this only allow new readers to come (so adding more unfairness to the rwlock, that already favor readers against writer[s])
Maybe allow one cpu to spin, and eventually other 'writers' be queued ?
> + val = __insn_tns((int *)&rwlock->lock); > + } > > - /* Extract my ticket value from the original word. */ > + /* Take out the next ticket and extract my ticket value. */ > + rwlock->lock = __insn_addb(val, 1 << WR_NEXT_SHIFT); > my_ticket_ = val >> WR_NEXT_SHIFT; > > - /* > - * Wait until the "current" field matches our ticket, and > - * there are no remaining readers. > - */ > + /* Wait until the "current" field matches our ticket. */ > for (;;) { > u32 curr_ = val >> WR_CURR_SHIFT; > - u32 readers = val >> RD_COUNT_SHIFT; > - u32 delta = ((my_ticket_ - curr_) & WR_MASK) + !!readers; > + u32 delta = ((my_ticket_ - curr_) & WR_MASK); > if (likely(delta == 0)) > break; >
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