Re: [PATCH] arch/tile: fix rwlock so would-be write lockers don'tblock new readers

From: Eric Dumazet
Date: Mon Nov 15 2010 - 09:52:23 EST


Le lundi 15 novembre 2010 Ã 09:18 -0500, Chris Metcalf a Ãcrit :
> 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@xxxxxxxxxx>
> ---
> 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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