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authorChris Metcalf <cmetcalf@tilera.com>2010-11-15 09:18:49 -0500
committerChris Metcalf <cmetcalf@tilera.com>2010-11-15 09:18:49 -0500
commit24f3f6b5eff92608a62449e33bfac0eed1447d02 (patch)
tree7f487b3989657b9c1c896d6c0b3916c60c3ddfa8
parentd02db4f8d79c5841ba32b326edb75ea6acd081ca (diff)
downloadblackbird-op-linux-24f3f6b5eff92608a62449e33bfac0eed1447d02.tar.gz
blackbird-op-linux-24f3f6b5eff92608a62449e33bfac0eed1447d02.zip
arch/tile: fix rwlock so would-be write lockers don't block new readers
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>
-rw-r--r--arch/tile/lib/spinlock_32.c29
1 files changed, 18 insertions, 11 deletions
diff --git a/arch/tile/lib/spinlock_32.c b/arch/tile/lib/spinlock_32.c
index 485e24d62c6b..5cd1c4004eca 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++);
+ 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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