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RDS: Use a generation counter to avoid rds_send_xmit loop 

rds_send_xmit is required to loop around after it releases the lock
because someone else could done a trylock, found someone working on the
list and backed off.

But, once we drop our lock, it is possible that someone else does come
in and make progress on the list.  We should detect this and not loop
around if another process is actually working on the list.

This patch adds a generation counter that is bumped every time we
get the lock and do some send work.  If the retry notices someone else
has bumped the generation counter, it does not need to loop around and
continue working.

Signed-off-by: Chris Mason <chris.mason@oracle.com>
Signed-off-by: Andy Grover <andy.grover@oracle.com>
This commit is contained in:
Chris Mason 2010-04-15 16:38:14 -04:00 committed by Andy Grover
parent acfcd4d4ec
commit 9e29db0e36
3 changed files with 9 additions and 4 deletions
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1
net/rds/connection.c
View File

@ -147,6 +147,7 @@ static struct rds_connection *__rds_conn_create(__be32 laddr, __be32 faddr,
conn->c_next_tx_seq = 1;
spin_lock_init(&conn->c_send_lock);
atomic_set(&conn->c_send_generation, 1);
INIT_LIST_HEAD(&conn->c_send_queue);
INIT_LIST_HEAD(&conn->c_retrans);

1
net/rds/rds.h
View File

@ -92,6 +92,7 @@ struct rds_connection {
struct rds_cong_map *c_fcong;
spinlock_t c_send_lock; /* protect send ring */
atomic_t c_send_generation;
struct rds_message *c_xmit_rm;
unsigned long c_xmit_sg;
unsigned int c_xmit_hdr_off;

11
net/rds/send.c
View File

@ -112,6 +112,7 @@ int rds_send_xmit(struct rds_connection *conn)
unsigned int tmp;
struct scatterlist *sg;
int ret = 0;
int gen = 0;
LIST_HEAD(to_be_dropped);
restart:
@ -134,6 +135,8 @@ restart:
if (conn->c_trans->xmit_prepare)
conn->c_trans->xmit_prepare(conn);
gen = atomic_inc_return(&conn->c_send_generation);
/*
* spin trying to push headers and data down the connection until
* the connection doesn't make forward progress.
@ -359,13 +362,13 @@ restart:
if (ret == 0) {
/* A simple bit test would be way faster than taking the
* spin lock */
spin_lock_irqsave(&conn->c_lock, flags);
smp_mb();
if (!list_empty(&conn->c_send_queue)) {
rds_stats_inc(s_send_lock_queue_raced);
spin_unlock_irqrestore(&conn->c_lock, flags);
goto restart;
if (gen == atomic_read(&conn->c_send_generation)) {
goto restart;
}
}
spin_unlock_irqrestore(&conn->c_lock, flags);
}
out:
return ret;