libata: take advantage of cmwq and remove concurrency limitations

libata has two concurrency related limitations.

a. ata_wq which is used for polling PIO has single thread per CPU.  If
   there are multiple devices doing polling PIO on the same CPU, they
   can't be executed simultaneously.

b. ata_aux_wq which is used for SCSI probing has single thread.  In
   cases where SCSI probing is stalled for extended period of time
   which is possible for ATAPI devices, this will stall all probing.

#a is solved by increasing maximum concurrency of ata_wq.  Please note
that polling PIO might be used under allocation path and thus needs to
be served by a separate wq with a rescuer.

#b is solved by using the default wq instead and achieving exclusion
via per-port mutex.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Jeff Garzik <jgarzik@pobox.com>

1 files changed, 1 insertions, 8 deletions

diff --git a/drivers/ata/libata-sff.c b/drivers/ata/libata-sff.c
index efa4a18cfb9d..674c1436491f 100644
--- a/drivers/ata/libata-sff.c
+++ b/drivers/ata/libata-sff.c
@@ -3318,14 +3318,7 @@ void ata_sff_port_init(struct ata_port *ap)
 
 int __init ata_sff_init(void)
 {
-        /*
-         * FIXME: In UP case, there is only one workqueue thread and if you
-         * have more than one PIO device, latency is bloody awful, with
-         * occasional multi-second "hiccups" as one PIO device waits for
-         * another.  It's an ugly wart that users DO occasionally complain
-         * about; luckily most users have at most one PIO polled device.
-         */
-        ata_sff_wq = create_workqueue("ata_sff");
+        ata_sff_wq = alloc_workqueue("ata_sff", WQ_RESCUER, WQ_MAX_ACTIVE);
         if (!ata_sff_wq)
                 return -ENOMEM;