[PATCH] ata: libata-sff: fix race between IRQ deferral and ata_sff_flush_pio_task()

[PATCH] ata: libata-sff: fix race between IRQ deferral and ata_sff_flush_pio_task()

[Niklas Cassel]

Commit 4a2e5409472e ("ata: libata-sff: don't busy-wait for PIO data-in
command completion") allowed the interrupt handler to queue
ap->sff_pio_task from ata_sff_hsm_move() when a PIO data-in command is
still busy after its final data block.

ata_sff_flush_pio_task(), however, was written under the assumption that
cancel_delayed_work_sync() is enough to prevent sff_pio_task from being
queued again before the HSM state is reset to HSM_ST_IDLE and
sff_pio_task_link is cleared.

The flush runs early during error handling, at a point where the timed
out command is still active, the port is not frozen yet, and interrupts
are still being serviced.  If the final data-block interrupt of a PIO
data-in command is serviced in the window between
cancel_delayed_work_sync() and the reset of hsm_task_state, the
interrupt handler queues sff_pio_task after the cancel has completed.
When that work item then executes on ata_sff_wq after the flush cleared
ap->sff_pio_task_link, ata_sff_pio_task() hits
BUG_ON(ap->sff_pio_task_link == NULL) while holding the host lock with
interrupts disabled; alternatively it proceeds on a stale link while
error handling owns the port.

Fix this by resetting hsm_task_state to HSM_ST_IDLE before cancelling
the work item.  Once the state is IDLE, __ata_sff_port_intr() can no
longer enter ata_sff_hsm_move(), so the interrupt handler can no longer
queue the pio task, and cancel_delayed_work_sync() drains whatever was
queued before the reset.

Also make ata_sff_pio_task() bail out quietly when the HSM state was
marked idle by the flush, both at entry and after re-acquiring the port
lock following ata_msleep(), since a work item can already be running
when the flush resets the state.

Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Link: https://lore.kernel.org/linux-ide/sashiko-outbox-177477@kernel.org/
Fixes: 4a2e5409472e ("ata: libata-sff: don't busy-wait for PIO data-in command completion")
Cc: stable@vger.kernel.org
Signed-off-by: Niklas Cassel <cassel@kernel.org>
---
The bug was reproduced on QEMU (piix3 IDE disk driven by ata_piix,
i.e. the real SFF/BMDMA interrupt-driven PIO path) with a
fault-injection patch that:

 - forced all rw commands to ATA_PROT_PIO (the emulated disk
   otherwise uses UDMA),
 - added mdelay(200) per interrupt in __ata_sff_port_intr(), so a
   multi-sector read stays in flight for seconds - long enough for
   the 3 s SCSI timeout to fire mid-transfer,
 - forced the new HSM_ST_LAST deferral branch to always queue
   sff_pio_task from IRQ context (emulated devices clear BSY/DRQ
   too fast to hit the ~100 us busy-wait), with a 150 ms delay so
   the work runs after the flush has cleared sff_pio_task_link,
 - made ata_sff_flush_pio_task() wait for the IRQ to requeue the
   work inside the cancel_delayed_work_sync() -> sff_pio_task_link
   = NULL window.

Trigger: echo 3 > /sys/block/sda/device/timeout;
dd if=/dev/sda bs=16k iflag=direct. On the third EH cycle the
deferred queue landed in the window, producing "kernel BUG at
drivers/ata/libata-sff.c" in ata_sff_pio_task()
(BUG_ON(ap->sff_pio_task_link == NULL)), the kworker exiting with
IRQs disabled and host->lock held, followed 60 s later by an RCU
stall with scsi_eh_0 spinning forever in ata_eh_thaw_port().

With the fix applied under identical instrumentation, the same
trigger produced five timeout -> soft-reset -> speed-downgrade ->
EH complete cycles with no BUG and no lockup; the read failed
cleanly with an I/O error.

 drivers/ata/libata-sff.c | 32 +++++++++++++++++++++++++++++---
 1 file changed, 29 insertions(+), 3 deletions(-)

diff --git a/drivers/ata/libata-sff.c b/drivers/ata/libata-sff.c
index 976e4e160494..73767d4a85dd 100644
--- a/drivers/ata/libata-sff.c
+++ b/drivers/ata/libata-sff.c
@@ -1196,8 +1196,6 @@ void ata_sff_flush_pio_task(struct ata_port *ap)
 {
 	trace_ata_sff_flush_pio_task(ap);
 
-	cancel_delayed_work_sync(&ap->sff_pio_task);
-
 	/*
 	 * We wanna reset the HSM state to IDLE.  If we do so without
 	 * grabbing the port lock, critical sections protected by it which
@@ -1205,11 +1203,26 @@ void ata_sff_flush_pio_task(struct ata_port *ap)
 	 * example, we may set IDLE in between the time
 	 * __ata_sff_port_intr() checks for HSM_ST_IDLE and before it calls
 	 * ata_sff_hsm_move() causing ata_sff_hsm_move() to BUG().
+	 *
+	 * Reset the state before cancelling the work item: once the state
+	 * is IDLE, __ata_sff_port_intr() can no longer enter
+	 * ata_sff_hsm_move(), which prevents the interrupt handler from
+	 * queueing sff_pio_task when deferring the completion of a PIO
+	 * data-in command.  The SCSI error handler cannot dispatch new
+	 * commands while error handling is in progress, so issue-time
+	 * queuing is not possible either.
 	 */
 	spin_lock_irq(ap->lock);
 	ap->hsm_task_state = HSM_ST_IDLE;
 	spin_unlock_irq(ap->lock);
 
+	/*
+	 * Now that the interrupt handler can no longer queue the pio task,
+	 * cancel it.  This also drains a work item that the interrupt
+	 * handler managed to queue before the HSM state was reset.
+	 */
+	cancel_delayed_work_sync(&ap->sff_pio_task);
+
 	ap->sff_pio_task_link = NULL;
 }
 
@@ -1233,7 +1246,13 @@ static void ata_sff_pio_task(struct work_struct *work)
 	}
 
 fsm_start:
-	WARN_ON_ONCE(ap->hsm_task_state == HSM_ST_IDLE);
+	/*
+	 * ata_sff_flush_pio_task() may have marked the HSM idle after this
+	 * work item was queued or while it was running.  There is nothing
+	 * left to do.
+	 */
+	if (ap->hsm_task_state == HSM_ST_IDLE)
+		goto out_unlock;
 
 	wait_mask = ATA_BUSY;
 	if (ap->hsm_task_state == HSM_ST_LAST)
@@ -1252,6 +1271,13 @@ static void ata_sff_pio_task(struct work_struct *work)
 		ata_msleep(ap, 2);
 		spin_lock_irq(ap->lock);
 
+		/*
+		 * ata_sff_flush_pio_task() may have marked the HSM idle
+		 * while the port lock was dropped for ata_msleep().
+		 */
+		if (ap->hsm_task_state == HSM_ST_IDLE)
+			goto out_unlock;
+
 		status = ata_sff_busy_wait(ap, wait_mask, 10);
 		if (status & wait_mask) {
 			ata_sff_queue_pio_task(link, ATA_SHORT_PAUSE);
-- 
2.56.0