[PATCH v2 5/6] usb: typec: tcpm: Fix AB BA lock inversion between tcpm code and the alt-mode drivers

[Hans de Goede <hdegoede@redhat.com>]

When we receive a PD data packet which ends up being for the alt-mode
driver we have the following lock order:

1. tcpm_pd_rx_handler take the tcpm-port lock
2. We call into the alt-mode driver which takes the alt-mode's lock

And when the alt-mode driver initiates communication we have the following
lock order:

3. alt-mode driver takes the alt-mode's lock
4. alt-mode driver calls tcpm_altmode_enter which takes the tcpm-port lock

This is a classic AB BA lock inversion issue.

With the refactoring of tcpm_handle_vdm_request() done before this patch,
we don't rely on, or need to make changes to the tcpm-port data by the
time we make call 2. from above. All data to be passed to the alt-mode
driver sits on our stack at this point, and thus does not need locking.

So after the refactoring we can simply fix this by releasing the
tcpm-port lock before calling into the alt-mode driver.

This fixes the following lockdep warning:

[  191.454238] ======================================================
[  191.454240] WARNING: possible circular locking dependency detected
[  191.454244] 5.8.0-rc5+ #1 Not tainted
[  191.454246] ------------------------------------------------------
[  191.454248] kworker/u8:5/794 is trying to acquire lock:
[  191.454251] ffff9bac8e30d4a8 (&dp->lock){+.+.}-{3:3}, at: dp_altmode_vdm+0x30/0xf0 [typec_displayport]
[  191.454263]
               but task is already holding lock:
[  191.454264] ffff9bac9dc240a0 (&port->lock#2){+.+.}-{3:3}, at: tcpm_pd_rx_handler+0x43/0x12c0 [tcpm]
[  191.454273]
               which lock already depends on the new lock.

[  191.454275]
               the existing dependency chain (in reverse order) is:
[  191.454277]
               -> #1 (&port->lock#2){+.+.}-{3:3}:
[  191.454286]        __mutex_lock+0x7b/0x820
[  191.454290]        tcpm_altmode_enter+0x23/0x90 [tcpm]
[  191.454293]        dp_altmode_work+0xca/0xe0 [typec_displayport]
[  191.454299]        process_one_work+0x23f/0x570
[  191.454302]        worker_thread+0x55/0x3c0
[  191.454305]        kthread+0x138/0x160
[  191.454309]        ret_from_fork+0x22/0x30
[  191.454311]
               -> #0 (&dp->lock){+.+.}-{3:3}:
[  191.454317]        __lock_acquire+0x1241/0x2090
[  191.454320]        lock_acquire+0xa4/0x3d0
[  191.454323]        __mutex_lock+0x7b/0x820
[  191.454326]        dp_altmode_vdm+0x30/0xf0 [typec_displayport]
[  191.454330]        tcpm_pd_rx_handler+0x11ae/0x12c0 [tcpm]
[  191.454333]        process_one_work+0x23f/0x570
[  191.454336]        worker_thread+0x55/0x3c0
[  191.454338]        kthread+0x138/0x160
[  191.454341]        ret_from_fork+0x22/0x30
[  191.454343]
               other info that might help us debug this:

[  191.454345]  Possible unsafe locking scenario:

[  191.454347]        CPU0                    CPU1
[  191.454348]        ----                    ----
[  191.454350]   lock(&port->lock#2);
[  191.454353]                                lock(&dp->lock);
[  191.454355]                                lock(&port->lock#2);
[  191.454357]   lock(&dp->lock);
[  191.454360]
                *** DEADLOCK ***

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
---
Changes in v2:
-Move the mutex_lock call to above the tcpm_queue_vdm() call, so that
 we can use the regular tcpm_queue_vdm() instead of having to call
 tcpm_queue_vdm_unlocked()
---
 drivers/usb/typec/tcpm/tcpm.c | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/drivers/usb/typec/tcpm/tcpm.c b/drivers/usb/typec/tcpm/tcpm.c
index 03a0c083ee9a..9b26b57a0172 100644
--- a/drivers/usb/typec/tcpm/tcpm.c
+++ b/drivers/usb/typec/tcpm/tcpm.c
@@ -1249,6 +1249,27 @@ static void tcpm_handle_vdm_request(struct tcpm_port *port,
 	if (PD_VDO_SVDM(p[0]))
 		rlen = tcpm_pd_svdm(port, adev, p, cnt, response, &adev_action);
 
+	/*
+	 * We are done with any state stored in the port struct now, except
+	 * for any port struct changes done by the tcpm_queue_vdm() call
+	 * below, which is a separate operation.
+	 *
+	 * So we can safely release the lock here; and we MUST release the
+	 * lock here to avoid an AB BA lock inversion:
+	 *
+	 * If we keep the lock here then the lock ordering in this path is:
+	 * 1. tcpm_pd_rx_handler take the tcpm port lock
+	 * 2. One of the typec_altmode_* calls below takes the alt-mode's lock
+	 *
+	 * And we also have this ordering:
+	 * 1. alt-mode driver takes the alt-mode's lock
+	 * 2. alt-mode driver calls tcpm_altmode_enter which takes the
+	 *    tcpm port lock
+	 *
+	 * Dropping our lock here avoids this.
+	 */
+	mutex_unlock(&port->lock);
+
 	if (adev) {
 		switch (adev_action) {
 		case ADEV_NONE:
@@ -1273,6 +1294,15 @@ static void tcpm_handle_vdm_request(struct tcpm_port *port,
 		}
 	}
 
+	/*
+	 * We must re-take the lock here to balance the unlock in
+	 * tcpm_pd_rx_handler, note that no changes, other then the
+	 * tcpm_queue_vdm call, are made while the lock is held again.
+	 * All that is done after the call is unwinding the call stack until
+	 * we return to tcpm_pd_rx_handler and do the unlock there.
+	 */
+	mutex_lock(&port->lock);
+
 	if (rlen > 0)
 		tcpm_queue_vdm(port, response[0], &response[1], rlen - 1);
 }
-- 
2.26.2