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Subject[PATCH 5.3 126/344] x86/platform/intel/iosf_mbi Rewrite locking
Date
From: Hans de Goede <hdegoede@redhat.com>

[ Upstream commit 00452ba9fdb5bf6fb5fea1dae5227b4bbed44fc4 ]

There are 2 problems with the old iosf PMIC I2C bus arbritration code which
need to be addressed:

1. The lockdep code complains about a possible deadlock in the
iosf_mbi_[un]block_punit_i2c_access code:

[ 6.712662] ======================================================
[ 6.712673] WARNING: possible circular locking dependency detected
[ 6.712685] 5.3.0-rc2+ #79 Not tainted
[ 6.712692] ------------------------------------------------------
[ 6.712702] kworker/0:1/7 is trying to acquire lock:
[ 6.712712] 00000000df1c5681 (iosf_mbi_block_punit_i2c_access_count_mutex){+.+.}, at: iosf_mbi_unblock_punit_i2c_access+0x13/0x90
[ 6.712739]
but task is already holding lock:
[ 6.712749] 0000000067cb23e7 (iosf_mbi_punit_mutex){+.+.}, at: iosf_mbi_block_punit_i2c_access+0x97/0x186
[ 6.712768]
which lock already depends on the new lock.

[ 6.712780]
the existing dependency chain (in reverse order) is:
[ 6.712792]
-> #1 (iosf_mbi_punit_mutex){+.+.}:
[ 6.712808] __mutex_lock+0xa8/0x9a0
[ 6.712818] iosf_mbi_block_punit_i2c_access+0x97/0x186
[ 6.712831] i2c_dw_acquire_lock+0x20/0x30
[ 6.712841] i2c_dw_set_reg_access+0x15/0xb0
[ 6.712851] i2c_dw_probe+0x57/0x473
[ 6.712861] dw_i2c_plat_probe+0x33e/0x640
[ 6.712874] platform_drv_probe+0x38/0x80
[ 6.712884] really_probe+0xf3/0x380
[ 6.712894] driver_probe_device+0x59/0xd0
[ 6.712905] bus_for_each_drv+0x84/0xd0
[ 6.712915] __device_attach+0xe4/0x170
[ 6.712925] bus_probe_device+0x9f/0xb0
[ 6.712935] deferred_probe_work_func+0x79/0xd0
[ 6.712946] process_one_work+0x234/0x560
[ 6.712957] worker_thread+0x50/0x3b0
[ 6.712967] kthread+0x10a/0x140
[ 6.712977] ret_from_fork+0x3a/0x50
[ 6.712986]
-> #0 (iosf_mbi_block_punit_i2c_access_count_mutex){+.+.}:
[ 6.713004] __lock_acquire+0xe07/0x1930
[ 6.713015] lock_acquire+0x9d/0x1a0
[ 6.713025] __mutex_lock+0xa8/0x9a0
[ 6.713035] iosf_mbi_unblock_punit_i2c_access+0x13/0x90
[ 6.713047] i2c_dw_set_reg_access+0x4d/0xb0
[ 6.713058] i2c_dw_probe+0x57/0x473
[ 6.713068] dw_i2c_plat_probe+0x33e/0x640
[ 6.713079] platform_drv_probe+0x38/0x80
[ 6.713089] really_probe+0xf3/0x380
[ 6.713099] driver_probe_device+0x59/0xd0
[ 6.713109] bus_for_each_drv+0x84/0xd0
[ 6.713119] __device_attach+0xe4/0x170
[ 6.713129] bus_probe_device+0x9f/0xb0
[ 6.713140] deferred_probe_work_func+0x79/0xd0
[ 6.713150] process_one_work+0x234/0x560
[ 6.713160] worker_thread+0x50/0x3b0
[ 6.713170] kthread+0x10a/0x140
[ 6.713180] ret_from_fork+0x3a/0x50
[ 6.713189]
other info that might help us debug this:

[ 6.713202] Possible unsafe locking scenario:

[ 6.713212] CPU0 CPU1
[ 6.713221] ---- ----
[ 6.713229] lock(iosf_mbi_punit_mutex);
[ 6.713239] lock(iosf_mbi_block_punit_i2c_access_count_mutex);
[ 6.713253] lock(iosf_mbi_punit_mutex);
[ 6.713265] lock(iosf_mbi_block_punit_i2c_access_count_mutex);
[ 6.713276]
*** DEADLOCK ***

In practice can never happen because only the first caller which
increments iosf_mbi_block_punit_i2c_access_count will also take
iosf_mbi_punit_mutex, that is the whole purpose of the counter, which
itself is protected by iosf_mbi_block_punit_i2c_access_count_mutex.

But there is no way to tell the lockdep code about this and we really
want to be able to run a kernel with lockdep enabled without these
warnings being triggered.

2. The lockdep warning also points out another real problem, if 2 threads
both are in a block of code protected by iosf_mbi_block_punit_i2c_access
and the first thread to acquire the block exits before the second thread
then the second thread will call mutex_unlock on iosf_mbi_punit_mutex,
but it is not the thread which took the mutex and unlocking by another
thread is not allowed.

Fix this by getting rid of the notion of holding a mutex for the entire
duration of the PMIC accesses, be it either from the PUnit side, or from an
in kernel I2C driver. In general holding a mutex after exiting a function
is a bad idea and the above problems show this case is no different.

Instead 2 counters are now used, one for PMIC accesses from the PUnit
and one for accesses from in kernel I2C code. When access is requested
now the code will wait (using a waitqueue) for the counter of the other
type of access to reach 0 and on release, if the counter reaches 0 the
wakequeue is woken.

Note that the counter approach is necessary to allow nested calls.
The main reason for this is so that a series of i2c transfers can be done
with the punit blocked from accessing the bus the whole time. This is
necessary to be able to safely read/modify/write a PMIC register without
racing with the PUNIT doing the same thing.

Allowing nested iosf_mbi_block_punit_i2c_access() calls also is desirable
from a performance pov since the whole dance necessary to block the PUnit
from accessing the PMIC I2C bus is somewhat expensive.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lkml.kernel.org/r/20190812102113.95794-1-hdegoede@redhat.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/x86/platform/intel/iosf_mbi.c | 100 ++++++++++++++++++-----------
1 file changed, 62 insertions(+), 38 deletions(-)

diff --git a/arch/x86/platform/intel/iosf_mbi.c b/arch/x86/platform/intel/iosf_mbi.c
index 2e796b54cbde1..9e24445004282 100644
--- a/arch/x86/platform/intel/iosf_mbi.c
+++ b/arch/x86/platform/intel/iosf_mbi.c
@@ -17,6 +17,7 @@
#include <linux/debugfs.h>
#include <linux/capability.h>
#include <linux/pm_qos.h>
+#include <linux/wait.h>

#include <asm/iosf_mbi.h>

@@ -201,23 +202,45 @@ EXPORT_SYMBOL(iosf_mbi_available);
#define PUNIT_SEMAPHORE_BIT BIT(0)
#define PUNIT_SEMAPHORE_ACQUIRE BIT(1)

-static DEFINE_MUTEX(iosf_mbi_punit_mutex);
-static DEFINE_MUTEX(iosf_mbi_block_punit_i2c_access_count_mutex);
+static DEFINE_MUTEX(iosf_mbi_pmic_access_mutex);
static BLOCKING_NOTIFIER_HEAD(iosf_mbi_pmic_bus_access_notifier);
-static u32 iosf_mbi_block_punit_i2c_access_count;
+static DECLARE_WAIT_QUEUE_HEAD(iosf_mbi_pmic_access_waitq);
+static u32 iosf_mbi_pmic_punit_access_count;
+static u32 iosf_mbi_pmic_i2c_access_count;
static u32 iosf_mbi_sem_address;
static unsigned long iosf_mbi_sem_acquired;
static struct pm_qos_request iosf_mbi_pm_qos;

void iosf_mbi_punit_acquire(void)
{
- mutex_lock(&iosf_mbi_punit_mutex);
+ /* Wait for any I2C PMIC accesses from in kernel drivers to finish. */
+ mutex_lock(&iosf_mbi_pmic_access_mutex);
+ while (iosf_mbi_pmic_i2c_access_count != 0) {
+ mutex_unlock(&iosf_mbi_pmic_access_mutex);
+ wait_event(iosf_mbi_pmic_access_waitq,
+ iosf_mbi_pmic_i2c_access_count == 0);
+ mutex_lock(&iosf_mbi_pmic_access_mutex);
+ }
+ /*
+ * We do not need to do anything to allow the PUNIT to safely access
+ * the PMIC, other then block in kernel accesses to the PMIC.
+ */
+ iosf_mbi_pmic_punit_access_count++;
+ mutex_unlock(&iosf_mbi_pmic_access_mutex);
}
EXPORT_SYMBOL(iosf_mbi_punit_acquire);

void iosf_mbi_punit_release(void)
{
- mutex_unlock(&iosf_mbi_punit_mutex);
+ bool do_wakeup;
+
+ mutex_lock(&iosf_mbi_pmic_access_mutex);
+ iosf_mbi_pmic_punit_access_count--;
+ do_wakeup = iosf_mbi_pmic_punit_access_count == 0;
+ mutex_unlock(&iosf_mbi_pmic_access_mutex);
+
+ if (do_wakeup)
+ wake_up(&iosf_mbi_pmic_access_waitq);
}
EXPORT_SYMBOL(iosf_mbi_punit_release);

@@ -256,34 +279,32 @@ static void iosf_mbi_reset_semaphore(void)
* already blocked P-Unit accesses because it wants them blocked over multiple
* i2c-transfers, for e.g. read-modify-write of an I2C client register.
*
- * The P-Unit accesses already being blocked is tracked through the
- * iosf_mbi_block_punit_i2c_access_count variable which is protected by the
- * iosf_mbi_block_punit_i2c_access_count_mutex this mutex is hold for the
- * entire duration of the function.
- *
- * If access is not blocked yet, this function takes the following steps:
+ * To allow safe PMIC i2c bus accesses this function takes the following steps:
*
* 1) Some code sends request to the P-Unit which make it access the PMIC
* I2C bus. Testing has shown that the P-Unit does not check its internal
* PMIC bus semaphore for these requests. Callers of these requests call
* iosf_mbi_punit_acquire()/_release() around their P-Unit accesses, these
- * functions lock/unlock the iosf_mbi_punit_mutex.
- * As the first step we lock the iosf_mbi_punit_mutex, to wait for any in
- * flight requests to finish and to block any new requests.
+ * functions increase/decrease iosf_mbi_pmic_punit_access_count, so first
+ * we wait for iosf_mbi_pmic_punit_access_count to become 0.
+ *
+ * 2) Check iosf_mbi_pmic_i2c_access_count, if access has already
+ * been blocked by another caller, we only need to increment
+ * iosf_mbi_pmic_i2c_access_count and we can skip the other steps.
*
- * 2) Some code makes such P-Unit requests from atomic contexts where it
+ * 3) Some code makes such P-Unit requests from atomic contexts where it
* cannot call iosf_mbi_punit_acquire() as that may sleep.
* As the second step we call a notifier chain which allows any code
* needing P-Unit resources from atomic context to acquire them before
* we take control over the PMIC I2C bus.
*
- * 3) When CPU cores enter C6 or C7 the P-Unit needs to talk to the PMIC
+ * 4) When CPU cores enter C6 or C7 the P-Unit needs to talk to the PMIC
* if this happens while the kernel itself is accessing the PMIC I2C bus
* the SoC hangs.
* As the third step we call pm_qos_update_request() to disallow the CPU
* to enter C6 or C7.
*
- * 4) The P-Unit has a PMIC bus semaphore which we can request to stop
+ * 5) The P-Unit has a PMIC bus semaphore which we can request to stop
* autonomous P-Unit tasks from accessing the PMIC I2C bus while we hold it.
* As the fourth and final step we request this semaphore and wait for our
* request to be acknowledged.
@@ -297,12 +318,18 @@ int iosf_mbi_block_punit_i2c_access(void)
if (WARN_ON(!mbi_pdev || !iosf_mbi_sem_address))
return -ENXIO;

- mutex_lock(&iosf_mbi_block_punit_i2c_access_count_mutex);
+ mutex_lock(&iosf_mbi_pmic_access_mutex);

- if (iosf_mbi_block_punit_i2c_access_count > 0)
+ while (iosf_mbi_pmic_punit_access_count != 0) {
+ mutex_unlock(&iosf_mbi_pmic_access_mutex);
+ wait_event(iosf_mbi_pmic_access_waitq,
+ iosf_mbi_pmic_punit_access_count == 0);
+ mutex_lock(&iosf_mbi_pmic_access_mutex);
+ }
+
+ if (iosf_mbi_pmic_i2c_access_count > 0)
goto success;

- mutex_lock(&iosf_mbi_punit_mutex);
blocking_notifier_call_chain(&iosf_mbi_pmic_bus_access_notifier,
MBI_PMIC_BUS_ACCESS_BEGIN, NULL);

@@ -330,10 +357,6 @@ int iosf_mbi_block_punit_i2c_access(void)
iosf_mbi_sem_acquired = jiffies;
dev_dbg(&mbi_pdev->dev, "P-Unit semaphore acquired after %ums\n",
jiffies_to_msecs(jiffies - start));
- /*
- * Success, keep iosf_mbi_punit_mutex locked till
- * iosf_mbi_unblock_punit_i2c_access() gets called.
- */
goto success;
}

@@ -344,15 +367,13 @@ int iosf_mbi_block_punit_i2c_access(void)
dev_err(&mbi_pdev->dev, "Error P-Unit semaphore timed out, resetting\n");
error:
iosf_mbi_reset_semaphore();
- mutex_unlock(&iosf_mbi_punit_mutex);
-
if (!iosf_mbi_get_sem(&sem))
dev_err(&mbi_pdev->dev, "P-Unit semaphore: %d\n", sem);
success:
if (!WARN_ON(ret))
- iosf_mbi_block_punit_i2c_access_count++;
+ iosf_mbi_pmic_i2c_access_count++;

- mutex_unlock(&iosf_mbi_block_punit_i2c_access_count_mutex);
+ mutex_unlock(&iosf_mbi_pmic_access_mutex);

return ret;
}
@@ -360,17 +381,20 @@ EXPORT_SYMBOL(iosf_mbi_block_punit_i2c_access);

void iosf_mbi_unblock_punit_i2c_access(void)
{
- mutex_lock(&iosf_mbi_block_punit_i2c_access_count_mutex);
+ bool do_wakeup = false;

- iosf_mbi_block_punit_i2c_access_count--;
- if (iosf_mbi_block_punit_i2c_access_count == 0) {
+ mutex_lock(&iosf_mbi_pmic_access_mutex);
+ iosf_mbi_pmic_i2c_access_count--;
+ if (iosf_mbi_pmic_i2c_access_count == 0) {
iosf_mbi_reset_semaphore();
- mutex_unlock(&iosf_mbi_punit_mutex);
dev_dbg(&mbi_pdev->dev, "punit semaphore held for %ums\n",
jiffies_to_msecs(jiffies - iosf_mbi_sem_acquired));
+ do_wakeup = true;
}
+ mutex_unlock(&iosf_mbi_pmic_access_mutex);

- mutex_unlock(&iosf_mbi_block_punit_i2c_access_count_mutex);
+ if (do_wakeup)
+ wake_up(&iosf_mbi_pmic_access_waitq);
}
EXPORT_SYMBOL(iosf_mbi_unblock_punit_i2c_access);

@@ -379,10 +403,10 @@ int iosf_mbi_register_pmic_bus_access_notifier(struct notifier_block *nb)
int ret;

/* Wait for the bus to go inactive before registering */
- mutex_lock(&iosf_mbi_punit_mutex);
+ iosf_mbi_punit_acquire();
ret = blocking_notifier_chain_register(
&iosf_mbi_pmic_bus_access_notifier, nb);
- mutex_unlock(&iosf_mbi_punit_mutex);
+ iosf_mbi_punit_release();

return ret;
}
@@ -403,9 +427,9 @@ int iosf_mbi_unregister_pmic_bus_access_notifier(struct notifier_block *nb)
int ret;

/* Wait for the bus to go inactive before unregistering */
- mutex_lock(&iosf_mbi_punit_mutex);
+ iosf_mbi_punit_acquire();
ret = iosf_mbi_unregister_pmic_bus_access_notifier_unlocked(nb);
- mutex_unlock(&iosf_mbi_punit_mutex);
+ iosf_mbi_punit_release();

return ret;
}
@@ -413,7 +437,7 @@ EXPORT_SYMBOL(iosf_mbi_unregister_pmic_bus_access_notifier);

void iosf_mbi_assert_punit_acquired(void)
{
- WARN_ON(!mutex_is_locked(&iosf_mbi_punit_mutex));
+ WARN_ON(iosf_mbi_pmic_punit_access_count == 0);
}
EXPORT_SYMBOL(iosf_mbi_assert_punit_acquired);

--
2.20.1


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