regulator: core: avoid regulator_resolve_supply() race condition

The final step in regulator_register() is to call
regulator_resolve_supply() for each registered regulator
(including the one in the process of being registered).  The
regulator_resolve_supply() function first checks if rdev->supply
is NULL, then it performs various steps to try to find the supply.
If successful, rdev->supply is set inside of set_supply().

This procedure can encounter a race condition if two concurrent
tasks call regulator_register() near to each other on separate CPUs
and one of the regulators has rdev->supply_name specified.  There
is currently nothing guaranteeing atomicity between the rdev->supply
check and set steps.  Thus, both tasks can observe rdev->supply==NULL
in their regulator_resolve_supply() calls.  This then results in
both creating a struct regulator for the supply.  One ends up
actually stored in rdev->supply and the other is lost (though still
present in the supply's consumer_list).

Here is a kernel log snippet showing the issue:

[   12.421768] gpu_cc_gx_gdsc: supplied by pm8350_s5_level
[   12.425854] gpu_cc_gx_gdsc: supplied by pm8350_s5_level
[   12.429064] debugfs: Directory 'regulator.4-SUPPLY' with parent
               '17a00000.rsc:rpmh-regulator-gfxlvl-pm8350_s5_level'
               already present!

Avoid this race condition by holding the rdev->mutex lock inside
of regulator_resolve_supply() while checking and setting
rdev->supply.

Signed-off-by: David Collins <collinsd@codeaurora.org>
Link: https://lore.kernel.org/r/1610068562-4410-1-git-send-email-collinsd@codeaurora.org
Signed-off-by: Mark Brown <broonie@kernel.org>

diff --git a/drivers/regulator/core.c b/drivers/regulator/core.c
index ca03d8e70bd1359f586e82aaa75878ecf58f75c7..cfc3bc9df93a0912bacf6790096e501bb49e892c 100644 (file)
--- a/drivers/regulator/core.c
+++ b/drivers/regulator/core.c
@@ -1813,23 +1813,34 @@ static int regulator_resolve_supply(struct regulator_dev *rdev)
 {
        struct regulator_dev *r;
        struct device *dev = rdev->dev.parent;
-       int ret;
+       int ret = 0;
 
        /* No supply to resolve? */
        if (!rdev->supply_name)
                return 0;
 
-       /* Supply already resolved? */
+       /* Supply already resolved? (fast-path without locking contention) */
        if (rdev->supply)
                return 0;
 
+       /*
+        * Recheck rdev->supply with rdev->mutex lock held to avoid a race
+        * between rdev->supply null check and setting rdev->supply in
+        * set_supply() from concurrent tasks.
+        */
+       regulator_lock(rdev);
+
+       /* Supply just resolved by a concurrent task? */
+       if (rdev->supply)
+               goto out;
+
        r = regulator_dev_lookup(dev, rdev->supply_name);
        if (IS_ERR(r)) {
                ret = PTR_ERR(r);
 
                /* Did the lookup explicitly defer for us? */
                if (ret == -EPROBE_DEFER)
-                       return ret;
+                       goto out;
 
                if (have_full_constraints()) {
                        r = dummy_regulator_rdev;
@@ -1837,15 +1848,18 @@ static int regulator_resolve_supply(struct regulator_dev *rdev)
                } else {
                        dev_err(dev, "Failed to resolve %s-supply for %s\n",
                                rdev->supply_name, rdev->desc->name);
-                       return -EPROBE_DEFER;
+                       ret = -EPROBE_DEFER;
+                       goto out;
                }
        }
 
        if (r == rdev) {
                dev_err(dev, "Supply for %s (%s) resolved to itself\n",
                        rdev->desc->name, rdev->supply_name);
-               if (!have_full_constraints())
-                       return -EINVAL;
+               if (!have_full_constraints()) {
+                       ret = -EINVAL;
+                       goto out;
+               }
                r = dummy_regulator_rdev;
                get_device(&r->dev);
        }
@@ -1859,7 +1873,8 @@ static int regulator_resolve_supply(struct regulator_dev *rdev)
        if (r->dev.parent && r->dev.parent != rdev->dev.parent) {
                if (!device_is_bound(r->dev.parent)) {
                        put_device(&r->dev);
-                       return -EPROBE_DEFER;
+                       ret = -EPROBE_DEFER;
+                       goto out;
                }
        }
 
@@ -1867,13 +1882,13 @@ static int regulator_resolve_supply(struct regulator_dev *rdev)
        ret = regulator_resolve_supply(r);
        if (ret < 0) {
                put_device(&r->dev);
-               return ret;
+               goto out;
        }
 
        ret = set_supply(rdev, r);
        if (ret < 0) {
                put_device(&r->dev);
-               return ret;
+               goto out;
        }
 
        /*
@@ -1886,11 +1901,13 @@ static int regulator_resolve_supply(struct regulator_dev *rdev)
                if (ret < 0) {
                        _regulator_put(rdev->supply);
                        rdev->supply = NULL;
-                       return ret;
+                       goto out;
                }
        }
 
-       return 0;
+out:
+       regulator_unlock(rdev);
+       return ret;
 }
 
 /* Internal regulator request function */