dpll: fix possible deadlock during netlink dump operation

Recently, I've been hitting following deadlock warning during dpll pin
dump:

[52804.637962] ======================================================
[52804.638536] WARNING: possible circular locking dependency detected
[52804.639111] 6.8.0-rc2jiri+ #1 Not tainted
[52804.639529] ------------------------------------------------------
[52804.640104] python3/2984 is trying to acquire lock:
[52804.640581] ffff88810e642678 (nlk_cb_mutex-GENERIC){+.+.}-{3:3}, at: netlink_dump+0xb3/0x780
[52804.641417]
               but task is already holding lock:
[52804.642010] ffffffff83bde4c8 (dpll_lock){+.+.}-{3:3}, at: dpll_lock_dumpit+0x13/0x20
[52804.642747]
               which lock already depends on the new lock.

[52804.643551]
               the existing dependency chain (in reverse order) is:
[52804.644259]
               -> #1 (dpll_lock){+.+.}-{3:3}:
[52804.644836]        lock_acquire+0x174/0x3e0
[52804.645271]        __mutex_lock+0x119/0x1150
[52804.645723]        dpll_lock_dumpit+0x13/0x20
[52804.646169]        genl_start+0x266/0x320
[52804.646578]        __netlink_dump_start+0x321/0x450
[52804.647056]        genl_family_rcv_msg_dumpit+0x155/0x1e0
[52804.647575]        genl_rcv_msg+0x1ed/0x3b0
[52804.648001]        netlink_rcv_skb+0xdc/0x210
[52804.648440]        genl_rcv+0x24/0x40
[52804.648831]        netlink_unicast+0x2f1/0x490
[52804.649290]        netlink_sendmsg+0x36d/0x660
[52804.649742]        __sock_sendmsg+0x73/0xc0
[52804.650165]        __sys_sendto+0x184/0x210
[52804.650597]        __x64_sys_sendto+0x72/0x80
[52804.651045]        do_syscall_64+0x6f/0x140
[52804.651474]        entry_SYSCALL_64_after_hwframe+0x46/0x4e
[52804.652001]
               -> #0 (nlk_cb_mutex-GENERIC){+.+.}-{3:3}:
[52804.652650]        check_prev_add+0x1ae/0x1280
[52804.653107]        __lock_acquire+0x1ed3/0x29a0
[52804.653559]        lock_acquire+0x174/0x3e0
[52804.653984]        __mutex_lock+0x119/0x1150
[52804.654423]        netlink_dump+0xb3/0x780
[52804.654845]        __netlink_dump_start+0x389/0x450
[52804.655321]        genl_family_rcv_msg_dumpit+0x155/0x1e0
[52804.655842]        genl_rcv_msg+0x1ed/0x3b0
[52804.656272]        netlink_rcv_skb+0xdc/0x210
[52804.656721]        genl_rcv+0x24/0x40
[52804.657119]        netlink_unicast+0x2f1/0x490
[52804.657570]        netlink_sendmsg+0x36d/0x660
[52804.658022]        __sock_sendmsg+0x73/0xc0
[52804.658450]        __sys_sendto+0x184/0x210
[52804.658877]        __x64_sys_sendto+0x72/0x80
[52804.659322]        do_syscall_64+0x6f/0x140
[52804.659752]        entry_SYSCALL_64_after_hwframe+0x46/0x4e
[52804.660281]
               other info that might help us debug this:

[52804.661077]  Possible unsafe locking scenario:

[52804.661671]        CPU0                    CPU1
[52804.662129]        ----                    ----
[52804.662577]   lock(dpll_lock);
[52804.662924]                                lock(nlk_cb_mutex-GENERIC);
[52804.663538]                                lock(dpll_lock);
[52804.664073]   lock(nlk_cb_mutex-GENERIC);
[52804.664490]

The issue as follows: __netlink_dump_start() calls control->start(cb)
with nlk->cb_mutex held. In control->start(cb) the dpll_lock is taken.
Then nlk->cb_mutex is released and taken again in netlink_dump(), while
dpll_lock still being held. That leads to ABBA deadlock when another
CPU races with the same operation.

Fix this by moving dpll_lock taking into dumpit() callback which ensures
correct lock taking order.

Fixes: 9d71b54 ("dpll: netlink: Add DPLL framework base functions")
Signed-off-by: Jiri Pirko <[email protected]>
Reviewed-by: Arkadiusz Kubalewski <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Jiri Pirko

Documentation/netlink/specs/dpll.yaml

@@ -384,8 +384,6 @@ operations:
             - type
 
       dump:
-        pre: dpll-lock-dumpit
-        post: dpll-unlock-dumpit
         reply: *dev-attrs
 
     -
@@ -473,8 +471,6 @@ operations:
             - fractional-frequency-offset
 
       dump:
-        pre: dpll-lock-dumpit
-        post: dpll-unlock-dumpit
         request:
           attributes:
             - id

drivers/dpll/dpll_netlink.c

@@ -1199,6 +1199,7 @@ int dpll_nl_pin_get_dumpit(struct sk_buff *skb, struct netlink_callback *cb)
 	unsigned long i;
 	int ret = 0;
 
+	mutex_lock(&dpll_lock);
 	xa_for_each_marked_start(&dpll_pin_xa, i, pin, DPLL_REGISTERED,
 				 ctx->idx) {
 		if (!dpll_pin_available(pin))
@@ -1218,6 +1219,8 @@ int dpll_nl_pin_get_dumpit(struct sk_buff *skb, struct netlink_callback *cb)
 		}
 		genlmsg_end(skb, hdr);
 	}
+	mutex_unlock(&dpll_lock);
+
 	if (ret == -EMSGSIZE) {
 		ctx->idx = i;
 		return skb->len;
@@ -1373,6 +1376,7 @@ int dpll_nl_device_get_dumpit(struct sk_buff *skb, struct netlink_callback *cb)
 	unsigned long i;
 	int ret = 0;
 
+	mutex_lock(&dpll_lock);
 	xa_for_each_marked_start(&dpll_device_xa, i, dpll, DPLL_REGISTERED,
 				 ctx->idx) {
 		hdr = genlmsg_put(skb, NETLINK_CB(cb->skb).portid,
@@ -1389,6 +1393,8 @@ int dpll_nl_device_get_dumpit(struct sk_buff *skb, struct netlink_callback *cb)
 		}
 		genlmsg_end(skb, hdr);
 	}
+	mutex_unlock(&dpll_lock);
+
 	if (ret == -EMSGSIZE) {
 		ctx->idx = i;
 		return skb->len;
@@ -1439,20 +1445,6 @@ dpll_unlock_doit(const struct genl_split_ops *ops, struct sk_buff *skb,
 	mutex_unlock(&dpll_lock);
 }
 
-int dpll_lock_dumpit(struct netlink_callback *cb)
-{
-	mutex_lock(&dpll_lock);
-
-	return 0;
-}
-
-int dpll_unlock_dumpit(struct netlink_callback *cb)
-{
-	mutex_unlock(&dpll_lock);
-
-	return 0;
-}
-
 int dpll_pin_pre_doit(const struct genl_split_ops *ops, struct sk_buff *skb,
 		      struct genl_info *info)
 {

drivers/dpll/dpll_nl.c

@@ -95,9 +95,7 @@ static const struct genl_split_ops dpll_nl_ops[] = {
 	},
 	{
 		.cmd	= DPLL_CMD_DEVICE_GET,
-		.start	= dpll_lock_dumpit,
 		.dumpit	= dpll_nl_device_get_dumpit,
-		.done	= dpll_unlock_dumpit,
 		.flags	= GENL_ADMIN_PERM | GENL_CMD_CAP_DUMP,
 	},
 	{
@@ -129,9 +127,7 @@ static const struct genl_split_ops dpll_nl_ops[] = {
 	},
 	{
 		.cmd		= DPLL_CMD_PIN_GET,
-		.start		= dpll_lock_dumpit,
 		.dumpit		= dpll_nl_pin_get_dumpit,
-		.done		= dpll_unlock_dumpit,
 		.policy		= dpll_pin_get_dump_nl_policy,
 		.maxattr	= DPLL_A_PIN_ID,
 		.flags		= GENL_ADMIN_PERM | GENL_CMD_CAP_DUMP,

drivers/dpll/dpll_nl.h

@@ -30,8 +30,6 @@ dpll_post_doit(const struct genl_split_ops *ops, struct sk_buff *skb,
 void
 dpll_pin_post_doit(const struct genl_split_ops *ops, struct sk_buff *skb,
 		   struct genl_info *info);
-int dpll_lock_dumpit(struct netlink_callback *cb);
-int dpll_unlock_dumpit(struct netlink_callback *cb);
 
 int dpll_nl_device_id_get_doit(struct sk_buff *skb, struct genl_info *info);
 int dpll_nl_device_get_doit(struct sk_buff *skb, struct genl_info *info);