linux-kernel - [PATCH v2] ptp: remove ptp->n_vclocks check logic in ptp_vclock_in

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Message-ID: <20250520160717.7350-1-aha310510@gmail.com>
Date: Wed, 21 May 2025 01:07:17 +0900
From: Jeongjun Park <aha310510@...il.com>
To: richardcochran@...il.com,
	andrew+netdev@...n.ch
Cc: davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	yangbo.lu@....com,
	netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Jeongjun Park <aha310510@...il.com>
Subject: [PATCH v2] ptp: remove ptp->n_vclocks check logic in ptp_vclock_in_use()

There is no disagreement that we should check both ptp->is_virtual_clock
and ptp->n_vclocks to check if the ptp virtual clock is in use.

However, when we acquire ptp->n_vclocks_mux to read ptp->n_vclocks in
ptp_vclock_in_use(), we observe a recursive lock in the call trace
starting from n_vclocks_store().

============================================
WARNING: possible recursive locking detected
6.15.0-rc6 #1 Not tainted
--------------------------------------------
syz.0.1540/13807 is trying to acquire lock:
ffff888035a24868 (&ptp->n_vclocks_mux){+.+.}-{4:4}, at:
 ptp_vclock_in_use drivers/ptp/ptp_private.h:103 [inline]
ffff888035a24868 (&ptp->n_vclocks_mux){+.+.}-{4:4}, at:
 ptp_clock_unregister+0x21/0x250 drivers/ptp/ptp_clock.c:415

but task is already holding lock:
ffff888030704868 (&ptp->n_vclocks_mux){+.+.}-{4:4}, at:
 n_vclocks_store+0xf1/0x6d0 drivers/ptp/ptp_sysfs.c:215

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(&ptp->n_vclocks_mux);
  lock(&ptp->n_vclocks_mux);

 *** DEADLOCK ***
....
============================================

The best way to solve this is to remove the logic that checks
ptp->n_vclocks in ptp_vclock_in_use().

The reason why this is appropriate is that any path that uses
ptp->n_vclocks must unconditionally check if ptp->n_vclocks is greater
than 0 before unregistering vclocks, and all functions are already
written this way. And in the function that uses ptp->n_vclocks, we
already get ptp->n_vclocks_mux before unregistering vclocks.

Therefore, we need to remove the redundant check for ptp->n_vclocks in
ptp_vclock_in_use() to prevent recursive locking.

Fixes: 73f37068d540 ("ptp: support ptp physical/virtual clocks conversion")
Signed-off-by: Jeongjun Park <aha310510@...il.com>
---
v2: Remove changes unrelated to the patch subject
- Link to v1: https://lore.kernel.org/all/20250519153735.66940-1-aha310510@gmail.com/
---
 drivers/ptp/ptp_private.h | 12 +-----------
 1 file changed, 1 insertion(+), 11 deletions(-)

diff --git a/drivers/ptp/ptp_private.h b/drivers/ptp/ptp_private.h
index 18934e28469e..528d86a33f37 100644
--- a/drivers/ptp/ptp_private.h
+++ b/drivers/ptp/ptp_private.h
@@ -98,17 +98,7 @@ static inline int queue_cnt(const struct timestamp_event_queue *q)
 /* Check if ptp virtual clock is in use */
 static inline bool ptp_vclock_in_use(struct ptp_clock *ptp)
 {
-	bool in_use = false;
-
-	if (mutex_lock_interruptible(&ptp->n_vclocks_mux))
-		return true;
-
-	if (!ptp->is_virtual_clock && ptp->n_vclocks)
-		in_use = true;
-
-	mutex_unlock(&ptp->n_vclocks_mux);
-
-	return in_use;
+	return !ptp->is_virtual_clock;
 }
 
 /* Check if ptp clock shall be free running */
--