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ptp: remove ptp->n_vclocks check logic in ptp_vclock_in_use()

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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: 73f37068d5 ("ptp: support ptp physical/virtual clocks conversion")
Signed-off-by: Jeongjun Park <aha310510@gmail.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Link: https://patch.msgid.link/20250520160717.7350-1-aha310510@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Jeongjun Park 2025-05-21 01:07:17 +09:00 • committed by Jakub Kicinski
parent 692eb9f8a5
commit 87f7ce260a
1 changed files with 1 additions and 11 deletions
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12
drivers/ptp/ptp_private.h
View file

@ -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 */