syzbot reported the following ABBA deadlock: CPU0 CPU1 ---- ---- n_vclocks_store() lock(&ptp->n_vclocks_mux) [1] (physical clock) pc_clock_adjtime() lock(&clk->rwsem) [2] (physical clock) ... ptp_clock_freerun() ptp_vclock_in_use() lock(&ptp->n_vclocks_mux) [3] (physical clock) ptp_clock_unregister() posix_clock_unregister() lock(&clk->rwsem) [4] (virtual clock) Since ptp virtual clock is registered only under ptp physical clock, both ptp_clock and posix_clock must be physical clocks for ptp_vclock_in_use() to lock &ptp->n_vclocks_mux and check ptp->n_vclocks. However, when unregistering vclocks in n_vclocks_store(), the locking ptp->n_vclocks_mux is a physical clock lock, but clk->rwsem of ptp_clock_unregister() called through device_for_each_child_reverse() is a virtual clock lock. Therefore, clk->rwsem used in CPU0 and clk->rwsem used in CPU1 are different locks, but in lockdep, a false positive occurs because the possibility of deadlock is determined through lock-class. To solve this, lock subclass annotation must be added to the posix_clock rwsem of the vclock. Reported-by: syzbot+7cfb66a237c4a5fb22ad@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=7cfb66a237c4a5fb22ad Fixes: 73f37068d540 ("ptp: support ptp physical/virtual clocks conversion") Signed-off-by: Jeongjun Park --- v4: Remove unnecessary lock class annotation and CC "POSIX CLOCKS and TIMERS" maintainer - Link to v3: https://lore.kernel.org/all/20250719124022.1536524-1-aha310510@gmail.com/ v3: Annotate lock subclass to prevent false positives of lockdep - Link to v2: https://lore.kernel.org/all/20250718114958.1473199-1-aha310510@gmail.com/ v2: Add CC Vladimir - Link to v1: https://lore.kernel.org/all/20250705145031.140571-1-aha310510@gmail.com/ --- drivers/ptp/ptp_private.h | 5 +++++ drivers/ptp/ptp_vclock.c | 7 +++++++ 2 files changed, 12 insertions(+) diff --git a/drivers/ptp/ptp_private.h b/drivers/ptp/ptp_private.h index a6aad743c282..b352df4cd3f9 100644 --- a/drivers/ptp/ptp_private.h +++ b/drivers/ptp/ptp_private.h @@ -24,6 +24,11 @@ #define PTP_DEFAULT_MAX_VCLOCKS 20 #define PTP_MAX_CHANNELS 2048 +enum { + PTP_LOCK_PHYSICAL = 0, + PTP_LOCK_VIRTUAL, +}; + struct timestamp_event_queue { struct ptp_extts_event buf[PTP_MAX_TIMESTAMPS]; int head; diff --git a/drivers/ptp/ptp_vclock.c b/drivers/ptp/ptp_vclock.c index 7febfdcbde8b..8ed4b8598924 100644 --- a/drivers/ptp/ptp_vclock.c +++ b/drivers/ptp/ptp_vclock.c @@ -154,6 +154,11 @@ static long ptp_vclock_refresh(struct ptp_clock_info *ptp) return PTP_VCLOCK_REFRESH_INTERVAL; } +static void ptp_vclock_set_subclass(struct ptp_clock *ptp) +{ + lockdep_set_subclass(&ptp->clock.rwsem, PTP_LOCK_VIRTUAL); +} + static const struct ptp_clock_info ptp_vclock_info = { .owner = THIS_MODULE, .name = "ptp virtual clock", @@ -213,6 +218,8 @@ struct ptp_vclock *ptp_vclock_register(struct ptp_clock *pclock) return NULL; } + ptp_vclock_set_subclass(vclock->clock); + timecounter_init(&vclock->tc, &vclock->cc, 0); ptp_schedule_worker(vclock->clock, PTP_VCLOCK_REFRESH_INTERVAL); --