From 714ed5b27bf70e47983236bfb4cee02690073826 Mon Sep 17 00:00:00 2001 From: Hans Petter Selasky Date: Mon, 21 Aug 2017 11:51:40 +0000 Subject: [PATCH] Fix for deadlock situation in the LinuxKPI's RCU synchronize API. Deadlock condition: The return value of TDQ_LOCKPTR(td) is the same for two threads. 1) The first thread signals a wakeup while keeping the rcu_read_lock(). This invokes sched_add() which in turn will try to lock TDQ_LOCK(). 2) The second thread is calling synchronize_rcu() calling mi_switch() over and over again trying to yield(). This prevents the first thread from running and releasing the RCU reader lock. Solution: Release the thread lock while yielding to allow other threads to acquire the lock pointed to by TDQ_LOCKPTR(td). Found by: KrishnamRaju ErapaRaju MFC after: 1 week Sponsored by: Mellanox Technologies --- sys/compat/linuxkpi/common/src/linux_rcu.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/sys/compat/linuxkpi/common/src/linux_rcu.c b/sys/compat/linuxkpi/common/src/linux_rcu.c index 3cf6aba7878..29944090af8 100644 --- a/sys/compat/linuxkpi/common/src/linux_rcu.c +++ b/sys/compat/linuxkpi/common/src/linux_rcu.c @@ -258,6 +258,15 @@ linux_synchronize_rcu_cb(ck_epoch_t *epoch __unused, ck_epoch_record_t *epoch_re sched_prio(td, prio); /* task switch */ mi_switch(SW_VOL | SWT_RELINQUISH, NULL); + + /* + * Release the thread lock while yielding to + * allow other threads to acquire the lock + * pointed to by TDQ_LOCKPTR(td). Else a + * deadlock like situation might happen. + */ + thread_unlock(td); + thread_lock(td); } } else { /*