svn commit: r322746 - head/sys/compat/linuxkpi/common/src

[Hans Petter Selasky]

Author: hselasky
Date: Mon Aug 21 11:51:40 2017
New Revision: 322746
URL: https://svnweb.freebsd.org/changeset/base/322746

Log:
  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 <Krishna2 at chelsio.com>
  MFC after:	1 week
  Sponsored by:	Mellanox Technologies

Modified:
  head/sys/compat/linuxkpi/common/src/linux_rcu.c

Modified: head/sys/compat/linuxkpi/common/src/linux_rcu.c
==============================================================================
--- head/sys/compat/linuxkpi/common/src/linux_rcu.c	Mon Aug 21 10:26:11 2017	(r322745)
+++ head/sys/compat/linuxkpi/common/src/linux_rcu.c	Mon Aug 21 11:51:40 2017	(r322746)
@@ -258,6 +258,15 @@ linux_synchronize_rcu_cb(ck_epoch_t *epoch __unused, c
 			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 {
 		/*