- Fix a race in sched_switch() of sched_4bsd.

  In the case of the thread being on a sleepqueue or a turnstile, the
  sched_lock was acquired (without the aid of the td_lock interface) and
  the td_lock was dropped. This was going to break locking rules on other
  threads willing to access to the thread (via the td_lock interface) and
  modify his flags (allowed as long as the container lock was different
  by the one used in sched_switch).
  In order to prevent this situation, while sched_lock is acquired there
  the td_lock gets blocked. [0]
- Merge the ULE's internal function thread_block_switch() into the global
  thread_lock_block() and make the former semantic as the default for
  thread_lock_block(). This means that thread_lock_block() will not
  disable interrupts when called (and consequently thread_unlock_block()
  will not re-enabled them when called). This should be done manually
  when necessary.
  Note, however, that ULE's thread_unblock_switch() is not reaped
  because it does reflect a difference in semantic due in ULE (the
  td_lock may not be necessarilly still blocked_lock when calling this).
  While asymmetric, it does describe a remarkable difference in semantic
  that is good to keep in mind.

[0] Reported by:	Kohji Okuno
			<okuno dot kohji at jp dot panasonic dot com>
Tested by:		Giovanni Trematerra
			<giovanni dot trematerra at gmail dot com>
MFC:			2 weeks

1 files changed, 5 insertions, 2 deletions

diff --git a/sys/kern/sched_4bsd.c b/sys/kern/sched_4bsd.c
index 99ea7b802380..bcec5b9205dd 100644
--- a/sys/kern/sched_4bsd.c
+++ b/sys/kern/sched_4bsd.c
@@ -920,9 +920,11 @@ sched_sleep(struct thread *td, int pri)
 void
 sched_switch(struct thread *td, struct thread *newtd, int flags)
 {
+	struct mtx *tmtx;
 	struct td_sched *ts;
 	struct proc *p;
 
+	tmtx = NULL;
 	ts = td->td_sched;
 	p = td->td_proc;
 
@@ -931,10 +933,11 @@ sched_switch(struct thread *td, struct thread *newtd, int flags)
 	/* 
 	 * Switch to the sched lock to fix things up and pick
 	 * a new thread.
+	 * Block the td_lock in order to avoid breaking the critical path.
 	 */
 	if (td->td_lock != &sched_lock) {
 		mtx_lock_spin(&sched_lock);
-		thread_unlock(td);
+		tmtx = thread_lock_block(td);
 	}
 
 	if ((td->td_flags & TDF_NOLOAD) == 0)
@@ -1004,7 +1007,7 @@ sched_switch(struct thread *td, struct thread *newtd, int flags)
 			(*dtrace_vtime_switch_func)(newtd);
 #endif
 
-		cpu_switch(td, newtd, td->td_lock);
+		cpu_switch(td, newtd, tmtx != NULL ? tmtx : td->td_lock);
 		lock_profile_obtain_lock_success(&sched_lock.lock_object,
 		    0, 0, __FILE__, __LINE__);
 		/*