8040 NFSv4 client: 3-way deadlock between nfs4_bio(), nfs4_do_delegreturn(), and nfs4_flush_pages()
Reviewed by: Arne Jansen <arne@die-jansens.de>
Reviewed by: Vitaliy Gusev <gusev.vitaliy@icloud.com>
Approved by: Gordon Ross <gordon.w.ross@gmail.com>

*** 462,500 ****
          mutex_exit(&mi->mi_lock);
  
          rp = VTOR4(vp);
          mutex_enter(&rp->r_statelock);
          was_serial = (rp->r_serial == curthread);
!         if (rp->r_serial && !was_serial) {
!                 klwp_t *lwp = ttolwp(curthread);
! 
                  /*
!                  * If we're the recovery thread, then purge current attrs
!                  * and bail out to avoid potential deadlock between another
!                  * thread caching attrs (r_serial thread), recov thread,
!                  * and an async writer thread.
                   */
-                 if (recov) {
                          PURGE_ATTRCACHE4_LOCKED(rp);
                          mutex_exit(&rp->r_statelock);
                          return;
                  }
  
-                 if (lwp != NULL)
-                         lwp->lwp_nostop++;
-                 while (rp->r_serial != NULL) {
-                         if (!cv_wait_sig(&rp->r_cv, &rp->r_statelock)) {
-                                 mutex_exit(&rp->r_statelock);
-                                 if (lwp != NULL)
-                                         lwp->lwp_nostop--;
-                                 return;
-                         }
-                 }
-                 if (lwp != NULL)
-                         lwp->lwp_nostop--;
-         }
- 
          /*
           * If there is a page flush thread, the current thread needs to
           * bail out, to prevent a possible deadlock between the current
           * thread (which might be in a start_op/end_op region), the
           * recovery thread, and the page flush thread.  Expire the
--- 462,482 ----
          mutex_exit(&mi->mi_lock);
  
          rp = VTOR4(vp);
          mutex_enter(&rp->r_statelock);
          was_serial = (rp->r_serial == curthread);
!         if (rp->r_serial != NULL && !was_serial) {
                  /*
!                  * Purge current attrs and bail out to avoid potential deadlock
!                  * between another thread caching attrs (r_serial thread), this
!                  * thread, and a thread trying to read or write pages.
                   */
                  PURGE_ATTRCACHE4_LOCKED(rp);
                  mutex_exit(&rp->r_statelock);
                  return;
          }
  
          /*
           * If there is a page flush thread, the current thread needs to
           * bail out, to prevent a possible deadlock between the current
           * thread (which might be in a start_op/end_op region), the
           * recovery thread, and the page flush thread.  Expire the