Disallow nesting of LITMUS^RT locks

Nesting of locks was never supported in LITMUS^RT since
the required analysis does not exist anyway. That is, as
defined in the literature, the protocols implemented
in LITMUS^RT have not been studied in conjunction with
nested critical sections.

In LITMUS^RT, attempting to nest locks could lead to
silent or not-so-silent bugs. This patch makes this
restriction explicit and returns EBUSY when a process
attempts to nest resources.

This is enforced on a protocol-by-protocol basis,
which means that adding protocols with support for
nesting in future versions is not affected by this
change.

Exception: PCP and SRP resources may be nested,
but not within global critical sections.

4 files changed, 66 insertions, 3 deletions

diff --git a/litmus/sched_gsn_edf.c b/litmus/sched_gsn_edf.c
index b8548b885b35..8fdc8f68fcfb 100644
--- a/litmus/sched_gsn_edf.c
+++ b/litmus/sched_gsn_edf.c
@@ -773,6 +773,10 @@ int gsnedf_fmlp_lock(struct litmus_lock* l)
         if (!is_realtime(t))
                 return -EPERM;
 
+        /* prevent nested lock acquisition --- not supported by FMLP */
+        if (tsk_rt(t)->num_locks_held)
+                return -EBUSY;
+
         spin_lock_irqsave(&sem->wait.lock, flags);
 
         if (sem->owner) {
@@ -817,6 +821,8 @@ int gsnedf_fmlp_lock(struct litmus_lock* l)
                 spin_unlock_irqrestore(&sem->wait.lock, flags);
         }
 
+        tsk_rt(t)->num_locks_held++;
+
         return 0;
 }
 
@@ -834,6 +840,8 @@ int gsnedf_fmlp_unlock(struct litmus_lock* l)
                 goto out;
         }
 
+        tsk_rt(t)->num_locks_held--;
+
         /* check if there are jobs waiting for this resource */
         next = __waitqueue_remove_first(&sem->wait);
         if (next) {
diff --git a/litmus/sched_pfp.c b/litmus/sched_pfp.c
index e3c3bd770efa..fc9a509f185d 100644
--- a/litmus/sched_pfp.c
+++ b/litmus/sched_pfp.c
@@ -555,6 +555,11 @@ int pfp_fmlp_lock(struct litmus_lock* l)
         if (!is_realtime(t))
                 return -EPERM;
 
+        /* prevent nested lock acquisition --- not supported by FMLP */
+        if (tsk_rt(t)->num_locks_held ||
+            tsk_rt(t)->num_local_locks_held)
+                return -EBUSY;
+
         spin_lock_irqsave(&sem->wait.lock, flags);
 
         /* tie-break by this point in time */
@@ -599,6 +604,8 @@ int pfp_fmlp_lock(struct litmus_lock* l)
                 spin_unlock_irqrestore(&sem->wait.lock, flags);
         }
 
+        tsk_rt(t)->num_locks_held++;
+
         return 0;
 }
 
@@ -616,6 +623,8 @@ int pfp_fmlp_unlock(struct litmus_lock* l)
                 goto out;
         }
 
+        tsk_rt(t)->num_locks_held--;
+
         /* we lose the benefit of priority boosting */
 
         unboost_priority(t);
@@ -790,6 +799,11 @@ int pfp_mpcp_lock(struct litmus_lock* l)
         if (!is_realtime(t))
                 return -EPERM;
 
+        /* prevent nested lock acquisition */
+        if (tsk_rt(t)->num_locks_held ||
+            tsk_rt(t)->num_local_locks_held)
+                return -EBUSY;
+
         preempt_disable();
 
         if (sem->vspin)
@@ -840,6 +854,8 @@ int pfp_mpcp_lock(struct litmus_lock* l)
                 spin_unlock_irqrestore(&sem->wait.lock, flags);
         }
 
+        tsk_rt(t)->num_locks_held++;
+
         return 0;
 }
 
@@ -857,6 +873,9 @@ int pfp_mpcp_unlock(struct litmus_lock* l)
                 goto out;
         }
 
+
+        tsk_rt(t)->num_locks_held--;
+
         /* we lose the benefit of priority boosting */
 
         unboost_priority(t);
@@ -1249,12 +1268,18 @@ int pfp_pcp_lock(struct litmus_lock* l)
         if (!is_realtime(t) || from != to)
                 return -EPERM;
 
+        /* prevent nested lock acquisition in global critical section */
+        if (tsk_rt(t)->num_locks_held)
+                return -EBUSY;
+
         preempt_disable();
 
         pcp_raise_ceiling(sem, eprio);
 
         preempt_enable();
 
+        tsk_rt(t)->num_local_locks_held++;
+
         return 0;
 }
 
@@ -1272,6 +1297,8 @@ int pfp_pcp_unlock(struct litmus_lock* l)
                 goto out;
         }
 
+        tsk_rt(t)->num_local_locks_held--;
+
         /* give it back */
         pcp_lower_ceiling(sem);
 
@@ -1428,6 +1455,11 @@ int pfp_dpcp_lock(struct litmus_lock* l)
         if (!is_realtime(t))
                 return -EPERM;
 
+        /* prevent nested lock accquisition */
+        if (tsk_rt(t)->num_locks_held ||
+            tsk_rt(t)->num_local_locks_held)
+                return -EBUSY;
+
         preempt_disable();
 
         /* Priority-boost ourself *before* we suspend so that
@@ -1444,6 +1476,8 @@ int pfp_dpcp_lock(struct litmus_lock* l)
 
         preempt_enable();
 
+        tsk_rt(t)->num_locks_held++;
+
         return 0;
 }
 
@@ -1461,6 +1495,8 @@ int pfp_dpcp_unlock(struct litmus_lock* l)
                 goto out;
         }
 
+        tsk_rt(t)->num_locks_held--;
+
         home = sem->owner_cpu;
 
         /* give it back */
diff --git a/litmus/sched_psn_edf.c b/litmus/sched_psn_edf.c
index 0e1675d2e572..c158f3532ba6 100644
--- a/litmus/sched_psn_edf.c
+++ b/litmus/sched_psn_edf.c
@@ -419,6 +419,11 @@ int psnedf_fmlp_lock(struct litmus_lock* l)
         if (!is_realtime(t))
                 return -EPERM;
 
+        /* prevent nested lock acquisition --- not supported by FMLP */
+        if (tsk_rt(t)->num_locks_held ||
+            tsk_rt(t)->num_local_locks_held)
+                return -EBUSY;
+
         spin_lock_irqsave(&sem->wait.lock, flags);
 
         if (sem->owner) {
@@ -459,6 +464,8 @@ int psnedf_fmlp_lock(struct litmus_lock* l)
                 spin_unlock_irqrestore(&sem->wait.lock, flags);
         }
 
+        tsk_rt(t)->num_locks_held++;
+
         return 0;
 }
 
@@ -476,6 +483,8 @@ int psnedf_fmlp_unlock(struct litmus_lock* l)
                 goto out;
         }
 
+        tsk_rt(t)->num_locks_held--;
+
         /* we lose the benefit of priority boosting */
 
         unboost_priority(t);
diff --git a/litmus/srp.c b/litmus/srp.c
index 2ed4ec12a9d3..c88dbf2f580f 100644
--- a/litmus/srp.c
+++ b/litmus/srp.c
@@ -98,11 +98,16 @@ static void srp_add_prio(struct srp* srp, struct srp_priority* prio)
 
 static int lock_srp_semaphore(struct litmus_lock* l)
 {
+        struct task_struct* t = current;
         struct srp_semaphore* sem = container_of(l, struct srp_semaphore, litmus_lock);
 
-        if (!is_realtime(current))
+        if (!is_realtime(t))
                 return -EPERM;
 
+        /* prevent acquisition of local locks in global critical sections */
+        if (tsk_rt(t)->num_locks_held)
+                return -EBUSY;
+
         preempt_disable();
 
         /* Update ceiling. */
@@ -111,9 +116,11 @@ static int lock_srp_semaphore(struct litmus_lock* l)
         /* SRP invariant: all resources available */
         BUG_ON(sem->owner != NULL);
 
-        sem->owner = current;
+        sem->owner = t;
         TRACE_CUR("acquired srp 0x%p\n", sem);
 
+        tsk_rt(t)->num_local_locks_held++;
+
         preempt_enable();
 
         return 0;
@@ -121,12 +128,13 @@ static int lock_srp_semaphore(struct litmus_lock* l)
 
 static int unlock_srp_semaphore(struct litmus_lock* l)
 {
+        struct task_struct* t = current;
         struct srp_semaphore* sem = container_of(l, struct srp_semaphore, litmus_lock);
         int err = 0;
 
         preempt_disable();
 
-        if (sem->owner != current) {
+        if (sem->owner != t) {
                 err = -EINVAL;
         } else {
                 /* Determine new system priority ceiling for this CPU. */
@@ -138,6 +146,8 @@ static int unlock_srp_semaphore(struct litmus_lock* l)
                 /* Wake tasks on this CPU, if they exceed current ceiling. */
                 TRACE_CUR("released srp 0x%p\n", sem);
                 wake_up_all(&__get_cpu_var(srp).ceiling_blocked);
+
+                tsk_rt(t)->num_local_locks_held--;
         }
 
         preempt_enable();