Added shmtx interface to forcibly unlock mutexes.

It is currently used from master process on abnormal worker termination to
unlock accept mutex (unlocking of accept mutex was broken in 1.0.2).  It is
expected to be used in the future to unlock other mutexes as well.

Shared mutex code was rewritten to make this possible in a safe way, i.e.
with a check if lock was actually held by the exited process.  We again use
pid to lock mutex, and use separate atomic variable for a count of processes
waiting in sem_wait().

6 files changed, 86 insertions, 49 deletions

diff --git a/src/core/ngx_cycle.c b/src/core/ngx_cycle.c
index db473571f..525ef41d7 100644
--- a/src/core/ngx_cycle.c
+++ b/src/core/ngx_cycle.c
@@ -952,7 +952,7 @@ ngx_init_zone_pool(ngx_cycle_t *cycle, ngx_shm_zone_t *zn)
 
 #endif
 
-    if (ngx_shmtx_create(&sp->mutex, (void *) &sp->lock, file) != NGX_OK) {
+    if (ngx_shmtx_create(&sp->mutex, &sp->lock, file) != NGX_OK) {
         return NGX_ERROR;
     }
 
diff --git a/src/core/ngx_shmtx.c b/src/core/ngx_shmtx.c
index 3b429c4f9..5a9242dfe 100644
--- a/src/core/ngx_shmtx.c
+++ b/src/core/ngx_shmtx.c
@@ -11,10 +11,13 @@
 #if (NGX_HAVE_ATOMIC_OPS)
 
 
+static void ngx_shmtx_wakeup(ngx_shmtx_t *mtx);
+
+
 ngx_int_t
-ngx_shmtx_create(ngx_shmtx_t *mtx, void *addr, u_char *name)
+ngx_shmtx_create(ngx_shmtx_t *mtx, ngx_shmtx_sh_t *addr, u_char *name)
 {
-    mtx->lock = addr;
+    mtx->lock = &addr->lock;
 
     if (mtx->spin == (ngx_uint_t) -1) {
         return NGX_OK;
@@ -24,6 +27,8 @@ ngx_shmtx_create(ngx_shmtx_t *mtx, void *addr, u_char *name)
 
 #if (NGX_HAVE_POSIX_SEM)
 
+    mtx->wait = &addr->wait;
+
     if (sem_init(&mtx->sem, 1, 0) == -1) {
         ngx_log_error(NGX_LOG_ALERT, ngx_cycle->log, ngx_errno,
                       "sem_init() failed");
@@ -56,12 +61,7 @@ ngx_shmtx_destory(ngx_shmtx_t *mtx)
 ngx_uint_t
 ngx_shmtx_trylock(ngx_shmtx_t *mtx)
 {
-    ngx_atomic_uint_t  val;
-
-    val = *mtx->lock;
-
-    return ((val & 0x80000000) == 0
-            && ngx_atomic_cmp_set(mtx->lock, val, val | 0x80000000));
+    return (*mtx->lock == 0 && ngx_atomic_cmp_set(mtx->lock, 0, ngx_pid));
 }
 
 
@@ -69,17 +69,12 @@ void
 ngx_shmtx_lock(ngx_shmtx_t *mtx)
 {
     ngx_uint_t         i, n;
-    ngx_atomic_uint_t  val;
 
     ngx_log_debug0(NGX_LOG_DEBUG_CORE, ngx_cycle->log, 0, "shmtx lock");
 
     for ( ;; ) {
 
-        val = *mtx->lock;
-
-        if ((val & 0x80000000) == 0
-            && ngx_atomic_cmp_set(mtx->lock, val, val | 0x80000000))
-        {
+        if (*mtx->lock == 0 && ngx_atomic_cmp_set(mtx->lock, 0, ngx_pid)) {
             return;
         }
 
@@ -91,10 +86,8 @@ ngx_shmtx_lock(ngx_shmtx_t *mtx)
                     ngx_cpu_pause();
                 }
 
-                val = *mtx->lock;
-
-                if ((val & 0x80000000) == 0
-                    && ngx_atomic_cmp_set(mtx->lock, val, val | 0x80000000))
+                if (*mtx->lock == 0
+                    && ngx_atomic_cmp_set(mtx->lock, 0, ngx_pid))
                 {
                     return;
                 }
@@ -104,24 +97,24 @@ ngx_shmtx_lock(ngx_shmtx_t *mtx)
 #if (NGX_HAVE_POSIX_SEM)
 
         if (mtx->semaphore) {
-            val = *mtx->lock;
+            ngx_atomic_fetch_add(mtx->wait, 1);
 
-            if ((val & 0x80000000)
-                && ngx_atomic_cmp_set(mtx->lock, val, val + 1))
-            {
-                ngx_log_debug1(NGX_LOG_DEBUG_CORE, ngx_cycle->log, 0,
-                               "shmtx wait %XA", val);
+            if (*mtx->lock == 0 && ngx_atomic_cmp_set(mtx->lock, 0, ngx_pid)) {
+                return;
+            }
 
-                while (sem_wait(&mtx->sem) == -1) {
-                    ngx_err_t  err;
+            ngx_log_debug1(NGX_LOG_DEBUG_CORE, ngx_cycle->log, 0,
+                           "shmtx wait %uA", *mtx->wait);
 
-                    err = ngx_errno;
+            while (sem_wait(&mtx->sem) == -1) {
+                ngx_err_t  err;
 
-                    if (err != NGX_EINTR) {
-                        ngx_log_error(NGX_LOG_ALERT, ngx_cycle->log, err,
-                                   "sem_wait() failed while waiting on shmtx");
-                        break;
-                    }
+                err = ngx_errno;
+
+                if (err != NGX_EINTR) {
+                    ngx_log_error(NGX_LOG_ALERT, ngx_cycle->log, err,
+                                  "sem_wait() failed while waiting on shmtx");
+                    break;
                 }
 
                 ngx_log_debug0(NGX_LOG_DEBUG_CORE, ngx_cycle->log, 0,
@@ -141,31 +134,56 @@ ngx_shmtx_lock(ngx_shmtx_t *mtx)
 void
 ngx_shmtx_unlock(ngx_shmtx_t *mtx)
 {
-    ngx_atomic_uint_t  val, old, wait;
-
     if (mtx->spin != (ngx_uint_t) -1) {
         ngx_log_debug0(NGX_LOG_DEBUG_CORE, ngx_cycle->log, 0, "shmtx unlock");
     }
 
-    for ( ;; ) {
+    if (ngx_atomic_cmp_set(mtx->lock, ngx_pid, 0)) {
+        ngx_shmtx_wakeup(mtx);
+    }
+}
 
-        old = *mtx->lock;
-        wait = old & 0x7fffffff;
-        val = wait ? wait - 1 : 0;
 
-        if (ngx_atomic_cmp_set(mtx->lock, old, val)) {
-            break;
-        }
+ngx_uint_t
+ngx_shmtx_force_unlock(ngx_shmtx_t *mtx, ngx_pid_t pid)
+{
+    ngx_log_debug0(NGX_LOG_DEBUG_CORE, ngx_cycle->log, 0,
+                   "shmtx forced unlock");
+
+    if (ngx_atomic_cmp_set(mtx->lock, pid, 0)) {
+        ngx_shmtx_wakeup(mtx);
+        return 1;
     }
 
+    return 0;
+}
+
+
+static void
+ngx_shmtx_wakeup(ngx_shmtx_t *mtx)
+{
 #if (NGX_HAVE_POSIX_SEM)
+    ngx_atomic_uint_t  wait;
 
-    if (wait == 0 || !mtx->semaphore) {
+    if (!mtx->semaphore) {
         return;
     }
 
+    for ( ;; ) {
+
+        wait = *mtx->wait;
+
+        if (wait == 0) {
+            return;
+        }
+
+        if (ngx_atomic_cmp_set(mtx->wait, wait, wait - 1)) {
+            break;
+        }
+    }
+
     ngx_log_debug1(NGX_LOG_DEBUG_CORE, ngx_cycle->log, 0,
-                   "shmtx wake %XA", old);
+                   "shmtx wake %uA", wait);
 
     if (sem_post(&mtx->sem) == -1) {
         ngx_log_error(NGX_LOG_ALERT, ngx_cycle->log, ngx_errno,
@@ -180,7 +198,7 @@ ngx_shmtx_unlock(ngx_shmtx_t *mtx)
 
 
 ngx_int_t
-ngx_shmtx_create(ngx_shmtx_t *mtx, void *addr, u_char *name)
+ngx_shmtx_create(ngx_shmtx_t *mtx, ngx_shmtx_sh_t *addr, u_char *name)
 {
     if (mtx->name) {
 
@@ -280,4 +298,11 @@ ngx_shmtx_unlock(ngx_shmtx_t *mtx)
     ngx_log_abort(err, ngx_unlock_fd_n " %s failed", mtx->name);
 }
 
+
+void
+ngx_shmtx_force_unlock(ngx_shmtx_t *mtx, ngx_pid_t pid)
+{
+    /* void */
+}
+
 #endif
diff --git a/src/core/ngx_shmtx.h b/src/core/ngx_shmtx.h
index 714f73aa6..3ee7d3a79 100644
--- a/src/core/ngx_shmtx.h
+++ b/src/core/ngx_shmtx.h
@@ -13,9 +13,18 @@
 
 
 typedef struct {
+    ngx_atomic_t   lock;
+#if (NGX_HAVE_POSIX_SEM)
+    ngx_atomic_t   wait;
+#endif
+} ngx_shmtx_sh_t;
+
+
+typedef struct {
 #if (NGX_HAVE_ATOMIC_OPS)
     ngx_atomic_t  *lock;
 #if (NGX_HAVE_POSIX_SEM)
+    ngx_atomic_t  *wait;
     ngx_uint_t     semaphore;
     sem_t          sem;
 #endif
@@ -27,11 +36,13 @@ typedef struct {
 } ngx_shmtx_t;
 
 
-ngx_int_t ngx_shmtx_create(ngx_shmtx_t *mtx, void *addr, u_char *name);
+ngx_int_t ngx_shmtx_create(ngx_shmtx_t *mtx, ngx_shmtx_sh_t *addr,
+    u_char *name);
 void ngx_shmtx_destory(ngx_shmtx_t *mtx);
 ngx_uint_t ngx_shmtx_trylock(ngx_shmtx_t *mtx);
 void ngx_shmtx_lock(ngx_shmtx_t *mtx);
 void ngx_shmtx_unlock(ngx_shmtx_t *mtx);
+ngx_uint_t ngx_shmtx_force_unlock(ngx_shmtx_t *mtx, ngx_pid_t pid);
 
 
 #endif /* _NGX_SHMTX_H_INCLUDED_ */
diff --git a/src/core/ngx_slab.h b/src/core/ngx_slab.h
index 291e1b51e..202fd9380 100644
--- a/src/core/ngx_slab.h
+++ b/src/core/ngx_slab.h
@@ -22,7 +22,7 @@ struct ngx_slab_page_s {
 
 
 typedef struct {
-    ngx_atomic_t      lock;
+    ngx_shmtx_sh_t    lock;
 
     size_t            min_size;
     size_t            min_shift;
diff --git a/src/event/ngx_event.c b/src/event/ngx_event.c
index 8a472ffe8..7d297d96b 100644
--- a/src/event/ngx_event.c
+++ b/src/event/ngx_event.c
@@ -521,7 +521,8 @@ ngx_event_module_init(ngx_cycle_t *cycle)
     ngx_accept_mutex_ptr = (ngx_atomic_t *) shared;
     ngx_accept_mutex.spin = (ngx_uint_t) -1;
 
-    if (ngx_shmtx_create(&ngx_accept_mutex, shared, cycle->lock_file.data)
+    if (ngx_shmtx_create(&ngx_accept_mutex, (ngx_shmtx_sh_t *) shared,
+                         cycle->lock_file.data)
         != NGX_OK)
     {
         return NGX_ERROR;
diff --git a/src/os/unix/ngx_process.c b/src/os/unix/ngx_process.c
index 08069c4b6..cbdae8be1 100644
--- a/src/os/unix/ngx_process.c
+++ b/src/os/unix/ngx_process.c
@@ -504,7 +504,7 @@ ngx_process_get_status(void)
              * held it
              */
 
-            ngx_atomic_cmp_set(ngx_accept_mutex_ptr, pid, 0);
+            ngx_shmtx_force_unlock(&ngx_accept_mutex, pid);
         }