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[PATCH 1 of 2] Added shmtx interface to forcibly unlock mutexes

Maxim Dounin
November 17, 2011 08:30PM
# HG changeset patch
# User Maxim Dounin <mdounin@mdounin.ru>
# Date 1321467405 -10800
# Node ID 98907592714e29982979010e310cd482d414b53f
# Parent 5a6ac21a44d49b71dfed08e362ea7a53e54d4f24
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().

diff --git a/src/core/ngx_cycle.c b/src/core/ngx_cycle.c
--- a/src/core/ngx_cycle.c
+++ b/src/core/ngx_cycle.c
@@ -952,7 +952,7 @@ ngx_init_zone_pool(ngx_cycle_t *cycle, n

#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
--- a/src/core/ngx_shmtx.c
+++ b/src/core/ngx_shmtx.c
@@ -12,9 +12,9 @@


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 +24,8 @@ ngx_shmtx_create(ngx_shmtx_t *mtx, void

#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 +58,7 @@ 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 +66,12 @@ 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 +83,8 @@ 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 +94,24 @@ 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 %XA", *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 +131,39 @@ ngx_shmtx_lock(ngx_shmtx_t *mtx)
void
ngx_shmtx_unlock(ngx_shmtx_t *mtx)
{
- ngx_atomic_uint_t val, old, wait;
+#if (NGX_HAVE_POSIX_SEM)
+ ngx_atomic_uint_t wait;
+#endif

if (mtx->spin != (ngx_uint_t) -1) {
ngx_log_debug0(NGX_LOG_DEBUG_CORE, ngx_cycle->log, 0, "shmtx unlock");
}

+ if (!ngx_atomic_cmp_set(mtx->lock, ngx_pid, 0)) {
+ return;
+ }
+
+#if (NGX_HAVE_POSIX_SEM)
+
+ if (!mtx->semaphore) {
+ return;
+ }
+
for ( ;; ) {

- old = *mtx->lock;
- wait = old & 0x7fffffff;
- val = wait ? wait - 1 : 0;
+ wait = *mtx->wait;

- if (ngx_atomic_cmp_set(mtx->lock, old, val)) {
+ if (wait == 0) {
+ return;
+ }
+
+ if (ngx_atomic_cmp_set(mtx->wait, wait, wait - 1)) {
break;
}
}

-#if (NGX_HAVE_POSIX_SEM)
-
- if (wait == 0 || !mtx->semaphore) {
- return;
- }
-
ngx_log_debug1(NGX_LOG_DEBUG_CORE, ngx_cycle->log, 0,
- "shmtx wake %XA", old);
+ "shmtx wake %XA", wait);

if (sem_post(&mtx->sem) == -1) {
ngx_log_error(NGX_LOG_ALERT, ngx_cycle->log, ngx_errno,
@@ -176,11 +174,58 @@ ngx_shmtx_unlock(ngx_shmtx_t *mtx)
}


+ngx_uint_t
+ngx_shmtx_force_unlock(ngx_shmtx_t *mtx, ngx_pid_t pid)
+{
+#if (NGX_HAVE_POSIX_SEM)
+ ngx_atomic_uint_t wait;
+#endif
+
+ ngx_log_debug0(NGX_LOG_DEBUG_CORE, ngx_cycle->log, 0,
+ "shmtx forced unlock");
+
+ if (!ngx_atomic_cmp_set(mtx->lock, pid, 0)) {
+ return 0;
+ }
+
+#if (NGX_HAVE_POSIX_SEM)
+
+ if (!mtx->semaphore) {
+ return 1;
+ }
+
+ for ( ;; ) {
+
+ wait = *mtx->wait;
+
+ if (wait == 0) {
+ return 1;
+ }
+
+ 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", wait);
+
+ if (sem_post(&mtx->sem) == -1) {
+ ngx_log_error(NGX_LOG_ALERT, ngx_cycle->log, ngx_errno,
+ "sem_post() failed while wake shmtx");
+ }
+
+#endif
+
+ return 1;
+}
+
+
#else


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 +325,11 @@ 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
--- 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
--- 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
--- 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
--- 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);
}



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Subject Author Views Posted

[PATCH 0 of 2] unlock mutexes on process crash

Maxim Dounin 995 November 17, 2011 08:30PM

[PATCH 1 of 2] Added shmtx interface to forcibly unlock mutexes

Maxim Dounin 572 November 17, 2011 08:30PM

[PATCH 2 of 2] Unlock of shared memory zones on process crash

Maxim Dounin 401 November 17, 2011 08:30PM



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