Joshua Hudson wrote:

>Since we are moving from semaphores to mutex, there should be a
>mutex_rw. I had a try
>at creating one (might as well convert from sem_rw). I'll bet somebody
>here can do a
>lot better, but this will do if need be.
>
>--- linux-2.6.16.1-stock/include/linux/mutex_rw.h       1969-12-31
>16:00:00.000000000 -0800
>+++ linux-2.6.16.1-nvl/include/linux/mutex_rw.h 2006-04-04
>18:11:56.000000000 -0700
>@@ -0,0 +1,60 @@
>+/* Linux RW mutex
>+ * This file: GNU GPL v2 or later, Joshua Hudson <joshudson@gmail.com>
>+ *
>+ * Somebody else can make this fast. I just made this work.
>+ *
>+ * DANGER! Change of this file will break module binaries if any
>+ *  rw mutex is shared between main kernel and modules or between
>+ *  modules with a different version.
>+ */
>+
>+#ifndef __KERNEL_MUTEX_RW
>+#define __KERNEL_MUTEX_RW
>+#ifdef __KERNEL__
>+
>+#include <linux/mutex.h>
>+
>+struct rw_mutex {
>+       struct mutex r_mutex;
>+       struct mutex w_mutex;
>+       unsigned n_readers;
>+};
>+
>+static inline void rw_mutex_init(struct rw_mutex *rw)
>+{
>+       mutex_init(&rw->r_mutex);
>+       mutex_init(&rw->w_mutex);
>+       rw->n_readers = 0;
>+}
>+
>+static inline void rw_mutex_destroy(struct rw_mutex *rw)
>+{
>+       mutex_destroy(&rw->r_mutex);
>+       mutex_destroy(&rw->w_mutex);
>+}
>+
>+static inline void mutex_lock_w(struct rw_mutex *rw)
>+{
>+       mutex_lock(&rw->w_mutex);
>+}
>+
>+static inline void mutex_unlock_w(struct rw_mutex *rw)
>+{
>+       mutex_unlock(&rw->w_mutex);
>+}
>+
>+static inline void mutex_lock_r(struct rw_mutex *rw)
>+{
>+       mutex_lock(&rw->r_mutex);
>+       if (++rw->n_readers == 1)
>+               mutex_lock(&rw->w_mutex);
>+       mutex_unlock(&rw->r_mutex);
>+}
>+
>+static inline void mutex_unlock_r(struct rw_mutex *rw)
>+{
>+       mutex_lock(&rw->r_mutex);
>+       if (--rw->n_readers == 0)
>+               mutex_unlock(&rw->w_mutex);
>+       mutex_unlock(&rw->r_mutex);
>+}
>+
>+#endif
>+#endif
>-
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>  
>
Race conditions here.

Try this code. These have been tested and I have been using the code for 
couple of years in another OS (which is just about done now and is in 
alpha and beta and little more SMP friendly).

Jeff