thanks for your explanation, this was skipped from my mind. -gd On 4/26/05, Jim Bauer <jfbauer@xxxxxxx> wrote: > On Tuesday 26 April 2005 11:55, Gaurav Dhiman wrote: > > why we need to keep a count of readers, does that matter ? > > You need to know when the number of readers is 0 so > a writer can get the lock. If you don't keep that > count, how to you tell at any one time if there are still > any readers? > > Without a count something like this would happen. > > - reader_a see the lock is not held and issues a read lock. > - reader_b see a read lock already held and proceeds > to access the data. > - reader_a releases the lock > - writer_c see the lock not held and modifies the data > but reader_b is still accessing the data. > > With the count > > - reader_a sees the lock is not held and increments the reader count > - reader_b increments the reader count > - reader_a decrements the reader count > - writer_c sees one or more readers have a lock and spins. > - reader_b decrements the reader count thus freeing the lock > - writer_c can now grab a write lock. > > > > > > -- > Kernelnewbies: Help each other learn about the Linux kernel. > Archive: http://mail.nl.linux.org/kernelnewbies/ > FAQ: http://kernelnewbies.org/faq/ > > -- -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
