Thanks to all of you who replied to this thread, it was good information for me and I also looked into "Understanding the Linux Kernel", which explains the things very clearly, so my doubts are clear now. Thanks once again. -gd On 4/26/05, K.R. Foley <kr@xxxxxxxxxx> wrote: > Gaurav Dhiman wrote: > > Thanks for your reply, it clears my doubt of keeping two locks, one > > for read and one for write. As this info can be stored in one element > > only, we dont need two. > > > > > >>You're right, but in fact, the implementation has been so that both > >>information are encoded inside a single integer. You don't really need > >>two counters, but : > >> - a counter of readers > > > > > > why we need to keep a count of readers, does that matter ? > > Because you can't get a write lock while there are any readers. > > From this and the rest of your message, I am not sure you are getting > how this works: > > readers and writers are mutually exclusive. While there are readers you > can't get the write lock. While there is a writer, there are no readers. > > > > > > >> - a bit indicating whether a writer owns the lock or not > > > > > >>So, in fact, one bit of the integer is reserved to "mark" the rwlock as > >>owned by a writer or not. Have a look at include/asm-i386/rwlock.h, and > >>particularly the definition of RW_LOCK_BIAS. > > > > > > Can you let me know whats the significance of this flag, is this flag > > used to mark the writer and other bits used for keeping a count of > > reades. > > This is the value that is subtracted to get the write lock. It also > happens to be the init/idle value of the lock. So, if subtracting > RW_LOCK_BIAS from the lock value yields 0x00000000 then you have > acquired the write lock. > > Since we are on the subject, read locks are also gotten by subtracting > from the value of the lock and are represented by any number less than > 0x01000000. > > > > > > >>Have a look at the implementation of __build_read_lock_const() and > >>__build_write_lock_const() is the same file. > > > > > > cud not understand this assembly code, where is the helper function > > defined, it is calling ? > > helper is the name of the function (in this case the spin function) that > is passed in like: > > __build_write_lock(rw, "__write_lock_failed"); > > Which is in spinlock.h > > > > > > >>Sincerly, > >> > >>Thomas > >>-- > >>Thomas Petazzoni > >>thomas.petazzoni@xxxxxxxx > >> > > > > > > -- > > > > -- > > Kernelnewbies: Help each other learn about the Linux kernel. > > Archive: http://mail.nl.linux.org/kernelnewbies/ > > FAQ: http://kernelnewbies.org/faq/ > > > > > > > -- > kr > -- -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
