read-write lock is defined as follws in kernel:
File: linux/include/asm-i386/spinlock.h
typedef struct {
volatile unsigned int lock;
#if SPINLOCK_DEBUG
unsigned magic;
#endif
} rwlock_t;
If we look at the rwlock_t structure shown above (which is actually
the read write lock), it is same as spinlock with only one lock
element (lock). Now, my question is, as there is only one lock element
in the structure, how can a second or third reader come to know if the
lock has been acquired by a reader or by a writer. If the lock has
been acquired by reader, then this second or third reader should be
allowed to enter c-region, else if it has already been acquired by
writer, this reader should not be allowed to enter, but how we will
come to know who actually acquired the lock (was that a reader or
writer). I think there should be two lock elemets in the read-write
spinlocks to differentiate between the concept of read lock and write
lock.
Correct me whereever I am wrong.
-gd
On 4/26/05, Thomas Petazzoni <thomas.petazzoni@xxxxxxxx> wrote:
> Hello,
>
> Gaurav Dhiman wrote:
>
> > I think if we allows the writer to be in critical section, no one else
> > (not even readers) should be allowed there in critical section,
> > because writer may effect what the reader are reading and a situation
> > like above (if I am correct) can lead to crashes.
>
> What the document says is : at a given time, you can have many readers
> in the critical section OR one writer in the critical section. So you're
> right, read/write locks can be taken by many readers at the same time OR
> by only ONE writer at a time if there aren't any readers.
>
> Sincerly,
>
> Thomas
> --
> Thomas Petazzoni
> thomas.petazzoni@xxxxxxxx
>
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