Hi Fabio,
My understanding: Semaphore are the tools to protect a region of code
by bloking access to other threads access while the first thead is
accessing it. Let it be any semaphore this is what it does.
global x;
up
/* critical section - any thing*/
*fpos+=count;
mystruct->size = *fpos;
mystruct->xxx = *fpos+ count;
down
What u say is:
What you are missing is that, in reading, there is NO manipulation of
the data you read. These are what are protected.
But, there is something like which from location u read and till which
location which are part of kernel file attributes. Is there no need to
protect them? or some location updates to my own structure which I
have to make for future use.
- Bhanu
On 4/1/08, Fabio De Francesco <[EMAIL PROTECTED]> wrote:
> On Tuesday 01 April 2008 23:57:47 bhanu nani wrote:
> >Hi guys,
> >
> >I was studying on reader-writer semaphores in LLD3. Refered to some
> >samaphore books. but some clarity is missing.
> >
> >Reader/Writer Semaphores:
> >The critical section between down_read, up_read semaphores provides
> >read-only access to the protected resources.
> >
> >I assume that any data in between thi calls is considered protected
> >resources.
> >As there is no data manipulation what it means is reader1/ reader2 can
> >execute the same instrction simultaniously. but how is that possible.
> >How can a perform a read a read aeithout data manipulation i.e.
> >updating user buffers, updatinf the fpos, count. Will these not need
> >protection between the readers themselves i.e. reader1, reader2,
> >...readerN
> >
> >How does this happen?
> >
> >Regards,
> >
> >Bhanu
>
> Dear Bhanu,
>
> What you are missing is that, in reading, there is NO manipulation of the
> data you read. These are what are protected. Buffers, fpos, counters and
> anything needed to comply are not part of the kernel structure the semaphore
> is protecting.
>
> You talked about "USER buffers...". May be you meant "PROCESS" ones. You
> forget that any process, in kernel mode, has his working memory: read 'Kernel
> Stack'.
>
> Regards,
>
> fabio de francesco
>
>
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