thread concurrent file operation

[Peter Teoh]

in ANY updates/changes, locking is always needed, to prevent multiple
parties from updating at the same time.   but there is another way:
 lockless updates.   one form done in linux kernel is called RCU:

http://en.wikipedia.org/wiki/Read-copy-update

the logic is whenever someone want to change, just write the changes
somewhere, so that reconstruction of the change is possible through reading
the changes + existing data.   (Oracle database, and indeed any database
does that too.).   so if multiple CPU want to write to the same place, then
u still need per-CPU locks for classic RCU:

http://lwn.net/Articles/305782/

But for reader, there is no need to lock:  just go ahead and read - if u
read AFTER the update has started, then u will be reading the older copy,
and the last reader will then kick off the merging of the older copy +
newer updates.

http://lwn.net/2001/features/OLS/pdf/pdf/read-copy.pdf

http://lwn.net/Articles/262464/

http://lwn.net/Articles/263130/  (see the picture here)

but these locking are done at the low level - harddisk is data block level.


For vfs_read() -  its purpose is to read...and it does not prevent u from
writing!!! yes, everything is left to the user at the userspace
level...locking/unlocking.   because it is done at the FILE level, and so
if u have multiple reads and then someone come in and write....yes, there
will be corruption.   but that is the logic corruption, not the
hardware/datablocks corruption, which the kernel aimed to protect.

On Tue, Jan 29, 2013 at 11:35 PM, Karaoui mohamed lamine <moharaka at gmail.com
> wrote:

> Hello,
>
> I was looking at how a syscall read/write was done, and i found this :
>
>    ....
>    loff_t pos = file_pos_read(f.file);
>    ret = vfs_read(f.file, buf, count, &pos);
>    file_pos_write(f.file, pos);
>    fdput(f);
>    ...
>
> My questions are :
>
> Where did the locking go? I would have imaginated something like :
>
>    ....
>    *lock(f);*
>    loff_t pos = file_pos_read(f.file);
>    ret = vfs_read(f.file, buf, count, &pos);
>    file_pos_write(f.file, pos);
>    fdput(f);
>    *unlock(f);*
>    ...
>
> If multiple threads try to read/write at the same time, they could
> read/write at the same offset ?
>
> If my understanding are correct, is this POSIX compliant ?
>
>
> thanks.
>
>
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>


-- 
Regards,
Peter Teoh
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