Re: [patch 13/13] lseek speedup

[Anton Altaparmakov]

At 08:16 22/07/02, Andrew Morton wrote:
>Anton Altaparmakov wrote:
> > At 17:49 17/07/02, Andrew Morton wrote:
> > >Anton Altaparmakov wrote:
> > > >
> > > > >  Now, why are we taking i_sem for lseek/readdir
> > > > >exclusion and not a per-file lock?
> > > >
> > > > Because it also excludes against directory modifications, etc. Just 
> imagine
> > > > what "rm somefile" or "mv somefile otherfile" or "touch newfile" 
> would do
> > > > to the directory contents and what a concurrent readdir() would do... A
> > > > very loud *BANG* is the only thing that springs to mind...
> > >
> > >That's different.  i_size, contents of things, yes - i_sem for
> > >those.
> > >
> > >But protection of struct file should not be via any per-inode thing.
> >
> > Oh, I see. But that would mean adding yet another lock to an existing
> > locking scheme? So both i_sem and the "per file lock" would nead to be held
> > over readdir(). That's doable but it would have to be a semaphore based
> > lock due to readdir()...
>
>Adding a sleeping lock to struct file for this would make sense; using
>i_sem to protect the innards of struct file ain't right.
>
>However I'm not sure that the VFS needs to be serialising lseek
>and readdir at all.  The fs can do that if it really needs to.
>
>And does it really need to?  Setting aside the non-atomic 64-bit
>read, it may be sufficient for the fs to do what ext2_readdir
>does:  read f_pos once on entry, write it once on exit and if
>there was a concurrent lseek then its results get stomped on.
>
>Can you see any problem with that?

No problem. NTFS can very easily be turned into doing that. It would need 
to happen to all file systems and perhaps a little documentation in the 
porting document to say so for out of kernel fs...

>Anyway.  It's not exactly a top-priority thing.  I'll park
>it for a while and just stop running that test ;)

Well, I will get ntfs_readdir changed to only read/write f_pos once on 
entry/exit respectively anyway...

Anton