[Devel] Re: [RFC PATCH 0/5] Resend - Use procfs to change a syscall behavior

[Pavel Machek]

Hi!

> This patchset is a part of an effort to change some syscalls behavior for
> checkpoint restart.
> 
> When restarting an object that has previously been checkpointed, its state
> should be unchanged compared to the checkpointed image.
> For example, a restarted process should have the same upid nr as the one it
> used to have when being checkpointed; an ipc object should have the same id
> as the one it had when the checkpoint occured.
> Also, talking about system V ipcs, they should be restored with the same
> state (e.g. in terms of pid of last operation).
> 
> This means that several syscalls should not behave in a default mode when
> they are called during a restart phase.
> 
> One solution consists in defining a new syscall for each syscall that is
> called during restart:
>  . sys_fork_with_id() would fork a process with a predefined id.
>  . sys_msgget_with_id() would create a msg queue with a predefined id
>  . sys_semget_with_id() would create a semaphore set with a predefined id
>  . etc,
> 
> This solution requires defining a new syscall each time we need an existing
> syscall to behave in a non-default way.

Yes, and I believe that's better than...

> An alternative to this solution consists in defining a new field in the
> task structure (let's call it next_syscall_data) that, if set, would change
> the behavior of next syscall to be called. The sys_fork_with_id() previously
> cited can be replaced by
>  1) set next_syscall_data to a target upid nr
>  2) call fork().

...bloat task struct and

> A new file is created in procfs: /proc/self/task/<my_tid>/next_syscall_data.
> This makes it possible to avoid races between several threads belonging to
> the same process.

...introducing this kind of uglyness.

Actually, there were proposals for sys_indirect(), which is slightly
less ugly, but IIRC we ended up with adding syscalls, too.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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