linux-kernel - Re: [patch 2/13] signal/timer/event fds v6

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Date:	Mon, 19 Mar 2007 12:11:01 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...sign.ru>
Subject: Re: [patch 2/13] signal/timer/event fds v6 - signalfd core ...

On Mon, 19 Mar 2007, Eric W. Biederman wrote:

> Davide Libenzi <davidel@...ilserver.org> writes:
> 
> > On Mon, 19 Mar 2007, Eric W. Biederman wrote:
> >
> >> Davide Libenzi <davidel@...ilserver.org> writes:
> >> 
> >> > struct signalfd_siginfo {
> >> > 	__u32 signo;	/* si_signo */
> >> > 	__s32 err;	/* si_errno */
> >> > 	__s32 code;	/* si_code */
> >> > 	__u32 pid;	/* si_pid */
> >> > 	__u32 uid;	/* si_uid */
> >> > 	__s32 fd;	/* si_fd */
> >> > 	__u32 tid;	/* si_fd */  
> >> > 	__u32 band;	/* si_band */
> >> > 	__u32 overrun;	/* si_overrun */
> >> > 	__u32 trapno;	/* si_trapno */
> >> > 	__s32 status;	/* si_status */
> >> > 	__s32 svint;	/* si_int */
> >> > 	__u64 svptr;	/* si_ptr */
> >> > 	__u64 utime;	/* si_utime */
> >> > 	__u64 stime;	/* si_stime */
> >> > 	__u64 addr;	/* si_addr */
> >> > };
> >> 
> >> Shouldn't we pad this to 128 bytes like we do siginfo in case there are
> >> more fields we need to include, or we need to extend the size of some
> >> field?
> >
> > Yes, I guess we can.
> 
> I'm just a little paranoid about ABI's.  There is always something
> that crops up.  And while we can probably cope by simply having another
> version of the signalfd or whatever your syscall is, but having to do
> that at the first sign of trouble sucks.  Especially since we would have
> to maintain two versions indefinitely.

Ok, I added the padding to 128 bytes to the struct.



> >> I think you want to use a struct pid *pid instead of a pointer to the
> >> task struct here.  It is slightly less efficient (one more
> >> dereference) but it means that we won't pin the task struct in memory
> >> indefinitely.  Pinning the task_struct like this makes for a very
> >> interesting way to get around the limits on the number of processes a
> >> user can have.
> >
> > Hmm, when the task is detached from the sighand, we get a notify, so I 
> > could do a put from there. This would avoid the extra de-reference. I need 
> > to verify locking though ...
> 
> Ok.  That sounds more efficient than playing with struct pid pointers,
> if it works.

I'm looking into this right now ...



- Davide


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