Re: git: 32114b639fa1 - main - Add PROC_COW_CHANGECOUNT and thread_cow_synced

On Fri, Feb 11, 2022 at 01:57:59PM +0000, Mateusz Guzik wrote:
> The branch main has been updated by mjg:
> 
> URL: https://cgit.FreeBSD.org/src/commit/?id=32114b639fa1ad777312eebe14a9f677bd7be2ea
> 
> commit 32114b639fa1ad777312eebe14a9f677bd7be2ea
> Author:     Mateusz Guzik <mjg@FreeBSD.org>
> AuthorDate: 2022-02-01 13:13:13 +0000
> Commit:     Mateusz Guzik <mjg@FreeBSD.org>
> CommitDate: 2022-02-11 11:44:07 +0000
> 
>     Add PROC_COW_CHANGECOUNT and thread_cow_synced
>     
>     Combined they can be used to avoid a proc lock/unlock cycle in the
>     syscall handler for curthread, see upcoming examples.
> ---
>  sys/kern/kern_thread.c | 13 +++++++++++++
>  sys/sys/proc.h         |  9 +++++++++
>  2 files changed, 22 insertions(+)
> 
> diff --git a/sys/kern/kern_thread.c b/sys/kern/kern_thread.c
> index dcb52b137b58..bb724a17803e 100644
> --- a/sys/kern/kern_thread.c
> +++ b/sys/kern/kern_thread.c
> @@ -868,6 +868,19 @@ thread_cow_update(struct thread *td)
>  		lim_free(oldlimit);
>  }
>  
> +void
> +thread_cow_synced(struct thread *td)
> +{
> +	struct proc *p;
> +
> +	p = td->td_proc;
> +	PROC_LOCK_ASSERT(p, MA_OWNED);
> +	MPASS(td->td_cowgen != p->p_cowgen);
> +	MPASS(td->td_ucred == p->p_ucred);
> +	MPASS(td->td_limit == p->p_limit);
> +	td->td_cowgen = p->p_cowgen;
This should be store-release, I think.
And corresponding loads in trap() needs to get acquire semantic.

This is probably a pre-existing bug.

> +}
> +
>  /*
>   * Discard the current thread and exit from its context.
>   * Always called with scheduler locked.
> diff --git a/sys/sys/proc.h b/sys/sys/proc.h
> index ff97bfbd54a9..0e33192303f4 100644
> --- a/sys/sys/proc.h
> +++ b/sys/sys/proc.h
> @@ -1009,6 +1009,14 @@ extern pid_t pid_max;
>  	(p)->p_cowgen++;						\
>  } while (0)
>  
> +#define	PROC_COW_CHANGECOUNT(td, p) ({					\
> +	struct thread *_td = (td);					\
> +	struct proc *_p = (p);						\
> +	MPASS(_td == curthread);					\
> +	PROC_LOCK_ASSERT(_p, MA_OWNED);					\
> +	_p->p_cowgen - _td->td_cowgen;					\
> +})
> +
>  /* Check whether a thread is safe to be swapped out. */
>  #define	thread_safetoswapout(td)	((td)->td_flags & TDF_CANSWAP)
>  
> @@ -1200,6 +1208,7 @@ void	thread_cow_get_proc(struct thread *newtd, struct proc *p);
>  void	thread_cow_get(struct thread *newtd, struct thread *td);
>  void	thread_cow_free(struct thread *td);
>  void	thread_cow_update(struct thread *td);
> +void	thread_cow_synced(struct thread *td);
>  int	thread_create(struct thread *td, struct rtprio *rtp,
>  	    int (*initialize_thread)(struct thread *, void *), void *thunk);
>  void	thread_exit(void) __dead2;