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 <[email protected]>
> AuthorDate: 2022-02-01 13:13:13 +0000
> Commit: Mateusz Guzik <[email protected]>
> 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;
