On Tue, Mar 01, 2022 at 05:41:55PM +0000, Mark Johnston wrote:
> The branch main has been updated by markj:
>
> URL:
> https://cgit.FreeBSD.org/src/commit/?id=12fb39ec3e6bc529feff3ba2862c6a4a30bd54eb
>
> commit 12fb39ec3e6bc529feff3ba2862c6a4a30bd54eb
> Author: Mark Johnston <[email protected]>
> AuthorDate: 2022-03-01 16:48:39 +0000
> Commit: Mark Johnston <[email protected]>
> CommitDate: 2022-03-01 17:40:35 +0000
>
> proc: Relax proc_rwmem()'s assertion on the process hold count
>
> This reference ensures that the process and its associated vmspace will
> not be destroyed while proc_rwmem() is executing. If, however, the
> calling thread belongs to the target process, then it is unnecessary to
> hold the process. In particular, fasttrap - a module which enables
> userspace dtrace - may frequently call proc_rwmem(), and we'd prefer to
> avoid the overhead of locking and bumping the hold count when possible.
In fact I am not sure it makes much sense to disable swap out for remote
process as well. With the current definition of p_hold, it only prevents
kstack pages reuse, which should be irrelevant for proc_rwmem().
>
> Thus, make the assertion conditional on "p != curproc". Also assert
> that the process is not already exiting. No functional change intended.
>
> MFC after: 2 weeks
> Sponsored by: The FreeBSD Foundation
> ---
> sys/kern/sys_process.c | 9 +++++----
> 1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/sys/kern/sys_process.c b/sys/kern/sys_process.c
> index 582bff962f1a..8d8c5a1d34ff 100644
> --- a/sys/kern/sys_process.c
> +++ b/sys/kern/sys_process.c
> @@ -336,11 +336,12 @@ proc_rwmem(struct proc *p, struct uio *uio)
> int error, fault_flags, page_offset, writing;
>
> /*
> - * Assert that someone has locked this vmspace. (Should be
> - * curthread but we can't assert that.) This keeps the process
> - * from exiting out from under us until this operation completes.
> + * Make sure that the process' vmspace remains live.
> */
> - PROC_ASSERT_HELD(p);
> + if (p != curproc)
> + PROC_ASSERT_HELD(p);
> + KASSERT((p->p_flag & P_WEXIT) == 0,
> + ("%s: process %p is exiting", __func__, p));
> PROC_LOCK_ASSERT(p, MA_NOTOWNED);
>
> /*
