Re: git: 12fb39ec3e6b - main - proc: Relax proc_rwmem()'s assertion on the process hold count

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 <markj@FreeBSD.org>
> AuthorDate: 2022-03-01 16:48:39 +0000
> Commit:     Mark Johnston <markj@FreeBSD.org>
> 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);
>  
>  	/*