svn commit: r367713 - head/sys/kern

[Konstantin Belousov]

On Mon, Nov 16, 2020 at 03:09:19AM +0000, Mateusz Guzik wrote:
> Author: mjg
> Date: Mon Nov 16 03:09:18 2020
> New Revision: 367713
> URL: https://svnweb.freebsd.org/changeset/base/367713
> 
> Log:
>   select: replace reference counting with memory barriers in selfd
>   
>   Refcounting was added to combat a race between selfdfree and doselwakup,
>   but it adds avoidable overhead.
>   
>   selfdfree detects it can free the object by ->sf_si == NULL, thus we can
>   ensure that the condition only holds after all accesses are completed.
> 
> Modified:
>   head/sys/kern/sys_generic.c
> 
> Modified: head/sys/kern/sys_generic.c
> ==============================================================================
> --- head/sys/kern/sys_generic.c	Sun Nov 15 22:49:28 2020	(r367712)
> +++ head/sys/kern/sys_generic.c	Mon Nov 16 03:09:18 2020	(r367713)
> @@ -156,7 +156,6 @@ struct selfd {
>  	struct mtx		*sf_mtx;	/* Pointer to selinfo mtx. */
>  	struct seltd		*sf_td;		/* (k) owning seltd. */
>  	void			*sf_cookie;	/* (k) fd or pollfd. */
> -	u_int			sf_refs;
>  };
>  
>  MALLOC_DEFINE(M_SELFD, "selfd", "selfd");
> @@ -1704,16 +1703,17 @@ static void
>  selfdfree(struct seltd *stp, struct selfd *sfp)
>  {
>  	STAILQ_REMOVE(&stp->st_selq, sfp, selfd, sf_link);
> -	if (sfp->sf_si != NULL) {
> +	/*
> +	 * Paired with doselwakeup.
> +	 */
> +	if (atomic_load_acq_ptr((uintptr_t *)&sfp->sf_si) != (uintptr_t)NULL) {
This could be != 0.

>  		mtx_lock(sfp->sf_mtx);
>  		if (sfp->sf_si != NULL) {
>  			TAILQ_REMOVE(&sfp->sf_si->si_tdlist, sfp, sf_threads);
> -			refcount_release(&sfp->sf_refs);
>  		}
>  		mtx_unlock(sfp->sf_mtx);
>  	}
> -	if (refcount_release(&sfp->sf_refs))
> -		free(sfp, M_SELFD);
> +	free(sfp, M_SELFD);
What guarantees that doselwakeup() finished with sfp ?

>  }
>  
>  /* Drain the waiters tied to all the selfd belonging the specified selinfo. */
> @@ -1766,7 +1766,6 @@ selrecord(struct thread *selector, struct selinfo *sip
>  	 */
>  	sfp->sf_si = sip;
>  	sfp->sf_mtx = mtxp;
> -	refcount_init(&sfp->sf_refs, 2);
>  	STAILQ_INSERT_TAIL(&stp->st_selq, sfp, sf_link);
>  	/*
>  	 * Now that we've locked the sip, check for initialization.
> @@ -1820,14 +1819,15 @@ doselwakeup(struct selinfo *sip, int pri)
>  		 * sf_si seltdclear will know to ignore this si.
>  		 */
>  		TAILQ_REMOVE(&sip->si_tdlist, sfp, sf_threads);
> -		sfp->sf_si = NULL;
>  		stp = sfp->sf_td;
> +		/*
> +		 * Paired with selfdfree.
> +		 */
> +		atomic_store_rel_ptr((uintptr_t *)&sfp->sf_si, (uintptr_t)NULL);
>  		mtx_lock(&stp->st_mtx);
>  		stp->st_flags |= SELTD_PENDING;
>  		cv_broadcastpri(&stp->st_wait, pri);
>  		mtx_unlock(&stp->st_mtx);
> -		if (refcount_release(&sfp->sf_refs))
> -			free(sfp, M_SELFD);
>  	}
>  	mtx_unlock(sip->si_mtx);
>  }