Date:   Fri, 4 Feb 2022 21:15:40 +0100
From:   Sam Ravnborg <sam@...nborg.org>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     DRI Development <dri-devel@...ts.freedesktop.org>,
        linux-fbdev@...r.kernel.org, Du Cheng <ducheng2@...il.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Claudio Suarez <cssk@...-c.es>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH 19/21] fbcon: Maintain a private array of fb_info

Hi Daniel,

On Mon, Jan 31, 2022 at 10:05:50PM +0100, Daniel Vetter wrote:
> Accessing the one in fbmem.c without taking the right locks is a bad
> idea. Instead maintain our own private copy, which is fully protected
> by console_lock() (like everything else in fbcon.c). That copy is
> serialized through fbcon_fb_registered/unregistered() calls.

I fail to see why we can make a private copy of registered_fb
just like that - are they not somehow shared between fbcon and fbmem.
So when fbmem updates it, then fbcon will use the entry or such?

I guess I am just ignorant of how registered_fb is used - but please
explain.

	Sam

> 
> Also this means we do not need to hold a full fb_info reference, which
> is nice because doing so would mean a refcount loop between the
> console and the fb_info. But it's also not nice since it means
> console_lock() must be held absolutely everywhere. Well strictly
> speaking we could still try to do some refcounting games again by
> calling get_fb_info before we drop the console_lock. But things will
> get tricky.
> 
> Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
> Cc: Daniel Vetter <daniel@...ll.ch>
> Cc: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> Cc: Claudio Suarez <cssk@...-c.es>
> Cc: Du Cheng <ducheng2@...il.com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> ---
>  drivers/video/fbdev/core/fbcon.c | 82 +++++++++++++++++---------------
>  1 file changed, 43 insertions(+), 39 deletions(-)
> 
> diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
> index 22581952b4fd..a0ca34b29615 100644
> --- a/drivers/video/fbdev/core/fbcon.c
> +++ b/drivers/video/fbdev/core/fbcon.c
> @@ -86,10 +86,6 @@
>   * - fbcon state itself is protected by the console_lock, and the code does a
>   *   pretty good job at making sure that lock is held everywhere it's needed.
>   *
> - * - access to the registered_fb array is entirely unprotected. This should use
> - *   proper object lifetime handling, i.e. get/put_fb_info. This also means
> - *   switching from indices to proper pointers for fb_info everywhere.
> - *
>   * - fbcon doesn't bother with fb_lock/unlock at all. This is buggy, since it
>   *   means concurrent access to the same fbdev from both fbcon and userspace
>   *   will blow up. To fix this all fbcon calls from fbmem.c need to be moved out
> @@ -107,6 +103,13 @@ enum {
>  
>  static struct fbcon_display fb_display[MAX_NR_CONSOLES];
>  
> +struct fb_info *fbcon_registered_fb[FB_MAX];
> +int fbcon_num_registered_fb;
> +
> +#define fbcon_for_each_registered_fb(i)		\
> +	for (i = 0; WARN_CONSOLE_UNLOCKED(), i < FB_MAX; i++)		\
> +		if (!fbcon_registered_fb[i]) {} else
> +
>  static signed char con2fb_map[MAX_NR_CONSOLES];
>  static signed char con2fb_map_boot[MAX_NR_CONSOLES];
>  
> @@ -114,12 +117,7 @@ static struct fb_info *fbcon_info_from_console(int console)
>  {
>  	WARN_CONSOLE_UNLOCKED();
>  
> -	/*
> -	 * Note that only con2fb_map is protected by the console lock,
> -	 * registered_fb is protected by a separate mutex. This lookup can
> -	 * therefore race.
> -	 */
> -	return registered_fb[con2fb_map[console]];
> +	return fbcon_registered_fb[con2fb_map[console]];
>  }
>  
>  static int logo_lines;
> @@ -516,7 +514,7 @@ static int do_fbcon_takeover(int show_logo)
>  {
>  	int err, i;
>  
> -	if (!num_registered_fb)
> +	if (!fbcon_num_registered_fb)
>  		return -ENODEV;
>  
>  	if (!show_logo)
> @@ -822,7 +820,7 @@ static int set_con2fb_map(int unit, int newidx, int user)
>  {
>  	struct vc_data *vc = vc_cons[unit].d;
>  	int oldidx = con2fb_map[unit];
> -	struct fb_info *info = registered_fb[newidx];
> +	struct fb_info *info = fbcon_registered_fb[newidx];
>  	struct fb_info *oldinfo = NULL;
>  	int found, err = 0, show_logo;
>  
> @@ -840,7 +838,7 @@ static int set_con2fb_map(int unit, int newidx, int user)
>  	}
>  
>  	if (oldidx != -1)
> -		oldinfo = registered_fb[oldidx];
> +		oldinfo = fbcon_registered_fb[oldidx];
>  
>  	found = search_fb_in_map(newidx);
>  
> @@ -932,13 +930,13 @@ static const char *fbcon_startup(void)
>  	 *  If num_registered_fb is zero, this is a call for the dummy part.
>  	 *  The frame buffer devices weren't initialized yet.
>  	 */
> -	if (!num_registered_fb || info_idx == -1)
> +	if (!fbcon_num_registered_fb || info_idx == -1)
>  		return display_desc;
>  	/*
>  	 * Instead of blindly using registered_fb[0], we use info_idx, set by
>  	 * fbcon_fb_registered();
>  	 */
> -	info = registered_fb[info_idx];
> +	info = fbcon_registered_fb[info_idx];
>  	if (!info)
>  		return NULL;
>  	
> @@ -1153,9 +1151,9 @@ static void fbcon_release_all(void)
>  	struct fb_info *info;
>  	int i, j, mapped;
>  
> -	for_each_registered_fb(i) {
> +	fbcon_for_each_registered_fb(i) {
>  		mapped = 0;
> -		info = registered_fb[i];
> +		info = fbcon_registered_fb[i];
>  
>  		for (j = first_fb_vc; j <= last_fb_vc; j++) {
>  			if (con2fb_map[j] == i) {
> @@ -1182,7 +1180,7 @@ static void fbcon_deinit(struct vc_data *vc)
>  	if (idx == -1)
>  		goto finished;
>  
> -	info = registered_fb[idx];
> +	info = fbcon_registered_fb[idx];
>  
>  	if (!info)
>  		goto finished;
> @@ -2118,9 +2116,9 @@ static int fbcon_switch(struct vc_data *vc)
>  	 *
>  	 * info->currcon = vc->vc_num;
>  	 */
> -	for_each_registered_fb(i) {
> -		if (registered_fb[i]->fbcon_par) {
> -			struct fbcon_ops *o = registered_fb[i]->fbcon_par;
> +	fbcon_for_each_registered_fb(i) {
> +		if (fbcon_registered_fb[i]->fbcon_par) {
> +			struct fbcon_ops *o = fbcon_registered_fb[i]->fbcon_par;
>  
>  			o->currcon = vc->vc_num;
>  		}
> @@ -2765,7 +2763,7 @@ int fbcon_mode_deleted(struct fb_info *info,
>  		j = con2fb_map[i];
>  		if (j == -1)
>  			continue;
> -		fb_info = registered_fb[j];
> +		fb_info = fbcon_registered_fb[j];
>  		if (fb_info != info)
>  			continue;
>  		p = &fb_display[i];
> @@ -2821,7 +2819,7 @@ void fbcon_fb_unbind(struct fb_info *info)
>  				set_con2fb_map(i, new_idx, 0);
>  		}
>  	} else {
> -		struct fb_info *info = registered_fb[idx];
> +		struct fb_info *info = fbcon_registered_fb[idx];
>  
>  		/* This is sort of like set_con2fb_map, except it maps
>  		 * the consoles to no device and then releases the
> @@ -2851,6 +2849,9 @@ void fbcon_fb_unregistered(struct fb_info *info)
>  
>  	console_lock();
>  
> +	fbcon_registered_fb[info->node] = NULL;
> +	fbcon_num_registered_fb--;
> +
>  	if (deferred_takeover) {
>  		console_unlock();
>  		return;
> @@ -2865,7 +2866,7 @@ void fbcon_fb_unregistered(struct fb_info *info)
>  	if (idx == info_idx) {
>  		info_idx = -1;
>  
> -		for_each_registered_fb(i) {
> +		fbcon_for_each_registered_fb(i) {
>  			info_idx = i;
>  			break;
>  		}
> @@ -2881,7 +2882,7 @@ void fbcon_fb_unregistered(struct fb_info *info)
>  	if (primary_device == idx)
>  		primary_device = -1;
>  
> -	if (!num_registered_fb)
> +	if (!fbcon_num_registered_fb)
>  		do_unregister_con_driver(&fb_con);
>  	console_unlock();
>  }
> @@ -2956,6 +2957,9 @@ int fbcon_fb_registered(struct fb_info *info)
>  	else
>  		atomic_inc(&ignore_console_lock_warning);
>  
> +	fbcon_registered_fb[info->node] = info;
> +	fbcon_num_registered_fb++;
> +
>  	idx = info->node;
>  	fbcon_select_primary(info);
>  
> @@ -3075,9 +3079,9 @@ int fbcon_set_con2fb_map_ioctl(void __user *argp)
>  		return -EINVAL;
>  	if (con2fb.framebuffer >= FB_MAX)
>  		return -EINVAL;
> -	if (!registered_fb[con2fb.framebuffer])
> +	if (!fbcon_registered_fb[con2fb.framebuffer])
>  		request_module("fb%d", con2fb.framebuffer);
> -	if (!registered_fb[con2fb.framebuffer]) {
> +	if (!fbcon_registered_fb[con2fb.framebuffer]) {
>  		return -EINVAL;
>  	}
>  
> @@ -3144,10 +3148,10 @@ static ssize_t store_rotate(struct device *device,
>  	console_lock();
>  	idx = con2fb_map[fg_console];
>  
> -	if (idx == -1 || registered_fb[idx] == NULL)
> +	if (idx == -1 || fbcon_registered_fb[idx] == NULL)
>  		goto err;
>  
> -	info = registered_fb[idx];
> +	info = fbcon_registered_fb[idx];
>  	rotate = simple_strtoul(buf, last, 0);
>  	fbcon_rotate(info, rotate);
>  err:
> @@ -3166,10 +3170,10 @@ static ssize_t store_rotate_all(struct device *device,
>  	console_lock();
>  	idx = con2fb_map[fg_console];
>  
> -	if (idx == -1 || registered_fb[idx] == NULL)
> +	if (idx == -1 || fbcon_registered_fb[idx] == NULL)
>  		goto err;
>  
> -	info = registered_fb[idx];
> +	info = fbcon_registered_fb[idx];
>  	rotate = simple_strtoul(buf, last, 0);
>  	fbcon_rotate_all(info, rotate);
>  err:
> @@ -3186,10 +3190,10 @@ static ssize_t show_rotate(struct device *device,
>  	console_lock();
>  	idx = con2fb_map[fg_console];
>  
> -	if (idx == -1 || registered_fb[idx] == NULL)
> +	if (idx == -1 || fbcon_registered_fb[idx] == NULL)
>  		goto err;
>  
> -	info = registered_fb[idx];
> +	info = fbcon_registered_fb[idx];
>  	rotate = fbcon_get_rotate(info);
>  err:
>  	console_unlock();
> @@ -3206,10 +3210,10 @@ static ssize_t show_cursor_blink(struct device *device,
>  	console_lock();
>  	idx = con2fb_map[fg_console];
>  
> -	if (idx == -1 || registered_fb[idx] == NULL)
> +	if (idx == -1 || fbcon_registered_fb[idx] == NULL)
>  		goto err;
>  
> -	info = registered_fb[idx];
> +	info = fbcon_registered_fb[idx];
>  	ops = info->fbcon_par;
>  
>  	if (!ops)
> @@ -3232,10 +3236,10 @@ static ssize_t store_cursor_blink(struct device *device,
>  	console_lock();
>  	idx = con2fb_map[fg_console];
>  
> -	if (idx == -1 || registered_fb[idx] == NULL)
> +	if (idx == -1 || fbcon_registered_fb[idx] == NULL)
>  		goto err;
>  
> -	info = registered_fb[idx];
> +	info = fbcon_registered_fb[idx];
>  
>  	if (!info->fbcon_par)
>  		goto err;
> @@ -3295,8 +3299,8 @@ static void fbcon_register_existing_fbs(struct work_struct *work)
>  	deferred_takeover = false;
>  	logo_shown = FBCON_LOGO_DONTSHOW;
>  
> -	for_each_registered_fb(i)
> -		fbcon_fb_registered(registered_fb[i]);
> +	fbcon_for_each_registered_fb(i)
> +		fbcon_fb_registered(fbcon_registered_fb[i]);
>  
>  	console_unlock();
>  }
> -- 
> 2.33.0

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