[PATCH] staging: fbtft: make dirty_lock IRQ-safe

[sh_def@163.com]

From: Hui Su <sh_def@163.com>

fbtft_mkdirty() can be reached from the fbcon rendering path while
processing printk() in hardirq context. Meanwhile, dirty_lock is also
taken by fbtft_deferred_io() in workqueue context with local interrupts
enabled.

Lockdep reports a possible IRQ lock inversion involving dirty_lock and
console_owner. A hardirq can interrupt a CPU holding dirty_lock and
enter the console rendering path, which can attempt to acquire
dirty_lock again.

The following lockdep report was observed on an RK3566 system with
CONFIG_PROVE_LOCKING enabled:

  WARNING: possible irq lock inversion dependency detected
  swapper/2/0 just changed the state of lock:
  (console_owner){-...}-{0:0}
  but this lock took another, HARDIRQ-unsafe lock in the past:
  (&par->dirty_lock){+.+.}-{2:2}

  CPU0                    CPU1
  ----                    ----
  lock(&par->dirty_lock);
                         local_irq_disable();
                         lock(console_owner);
                         lock(&par->dirty_lock);
  <Interrupt>
    lock(console_owner);

  *** DEADLOCK ***

Use spin_lock_irqsave() for both dirty_lock critical sections. They
only access the dirty line range, so the IRQ-off regions remain short.

Fixes: c296d5f9957c ("staging: fbtft: core support")
Signed-off-by: Hui Su <sh_def@163.com>
---
 drivers/staging/fbtft/fbtft-core.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/drivers/staging/fbtft/fbtft-core.c b/drivers/staging/fbtft/fbtft-core.c
index ca0c38221c16..193643d0329d 100644
--- a/drivers/staging/fbtft/fbtft-core.c
+++ b/drivers/staging/fbtft/fbtft-core.c
@@ -298,14 +298,20 @@ static void fbtft_mkdirty(struct fb_info *info, int y, int height)
 {
 	struct fbtft_par *par = info->par;
 	struct fb_deferred_io *fbdefio = info->fbdefio;
+	unsigned long flags;
 
 	/* Mark display lines/area as dirty */
-	spin_lock(&par->dirty_lock);
+	/*
+	 * fbcon takes dirty_lock while holding console_owner. Disable local
+	 * interrupts here so a printk hardirq cannot acquire console_owner
+	 * while dirty_lock is held and create the inverse lock ordering.
+	 */
+	spin_lock_irqsave(&par->dirty_lock, flags);
 	if (y < par->dirty_lines_start)
 		par->dirty_lines_start = y;
 	if (y + height - 1 > par->dirty_lines_end)
 		par->dirty_lines_end = y + height - 1;
-	spin_unlock(&par->dirty_lock);
+	spin_unlock_irqrestore(&par->dirty_lock, flags);
 
 	/* Schedule deferred_io to update display (no-op if already on queue)*/
 	schedule_delayed_work(&info->deferred_work, fbdefio->delay);
@@ -317,14 +323,15 @@ static void fbtft_deferred_io(struct fb_info *info, struct list_head *pagereflis
 	unsigned int dirty_lines_start, dirty_lines_end;
 	struct fb_deferred_io_pageref *pageref;
 	unsigned int y_low = 0, y_high = 0;
+	unsigned long flags;
 
-	spin_lock(&par->dirty_lock);
+	spin_lock_irqsave(&par->dirty_lock, flags);
 	dirty_lines_start = par->dirty_lines_start;
 	dirty_lines_end = par->dirty_lines_end;
 	/* set display line markers as clean */
 	par->dirty_lines_start = par->info->var.yres - 1;
 	par->dirty_lines_end = 0;
-	spin_unlock(&par->dirty_lock);
+	spin_unlock_irqrestore(&par->dirty_lock, flags);
 
 	/* Mark display lines as dirty */
 	list_for_each_entry(pageref, pagereflist, list) {
-- 
2.43.0