Re: [PATCH v2] video: fbdev: pm2fb: avoid stall on fb_sync

From: Randy Dunlap
Date: Sat Feb 20 2021 - 19:45:46 EST

On 2/20/21 4:00 PM, Tong Zhang wrote:
> pm2fb_sync is called when doing /dev/fb read or write.
> The original pm2fb_sync wait indefinitely on hardware flags which can
> possibly stall kernel and make everything unresponsive.
> Instead of waiting indefinitely, we can timeout to give user a chance to
> get back control.
>
> [ 39.503356] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.13.0-48-gd9c812dda5194
> [ 39.503947] Call Trace:
> [ 39.504081] <IRQ>
> [ 39.504193] dump_stack+0x7d/0xa3
> [ 39.504377] nmi_cpu_backtrace.cold+0x32/0x7e
> [ 39.504613] ? lapic_can_unplug_cpu+0x70/0x70
> [ 39.504850] nmi_trigger_cpumask_backtrace+0xdf/0x100
> [ 39.505121] rcu_dump_cpu_stacks+0xed/0x130
> [ 39.505349] rcu_sched_clock_irq.cold+0x3b1/0x61d
> [ 39.505602] ? hrtimer_run_queues+0x2c/0x1b0
> [ 39.505833] ? __acct_update_integrals+0x136/0x160
> [ 39.506091] update_process_times+0xb9/0xf0
> [ 39.506317] tick_sched_handle.isra.0+0x5c/0x80
> [ 39.506562] tick_sched_timer+0x70/0x90
> [ 39.506770] __hrtimer_run_queues+0x1c6/0x3e0
> [ 39.517095] ? tick_sched_handle.isra.0+0x80/0x80
> [ 39.517349] ? enqueue_hrtimer+0xd0/0xd0
> [ 39.517561] ? _raw_write_lock_irqsave+0xd0/0xd0
> [ 39.517812] ? ktime_get+0x45/0xb0
> [ 39.517997] ? ktime_get_update_offsets_now+0x96/0x150
> [ 39.518273] hrtimer_interrupt+0x1a0/0x340
> [ 39.518496] __sysvec_apic_timer_interrupt+0x7f/0x160
> [ 39.518768] asm_call_irq_on_stack+0xf/0x20
> [ 39.518997] </IRQ>
> [ 39.519114] sysvec_apic_timer_interrupt+0x6f/0x80
> [ 39.519372] asm_sysvec_apic_timer_interrupt+0x12/0x20
> [ 39.519647] RIP: 0010:pm2fb_sync+0x47/0x70 [pm2fb]
> [ 39.519907] Code: 89 ef e8 0c 87 2c c1 48 8b 53 08 31 c0 89 82 40 8c 00 00 0f ae f0 48 8b 53 08 1
> [ 39.520885] RSP: 0018:ffff88810a1f7df8 EFLAGS: 00000202
> [ 39.521165] RAX: 0000000072d5d49f RBX: ffff88810a034418 RCX: ffffc900000b0020
> [ 39.521542] RDX: ffffc900000b0000 RSI: 0000000000000246 RDI: ffff88810a034420
> [ 39.521920] RBP: ffff88810a034420 R08: 0000000000000000 R09: ffffed102143ef64
> [ 39.522297] R10: 0000000000000003 R11: ffffed102143ef63 R12: ffff88810a1f7ed0
> [ 39.522673] R13: ffff88810a034000 R14: ffffc90002800000 R15: ffff888109e5e000
> [ 39.523053] ? pm2fb_sync+0x24/0x70 [pm2fb]
> [ 39.523280] fb_write+0x1c2/0x2d0
> [ 39.523461] vfs_write+0x108/0x380
> [ 39.523647] ksys_write+0xb4/0x150
> [ 39.523832] ? __ia32_sys_read+0x40/0x40
> [ 39.524043] ? fpregs_assert_state_consistent+0x4d/0x60
> [ 39.524322] do_syscall_64+0x33/0x40
> [ 39.524517] entry_SYSCALL_64_after_hwframe+0x44/0xa9
> [ 39.524788] RIP: 0033:0x7f5e50097970
> [ 39.524981] Code: 73 01 c3 48 8b 0d 28 d5 2b 00 f7 d8 64 89 01 48 83 c8 ff c3 66 0f 1f 44 00 00 4
> [ 39.525952] RSP: 002b:00007ffec6895b38 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
> [ 39.526352] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f5e50097970
> [ 39.526726] RDX: 0000000000000200 RSI: 000056491a6ad000 RDI: 0000000000000001
> [ 39.537261] RBP: 0000000000000200 R08: 0000000000000000 R09: 000056491a6ad030
> [ 39.537633] R10: 0000000000000871 R11: 0000000000000246 R12: 0000000000000800
> [ 39.538008] R13: 000056491a6ad000 R14: 0000000000000200 R15: 0000000000000000
>
> Signed-off-by: Tong Zhang <ztong0001@xxxxxxxxx>
> ---
> v2: fix typo and add console log according to Randy's <rdunlap@xxxxxxxxxxxxx> comment
>
> drivers/video/fbdev/pm2fb.c | 29 ++++++++++++++++++++++++++---
> 1 file changed, 26 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/video/fbdev/pm2fb.c b/drivers/video/fbdev/pm2fb.c
> index 27893fa139b0..d6731e04252f 100644
> --- a/drivers/video/fbdev/pm2fb.c
> +++ b/drivers/video/fbdev/pm2fb.c
> @@ -183,12 +183,23 @@ static inline void pm2v_RDAC_WR(struct pm2fb_par *p, s32 idx, u32 v)
>
> #ifdef CONFIG_FB_PM2_FIFO_DISCONNECT
> #define WAIT_FIFO(p, a)
> +#define WAIT_FIFO_TIMEOUT(p, a) (0)
> #else
> static inline void WAIT_FIFO(struct pm2fb_par *p, u32 a)
> {
> while (pm2_RD(p, PM2R_IN_FIFO_SPACE) < a)
> cpu_relax();
> }

/* return 1 for timeout, otherwise 0 */
> +static inline int WAIT_FIFO_TIMEOUT(struct pm2fb_par *p, u32 a)
> +{
> + int timeout = 10000;
> + while (pm2_RD(p, PM2R_IN_FIFO_SPACE) < a) {
> + cpu_relax();
> + if (--timeout == 0)
> + return 1;
> + }
> + return 0;
> +}
> #endif
>
> /*
> @@ -1031,15 +1042,27 @@ static int pm2fb_blank(int blank_mode, struct fb_info *info)
> static int pm2fb_sync(struct fb_info *info)
> {
> struct pm2fb_par *par = info->par;
> + int timeout_sync = 10000;
> + int timeout_fifo;
>
> - WAIT_FIFO(par, 1);
> + if (WAIT_FIFO_TIMEOUT(par, 1))
> + goto end;

if the above goto happens, timeout_fifo is used but not initialized
at label end:

> pm2_WR(par, PM2R_SYNC, 0);
> mb();
> do {
> - while (pm2_RD(par, PM2R_OUT_FIFO_WORDS) == 0)
> + timeout_fifo = 10000;
> + while (pm2_RD(par, PM2R_OUT_FIFO_WORDS) == 0) {
> cpu_relax();
> - } while (pm2_RD(par, PM2R_OUT_FIFO) != PM2TAG(PM2R_SYNC));
> + if (--timeout_fifo == 0)
> + goto end;
> + }
> + if (pm2_RD(par, PM2R_OUT_FIFO) == PM2TAG(PM2R_SYNC))
> + break;
> + } while (--timeout_sync > 0);
>
> +end:
> + if ((!timeout_sync) || (!timeout_fifo))
> + printk_ratelimited(KERN_WARNING "pm2fb: sync timeout!\n");
> return 0;
> }
>
>

I suppose the rest of the patch is OK since it works for you.

I don't know any of this code. It would be nice to know what some
of those pm2_RD(par, SOME_REG) mean so that I could sort of
understand what it is doing, but don't go spending time on it
just for me. It's not worth doing that IMO.


--
~Randy