linux-kernel - Re: [PATCH RT] kernel/hrtimer: be non-freezeable in cpu

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Message-ID: <20140219102700.04de1195@gandalf.local.home>
Date:	Wed, 19 Feb 2014 10:27:00 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Clark Williams <williams@...hat.com>,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
	John Kacur <jkacur@...hat.com>,
	Ulrich Obergfell <uobergfe@...hat.com>
Subject: Re: [PATCH RT] kernel/hrtimer: be non-freezeable in cpu_chill()

On Wed, 19 Feb 2014 12:53:53 +0100
Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:

> Since we replaced msleep() by hrtimer I see now and then (rarely) this:
> 
> | [....] Waiting for /dev to be fully populated...
> | =====================================
> | [ BUG: udevd/229 still has locks held! ]
> | 3.12.11-rt17 #23 Not tainted
> | -------------------------------------
> | 1 lock held by udevd/229:
> |  #0:  (&type->i_mutex_dir_key#2){+.+.+.}, at: lookup_slow+0x28/0x98
> |
> | stack backtrace:
> | CPU: 0 PID: 229 Comm: udevd Not tainted 3.12.11-rt17 #23
> | (unwind_backtrace+0x0/0xf8) from (show_stack+0x10/0x14)
> | (show_stack+0x10/0x14) from (dump_stack+0x74/0xbc)
> | (dump_stack+0x74/0xbc) from (do_nanosleep+0x120/0x160)
> | (do_nanosleep+0x120/0x160) from (hrtimer_nanosleep+0x90/0x110)
> | (hrtimer_nanosleep+0x90/0x110) from (cpu_chill+0x30/0x38)
> | (cpu_chill+0x30/0x38) from (dentry_kill+0x158/0x1ec)
> | (dentry_kill+0x158/0x1ec) from (dput+0x74/0x15c)
> | (dput+0x74/0x15c) from (lookup_real+0x4c/0x50)
> | (lookup_real+0x4c/0x50) from (__lookup_hash+0x34/0x44)
> | (__lookup_hash+0x34/0x44) from (lookup_slow+0x38/0x98)
> | (lookup_slow+0x38/0x98) from (path_lookupat+0x208/0x7fc)
> | (path_lookupat+0x208/0x7fc) from (filename_lookup+0x20/0x60)
> | (filename_lookup+0x20/0x60) from (user_path_at_empty+0x50/0x7c)
> | (user_path_at_empty+0x50/0x7c) from (user_path_at+0x14/0x1c)
> | (user_path_at+0x14/0x1c) from (vfs_fstatat+0x48/0x94)
> | (vfs_fstatat+0x48/0x94) from (SyS_stat64+0x14/0x30)
> | (SyS_stat64+0x14/0x30) from (ret_fast_syscall+0x0/0x48)
> 
> For now I see no better way but to disable the freezer for the sleep period.

Hmm, I wonder if this is because msleep() sets the tasks state to
TASK_UNINTERRUPTIBLE, and hrtimer_nanosleep sets it to
TASK_INTERRUPTIBLE.

> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  kernel/hrtimer.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
> index 5c26d2c..083815d 100644
> --- a/kernel/hrtimer.c
> +++ b/kernel/hrtimer.c
> @@ -1899,8 +1899,12 @@ void cpu_chill(void)
>  	struct timespec tu = {
>  		.tv_nsec = NSEC_PER_MSEC,
>  	};
> +	unsigned int freeze_flag = current->flags & PF_NOFREEZE;
>  
> +	current->flags |= PF_NOFREEZE;
>  	hrtimer_nanosleep(&tu, NULL, HRTIMER_MODE_REL, CLOCK_MONOTONIC);
> +	if (!freeze_flag)
> +		current->flags &= ~PF_NOFREEZE;

We could remove the branch with:

	current->flags &= ~PF_NOFREEZE | freeze_flag;

But as we just slept for a millisecond, that optimization may not be
worth anything ;-)

Acked-by: Steven Rostedt <rostedt@...dmis.org>

-- Steve

>  }
>  EXPORT_SYMBOL(cpu_chill);
>  #endif

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