From fccac47c4972dfaef50b0e080c60879a9d97bd25 Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Wed, 7 Mar 2012 20:51:03 +0100 Subject: [PATCH 138/191] rt: Introduce cpu_chill() Retry loops on RT might loop forever when the modifying side was preempted. Add cpu_chill() to replace cpu_relax(). cpu_chill() defaults to cpu_relax() for non RT. On RT it puts the looping task to sleep for a tick so the preempted task can make progress. Steven Rostedt changed it to use a hrtimer instead of msleep(): | |Ulrich Obergfell pointed out that cpu_chill() calls msleep() which is woken |up by the ksoftirqd running the TIMER softirq. But as the cpu_chill() is |called from softirq context, it may block the ksoftirqd() from running, in |which case, it may never wake up the msleep() causing the deadlock. + bigeasy later changed to schedule_hrtimeout() |If a task calls cpu_chill() and gets woken up by a regular or spurious |wakeup and has a signal pending, then it exits the sleep loop in |do_nanosleep() and sets up the restart block. If restart->nanosleep.type is |not TI_NONE then this results in accessing a stale user pointer from a |previously interrupted syscall and a copy to user based on the stale |pointer or a BUG() when 'type' is not supported in nanosleep_copyout(). + bigeasy: add PF_NOFREEZE: | [....] 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) Signed-off-by: Thomas Gleixner Signed-off-by: Steven Rostedt Signed-off-by: Sebastian Andrzej Siewior --- include/linux/hrtimer.h | 6 ++++++ kernel/time/hrtimer.c | 30 ++++++++++++++++++++++++++++++ 2 files changed, 36 insertions(+) diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h index bb5e7b0a4274..e425a26a5ed8 100644 --- a/include/linux/hrtimer.h +++ b/include/linux/hrtimer.h @@ -540,4 +540,10 @@ int hrtimers_dead_cpu(unsigned int cpu); #define hrtimers_dead_cpu NULL #endif +#ifdef CONFIG_PREEMPT_RT +extern void cpu_chill(void); +#else +# define cpu_chill() cpu_relax() +#endif + #endif diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c index 788b9d137de4..3fa18a01f5b2 100644 --- a/kernel/time/hrtimer.c +++ b/kernel/time/hrtimer.c @@ -2006,6 +2006,36 @@ SYSCALL_DEFINE2(nanosleep_time32, struct old_timespec32 __user *, rqtp, } #endif +#ifdef CONFIG_PREEMPT_RT +/* + * Sleep for 1 ms in hope whoever holds what we want will let it go. + */ +void cpu_chill(void) +{ + unsigned int freeze_flag = current->flags & PF_NOFREEZE; + struct task_struct *self = current; + ktime_t chill_time; + + raw_spin_lock_irq(&self->pi_lock); + self->saved_state = self->state; + __set_current_state_no_track(TASK_UNINTERRUPTIBLE); + raw_spin_unlock_irq(&self->pi_lock); + + chill_time = ktime_set(0, NSEC_PER_MSEC); + + current->flags |= PF_NOFREEZE; + schedule_hrtimeout(&chill_time, HRTIMER_MODE_REL_HARD); + if (!freeze_flag) + current->flags &= ~PF_NOFREEZE; + + raw_spin_lock_irq(&self->pi_lock); + __set_current_state_no_track(self->saved_state); + self->saved_state = TASK_RUNNING; + raw_spin_unlock_irq(&self->pi_lock); +} +EXPORT_SYMBOL(cpu_chill); +#endif + /* * Functions related to boot-time initialization: */ -- 2.19.1