linux.git/kernel/posix-cpu-timers.c, branch v3.14.3

posix-timers: Convert abuses of BUG_ON to WARN_ON

2013-12-09T15:56:29+00:00

The posix cpu timers code makes a heavy use of BUG_ON()
but none of these concern fatal issues that require
to stop the machine. So let's just warn the user when
some internal state slips out of our hands.

Signed-off-by: Frederic Weisbecker 
Cc: Thomas Gleixner 
Cc: Ingo Molnar 
Cc: Peter Zijlstra 
Cc: Oleg Nesterov 
Cc: Kosaki Motohiro 
Cc: Andrew Morton

posix-timers: Remove remaining uses of tasklist_lock

2013-12-09T15:56:28+00:00

The remaining uses of tasklist_lock were mostly about synchronizing
against sighand modifications, getting coherent and safe group samples
and also thread/process wide timers list handling.

All of this is already safely synchronizable with the target's
sighand lock. Let's use it on these places instead.

Also update the comments about locking.

Signed-off-by: Frederic Weisbecker 
Cc: Thomas Gleixner 
Cc: Ingo Molnar 
Cc: Peter Zijlstra 
Cc: Oleg Nesterov 
Cc: Kosaki Motohiro 
Cc: Andrew Morton

posix-timers: Use sighand lock instead of tasklist_lock on timer deletion

2013-12-09T15:53:51+00:00

Timer deletion doesn't need the tasklist lock.
We need to protect against:

* concurrent access to the lists p->cputime_expires and
  p->sighand->cputime_expires

* task reaping that may also delete the timer list entry

* timer firing

We already hold the timer lock which protects us against concurrent
timer firing.

The rest only need the targets sighand to be locked.
So hold it and drop the use of tasklist_lock there.

Signed-off-by: Frederic Weisbecker 
Cc: Thomas Gleixner 
Cc: Ingo Molnar 
Cc: Peter Zijlstra 
Cc: Oleg Nesterov 
Cc: Kosaki Motohiro 
Cc: Andrew Morton

posix-timers: Use sighand lock instead of tasklist_lock for task clock sample

2013-12-09T15:53:51+00:00

There is no need for the tasklist_lock just to take a process
wide clock sample.

All we need is to get a coherent sample that doesn't race with
exit() and exec():

* exit() may be concurrently reaping a task and flushing its time

* sighand is unstable under exit() and exec(), and the latter also
  result in group leader that can change

To protect against these, locking the target's sighand is enough.

Signed-off-by: Frederic Weisbecker 
Cc: Thomas Gleixner 
Cc: Ingo Molnar 
Cc: Peter Zijlstra 
Cc: Oleg Nesterov 
Cc: Kosaki Motohiro 
Cc: Andrew Morton

posix-timers: Consolidate posix_cpu_clock_get()

2013-12-09T15:53:51+00:00

Consolidate the clock sampling common code used for both local
and remote targets.

Note that this introduces a tiny user ABI change: if a
PID is passed to clock_gettime() along the clockid,
we used to forbid a process wide clock sample when that
PID doesn't belong to a group leader. Now after this patch
we allow process wide clock samples if that PID belongs to
the current task, even if the current task is not the
group leader.

But local process wide clock samples are allowed if PID == 0
(current task) even if the current task is not the group leader.
So in the end this should be no big deal as this actually harmonize
the behaviour when the remote sample is actually a local one.

Signed-off-by: Frederic Weisbecker 
Cc: Thomas Gleixner 
Cc: Ingo Molnar 
Cc: Peter Zijlstra 
Cc: Oleg Nesterov 
Cc: Kosaki Motohiro 
Cc: Andrew Morton

posix-timers: Remove useless clock sample on timers cleanup

2013-12-09T15:53:50+00:00

a0b2062b0904ef07944c4a6e4d0f88ee44f1e9f2
("posix_timers: fix racy timer delta caching on task exit") forgot
to remove the arguments used for timer caching.

Fix this leftover.

Signed-off-by: Frederic Weisbecker 
Cc: Thomas Gleixner 
Cc: Ingo Molnar 
Cc: Peter Zijlstra 
Cc: Oleg Nesterov 
Cc: Kosaki Motohiro 
Cc: Andrew Morton

posix-timers: Remove dead task special case

2013-12-09T15:53:50+00:00

Now that we've removed all the optimizations that could
result in NULL timer's targets, we can remove all the
associated special case handling.

Also add some warnings on NULL targets to spot any possible
leftover.

Signed-off-by: Frederic Weisbecker 
Cc: Thomas Gleixner 
Cc: Ingo Molnar 
Cc: Peter Zijlstra 
Cc: Oleg Nesterov 
Cc: Kosaki Motohiro 
Cc: Andrew Morton

posix-timers: Cleanup reaped target handling

2013-12-09T15:53:50+00:00

When a timer's target is seen to be buried, for example on calls
to timer_gettime(), the posix cpu timers code behaves a bit
like a garbage collector and releases early the reference to the
task.

Then again, this optimization complicates the code for no much
value: it's up to the user to release the timer and its associated
ressources by calling timer_delete() after it buries the target
tasks.

Remove this to simplify the code.

Signed-off-by: Frederic Weisbecker 
Cc: Thomas Gleixner 
Cc: Ingo Molnar 
Cc: Peter Zijlstra 
Cc: Oleg Nesterov 
Cc: Kosaki Motohiro 
Cc: Andrew Morton

posix-timers: Remove dead process posix cpu timers caching

2013-12-09T15:53:49+00:00

Now that we removed dead thread posix cpu timers caching,
lets remove the dead process wide version. This caching
is similar to the per thread version but it should be even
more rare:

* If the process id dead, we are not reading its timers
status from a thread belonging to its group since they
are all dead. So this caching only concern remote process
timers reads. Now posix cpu timers using itimers or timer_settime()
can't do remote process timers anyway so it's not even clear if there
is actually a user for this caching.

* Unlike per thread timers caching, this only applies to
zombies targets. Buried targets' process wide timers return
0 values. But then again, timer_gettime() can't read remote
process timers, so if the process is dead, there can't be
any reader left anyway.

Then again this caching seem to complicate the code for
corner cases that are probably not worth it. So lets get
rid of it.

Also remove the sample snapshot on dying process timer
that is now useless, as suggested by Kosaki.

Signed-off-by: Frederic Weisbecker 
Cc: Thomas Gleixner 
Cc: Ingo Molnar 
Cc: Peter Zijlstra 
Cc: Oleg Nesterov 
Cc: Kosaki Motohiro 
Cc: Andrew Morton

posix-timers: Remove dead thread posix cpu timers caching

2013-12-09T15:50:57+00:00

When a task is exiting or has exited, its posix cpu timers
don't tick anymore and won't elapse further. It's too late
for them to expire.

So any further call to timer_gettime() on these timers will
return the same remaining expiry time.

The current code optimize this by caching the remaining delta
and storing it where we use to save the absolute expiration time.
This way, the future calls to timer_gettime() won't need to
compute the difference between the absolute expiration time and
the current time anymore.

Now this optimization doesn't seem to bring much value. Computing
the timer remaining delta is not very costly. Fetching the timer
value OTOH can be costly in two ways:

* CPUCLOCK_SCHED read requires to lock the target's rq. But some
optimizations are on the way to make task_sched_runtime() not holding
the rq lock of a non-running target.

* CPUCLOCK_VIRT/CPUCLOCK_PROF read simply consist in fetching
current->utime/current->stime except when the system uses full
dynticks cputime accounting. The latter requires a per task lock
in order to correctly compute user and system time. But once the
target is dead, this lock shouldn't be contended anyway.

All in one this caching doesn't seem to be justified.
Given that it complicates the code significantly for
few wins, let's remove it on single thread timers.

Signed-off-by: Frederic Weisbecker 
Cc: Thomas Gleixner 
Cc: Ingo Molnar 
Cc: Peter Zijlstra 
Cc: Oleg Nesterov 
Cc: Kosaki Motohiro 
Cc: Andrew Morton