Messages in this thread Patch in this message | | | Date | Mon, 19 Aug 2019 16:31:48 +0200 | From | Thomas Gleixner <> | Subject | [patch 07/44] posix-cpu-timers: Simplify sighand locking in run_posix_cpu_timers() |
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run_posix_cpu_timers() is called from the timer interrupt. The posix timer expiry always affects the current task which got interrupted.
sighand locking is only racy when done on a foreign task, which must use lock_task_sighand(). But in case of run_posix_cpu_timers() that's pointless.
sighand of a task can only be dropped or changed by the task itself. Drop happens in do_exit(), changing sighand happens in execve().
So the timer interrupt can see one of the following states of the current task which it interrupted:
- executing: sighand is set - exiting: sighand is either set or NULL - execing: sighand is either the original one or the new one
The 'check -> lock -> check again' logic in lock_task_sighand() is not required here at all. The state cannot change as the interrupted task context obviously is not executing instructions.
So the only interesting case here is to check whether current->sighand is NULL or not.
Add the check for sighand == NULL right at the beginning and replace lock_task_sighand() with a regular spin_lock() invocation along with proper comments why this is safe.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de> --- kernel/time/posix-cpu-timers.c | 26 ++++++++++++++++++++------ 1 file changed, 20 insertions(+), 6 deletions(-)
--- a/kernel/time/posix-cpu-timers.c +++ b/kernel/time/posix-cpu-timers.c @@ -1141,20 +1141,34 @@ void run_posix_cpu_timers(void) { struct task_struct *tsk = current; struct k_itimer *timer, *next; - unsigned long flags; LIST_HEAD(firing); lockdep_assert_irqs_disabled(); /* - * The fast path checks that there are no expired thread or thread - * group timers. If that's so, just return. + * Verify that sighand exits. If not, the task is exiting and has + * dropped sighand already. Without sighand, timer expiry is + * pointless. */ - if (!fastpath_timer_check(tsk)) + if (!tsk->sighand) return; - if (!lock_task_sighand(tsk, &flags)) + /* + * Now do a fast check for expired task and group timers. If + * no expired timers found, return. + */ + if(!fastpath_timer_check(tsk)) return; + + /* + * The timer interrupt hit current. It's verified that sighand + * exists, so it cannot go away before the timer interrupt returns + * as only current can remove or change its own sighand in exit() + * or exec() with sighand lock held and interrupts disabled. Ergo + * the interrupt can only observe a valid sighand or NULL. + */ + spin_lock(&tsk->sighand->siglock); + /* * Here we take off tsk->signal->cpu_timers[N] and * tsk->cpu_timers[N] all the timers that are firing, and @@ -1172,7 +1186,7 @@ void run_posix_cpu_timers(void) * that gets the timer lock before we do will give it up and * spin until we've taken care of that timer below. */ - unlock_task_sighand(tsk, &flags); + spin_unlock(&tsk->sighand->siglock); /* * Now that all the timers on our list have the firing flag,
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