diff options
| -rw-r--r-- | kernel/sys.c | 42 |
1 files changed, 34 insertions, 8 deletions
diff --git a/kernel/sys.c b/kernel/sys.c index f91218a5463..4941b9b14b9 100644 --- a/kernel/sys.c +++ b/kernel/sys.c @@ -1677,9 +1677,6 @@ asmlinkage long sys_setrlimit(unsigned int resource, struct rlimit __user *rlim) * a lot simpler! (Which we're not doing right now because we're not * measuring them yet). * - * This expects to be called with tasklist_lock read-locked or better, - * and the siglock not locked. It may momentarily take the siglock. - * * When sampling multiple threads for RUSAGE_SELF, under SMP we might have * races with threads incrementing their own counters. But since word * reads are atomic, we either get new values or old values and we don't @@ -1687,6 +1684,25 @@ asmlinkage long sys_setrlimit(unsigned int resource, struct rlimit __user *rlim) * the c* fields from p->signal from races with exit.c updating those * fields when reaping, so a sample either gets all the additions of a * given child after it's reaped, or none so this sample is before reaping. + * + * tasklist_lock locking optimisation: + * If we are current and single threaded, we do not need to take the tasklist + * lock or the siglock. No one else can take our signal_struct away, + * no one else can reap the children to update signal->c* counters, and + * no one else can race with the signal-> fields. + * If we do not take the tasklist_lock, the signal-> fields could be read + * out of order while another thread was just exiting. So we place a + * read memory barrier when we avoid the lock. On the writer side, + * write memory barrier is implied in __exit_signal as __exit_signal releases + * the siglock spinlock after updating the signal-> fields. + * + * We don't really need the siglock when we access the non c* fields + * of the signal_struct (for RUSAGE_SELF) even in multithreaded + * case, since we take the tasklist lock for read and the non c* signal-> + * fields are updated only in __exit_signal, which is called with + * tasklist_lock taken for write, hence these two threads cannot execute + * concurrently. + * */ static void k_getrusage(struct task_struct *p, int who, struct rusage *r) @@ -1694,13 +1710,23 @@ static void k_getrusage(struct task_struct *p, int who, struct rusage *r) struct task_struct *t; unsigned long flags; cputime_t utime, stime; + int need_lock = 0; memset((char *) r, 0, sizeof *r); + utime = stime = cputime_zero; - if (unlikely(!p->signal)) - return; + if (p != current || !thread_group_empty(p)) + need_lock = 1; - utime = stime = cputime_zero; + if (need_lock) { + read_lock(&tasklist_lock); + if (unlikely(!p->signal)) { + read_unlock(&tasklist_lock); + return; + } + } else + /* See locking comments above */ + smp_rmb(); switch (who) { case RUSAGE_BOTH: @@ -1740,6 +1766,8 @@ static void k_getrusage(struct task_struct *p, int who, struct rusage *r) BUG(); } + if (need_lock) + read_unlock(&tasklist_lock); cputime_to_timeval(utime, &r->ru_utime); cputime_to_timeval(stime, &r->ru_stime); } @@ -1747,9 +1775,7 @@ static void k_getrusage(struct task_struct *p, int who, struct rusage *r) int getrusage(struct task_struct *p, int who, struct rusage __user *ru) { struct rusage r; - read_lock(&tasklist_lock); k_getrusage(p, who, &r); - read_unlock(&tasklist_lock); return copy_to_user(ru, &r, sizeof(r)) ? -EFAULT : 0; } |