From: Ingo Molnar <mingo@elte.hu>
lock->break_lock is set when a lock is contended, but cleared only in
cond_resched_lock. Users of need_lockbreak (journal_commit_transaction,
copy_pte_range, unmap_vmas) don't necessarily use cond_resched_lock on it.
So, if the lock has been contended at some time in the past, break_lock
remains set thereafter, and the fastpath keeps dropping lock unnecessarily.
Hanging the system if you make a change like I did, forever restarting a
loop before making any progress. And even users of cond_resched_lock may
well suffer an initial unnecessary lockbreak.
There seems to be no point at which break_lock can be cleared when
unlocking, any point being either too early or too late; but that's okay,
it's only of interest while the lock is held. So clear it whenever the
lock is acquired - and any waiting contenders will quickly set it again.
Additional locking overhead? well, this is only when CONFIG_PREEMPT is on.
Since cond_resched_lock's spin_lock clears break_lock, no need to clear it
itself; and use need_lockbreak there too, preferring optimizer to #ifdefs.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---
25-akpm/kernel/sched.c | 5 +----
25-akpm/kernel/spinlock.c | 2 ++
2 files changed, 3 insertions(+), 4 deletions(-)
diff -puN kernel/sched.c~break_lock-fix-2 kernel/sched.c
--- 25/kernel/sched.c~break_lock-fix-2 Mon Mar 14 16:20:47 2005
+++ 25-akpm/kernel/sched.c Mon Mar 14 16:20:47 2005
@@ -3746,14 +3746,11 @@ EXPORT_SYMBOL(cond_resched);
*/
int cond_resched_lock(spinlock_t * lock)
{
-#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPT)
- if (lock->break_lock) {
- lock->break_lock = 0;
+ if (need_lockbreak(lock)) {
spin_unlock(lock);
cpu_relax();
spin_lock(lock);
}
-#endif
if (need_resched()) {
_raw_spin_unlock(lock);
preempt_enable_no_resched();
diff -puN kernel/spinlock.c~break_lock-fix-2 kernel/spinlock.c
--- 25/kernel/spinlock.c~break_lock-fix-2 Mon Mar 14 16:20:47 2005
+++ 25-akpm/kernel/spinlock.c Mon Mar 14 16:20:47 2005
@@ -187,6 +187,7 @@ void __lockfunc _##op##_lock(locktype##_
cpu_relax(); \
preempt_disable(); \
} \
+ (lock)->break_lock = 0; \
} \
\
EXPORT_SYMBOL(_##op##_lock); \
@@ -209,6 +210,7 @@ unsigned long __lockfunc _##op##_lock_ir
cpu_relax(); \
preempt_disable(); \
} \
+ (lock)->break_lock = 0; \
return flags; \
} \
\
_