Re: [PATCH] break_lock forever broken

From: Hugh Dickins
Date: Sat Mar 12 2005 - 18:41:20 EST

On Fri, 11 Mar 2005, Andrew Morton wrote:
> This patch causes a CONFIG_PREEMPT=y, CONFIG_PREEMPT_BKL=y,
> CONFIG_DEBUG_PREEMPT=y kernel on a ppc64 G5 to hang immediately after
> displaying the penguins, but apparently not before having set the hardware
> clock backwards 101 years.
> After having carefully reviewed the above description and having decided
> that these effects were not a part of the patch's design intent I have
> temporarily set it aside, thanks.

That was a Klingon patch, sorry, it escaped. Hence the time warp. Did
terrible things on i386 too once I tested it differently. When dealing
with something like a completion, disaster to touch a field of the lock
once it's been unlocked. Here's a replacement...

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@xxxxxxxxxxx>

--- 2.6.11-bk8/kernel/sched.c 2005-03-11 13:33:09.000000000 +0000
+++ linux/kernel/sched.c 2005-03-11 17:46:50.000000000 +0000
@@ -3753,14 +3753,11 @@ 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)) {
if (need_resched()) {
--- 2.6.11-bk8/kernel/spinlock.c 2005-03-02 07:38:52.000000000 +0000
+++ linux/kernel/spinlock.c 2005-03-12 22:52:41.000000000 +0000
@@ -187,6 +187,8 @@ void __lockfunc _##op##_lock(locktype##_
cpu_relax(); \
preempt_disable(); \
} \
+ if ((lock)->break_lock) \
+ (lock)->break_lock = 0; \
} \
EXPORT_SYMBOL(_##op##_lock); \
@@ -209,6 +211,8 @@ unsigned long __lockfunc _##op##_lock_ir
cpu_relax(); \
preempt_disable(); \
} \
+ if ((lock)->break_lock) \
+ (lock)->break_lock = 0; \
return flags; \
} \
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