linux-kernel - Re: [PATCH RFC kenrel/rcu] Eliminate BUG

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Date:   Wed, 31 Oct 2018 10:33:49 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     peterz@...radead.org, linux-kernel@...r.kernel.org,
        josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com
Subject: Re: [PATCH RFC kenrel/rcu] Eliminate BUG_ON() for sync.c

On Wed, Oct 31, 2018 at 06:26:05PM +0100, Oleg Nesterov wrote:
> On 10/30, Paul E. McKenney wrote:
> >
> > On Mon, Oct 22, 2018 at 06:14:40PM +0200, Oleg Nesterov wrote:
> > > > > ----------------------------------------------------------------------------
> > > > > Damn.
> > > > >
> > > > > This suddenly reminds me that I rewrote this code completely, and you even
> > > > > reviewed the new implementation and (iirc) acked it!
> > > > >
> > > > > However, I failed to force myself to rewrite the comments, and that is why
> > > > > I didn't send the "official" patch :/
> > > > >
> > > > > May be some time...
> > > >
> > > > Could you please point me at the last email thread?  Yes, I should be
> > > > able to find it, but I would probably get the wrong one.  :-/
> > >
> > > probably this one,
> > >
> > > 	[PATCH] rcu_sync: simplify the state machine, introduce __rcu_sync_enter()
> > > 	https://lkml.org/lkml/2016/7/16/150
> > >
> > > but I am not sure, will recheck tomorrow.
> >
> > Just following up...  Here is what I currently have.
> 
> Hmm. Are you sure you replied to the correct message? ;)
> 
> the patch below looks absolutely unrelated...

Oy...  Right message, wrong commit.

Does the one below look somewhat more relevant?  ;-)

							Thanx, Paul

------------------------------------------------------------------------

commit 10314fde6fcdbaaf7c21b539dd2db5933344344f
Author: Paul E. McKenney <paulmck@...ux.ibm.com>
Date:   Mon Oct 22 07:43:22 2018 -0700

    rcu: Eliminate BUG_ON() for sync.c
    
    The sync.c file has a number of calls to BUG_ON(), which panics the
    kernel, which is not a good strategy for devices (like embedded) that
    don't have a way to capture console output.  This commit therefore
    changes these BUG_ON() calls to WARN_ON_ONCE(), but does so quite naively.
    
    Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
    Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
    Cc: Oleg Nesterov <oleg@...hat.com>
    Cc: Peter Zijlstra <peterz@...radead.org>

diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
index 3f943efcf61c..a6ba446a9693 100644
--- a/kernel/rcu/sync.c
+++ b/kernel/rcu/sync.c
@@ -125,8 +125,7 @@ void rcu_sync_enter(struct rcu_sync *rsp)
 		rsp->gp_state = GP_PENDING;
 	spin_unlock_irq(&rsp->rss_lock);
 
-	BUG_ON(need_wait && need_sync);
-
+	WARN_ON_ONCE(need_wait && need_sync);
 	if (need_sync) {
 		gp_ops[rsp->gp_type].sync();
 		rsp->gp_state = GP_PASSED;
@@ -139,7 +138,7 @@ void rcu_sync_enter(struct rcu_sync *rsp)
 		 * Nobody has yet been allowed the 'fast' path and thus we can
 		 * avoid doing any sync(). The callback will get 'dropped'.
 		 */
-		BUG_ON(rsp->gp_state != GP_PASSED);
+		WARN_ON_ONCE(rsp->gp_state != GP_PASSED);
 	}
 }
 
@@ -166,8 +165,8 @@ static void rcu_sync_func(struct rcu_head *rhp)
 	struct rcu_sync *rsp = container_of(rhp, struct rcu_sync, cb_head);
 	unsigned long flags;
 
-	BUG_ON(rsp->gp_state != GP_PASSED);
-	BUG_ON(rsp->cb_state == CB_IDLE);
+	WARN_ON_ONCE(rsp->gp_state != GP_PASSED);
+	WARN_ON_ONCE(rsp->cb_state == CB_IDLE);
 
 	spin_lock_irqsave(&rsp->rss_lock, flags);
 	if (rsp->gp_count) {
@@ -225,7 +224,7 @@ void rcu_sync_dtor(struct rcu_sync *rsp)
 {
 	int cb_state;
 
-	BUG_ON(rsp->gp_count);
+	WARN_ON_ONCE(rsp->gp_count);
 
 	spin_lock_irq(&rsp->rss_lock);
 	if (rsp->cb_state == CB_REPLAY)
@@ -235,6 +234,6 @@ void rcu_sync_dtor(struct rcu_sync *rsp)
 
 	if (cb_state != CB_IDLE) {
 		gp_ops[rsp->gp_type].wait();
-		BUG_ON(rsp->cb_state != CB_IDLE);
+		WARN_ON_ONCE(rsp->cb_state != CB_IDLE);
 	}
 }