Date: Thu, 10 Aug 2017 21:17:37 +0800
From: Boqun Feng <boqun.feng@gmail.com>
To: Byungchul Park <byungchul.park@lge.com>
Cc: peterz@infradead.org, mingo@kernel.org, tglx@linutronix.de,
        walken@google.com, kirill@shutemov.name, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, akpm@linux-foundation.org, willy@infradead.org,
        npiggin@gmail.com, kernel-team@lge.com
Subject: Re: [PATCH v8 06/14] lockdep: Detect and handle hist_lock ring
 buffer overwrite
Message-ID: <20170810131737.skdyy4qcxlikbyeh@tardis>
References: <1502089981-21272-1-git-send-email-byungchul.park@lge.com>
 <1502089981-21272-7-git-send-email-byungchul.park@lge.com>
 <20170810115922.kegrfeg6xz7mgpj4@tardis>
 <016b01d311d1$d02acfa0$70806ee0$@lge.com>
 <20170810125133.2poixhni4d5aqkpy@tardis>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20170810125133.2poixhni4d5aqkpy@tardis>
User-Agent: NeoMutt/20170609 (1.8.3)
Sender: linux-kernel-owner@vger.kernel.org
Content-Transfer-Encoding: 8bit
Content-Length: 4915
Lines: 153

On Thu, Aug 10, 2017 at 08:51:33PM +0800, Boqun Feng wrote:
[...]
> > > > +		/* Check if the ring was overwritten. */
> > > > +		if (h->hist_id != cur->hist_id_save[c])
> > > 
> > > Could we use:
> > > 
> > > 		if (h->hist_id != idx)
> > 
> > No, we cannot.
> > 
> 
> Hey, I'm not buying it. task_struct::hist_id and task_struct::xhlock_idx
> are increased at the same place(in add_xhlock()), right?
> 
> And, yes, xhlock_idx will get decreased when we do ring-buffer
> unwinding, but that's OK, because we need to throw away those recently
> added items.
> 
> And xhlock_idx always points to the most recently added valid item,
> right?  Any other item's idx must "before()" the most recently added
> one's, right? So ::xhlock_idx acts just like a timestamp, doesn't it?
> 
> Maybe I'm missing something subtle, but could you show me an example,
> that could end up being a problem if we use xhlock_idx as the hist_id?
> 
> > hist_id is a kind of timestamp and used to detect overwriting
> > data into places of same indexes of the ring buffer. And idx is
> > just an index. :) IOW, they mean different things.
> > 
> > > 
> > > here, and
> > > 
> > > > +			invalidate_xhlock(h);
> > > > +	}
> > > >  }
> > > >
> > > >  static int cross_lock(struct lockdep_map *lock)
> > > > @@ -4826,6 +4851,7 @@ static inline int depend_after(struct held_lock
> > > *hlock)
> > > >   * Check if the xhlock is valid, which would be false if,
> > > >   *
> > > >   *    1. Has not used after initializaion yet.
> > > > + *    2. Got invalidated.
> > > >   *
> > > >   * Remind hist_lock is implemented as a ring buffer.
> > > >   */
> > > > @@ -4857,6 +4883,7 @@ static void add_xhlock(struct held_lock *hlock)
> > > >
> > > >  	/* Initialize hist_lock's members */
> > > >  	xhlock->hlock = *hlock;
> > > > +	xhlock->hist_id = current->hist_id++;
> 
> Besides, is this code correct? Does this just make xhlock->hist_id
> one-less-than the curr->hist_id, which cause the invalidation every time
> you do ring buffer unwinding?
> 
> Regards,
> Boqun
> 

So basically, I'm suggesting do this on top of your patch, there is also
a fix in commit_xhlocks(), which I think you should swap the parameters
in before(...), no matter using task_struct::hist_id or using
task_struct::xhlock_idx as the timestamp.

Hope this could make my point more clear, and if I do miss something,
please point it out, thanks ;-)

Regards,
Boqun
------------>8

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 074872f016f8..886ba79bfc38 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -854,9 +854,6 @@ struct task_struct {
 	unsigned int xhlock_idx;
 	/* For restoring at history boundaries */
 	unsigned int xhlock_idx_hist[XHLOCK_NR];
-	unsigned int hist_id;
-	/* For overwrite check at each context exit */
-	unsigned int hist_id_save[XHLOCK_NR];
 #endif
 
 #ifdef CONFIG_UBSAN
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 699fbeab1920..04c6c8d68e18 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -4752,10 +4752,8 @@ void crossrelease_hist_start(enum xhlock_context_t c)
 {
 	struct task_struct *cur = current;
 
-	if (cur->xhlocks) {
+	if (cur->xhlocks)
 		cur->xhlock_idx_hist[c] = cur->xhlock_idx;
-		cur->hist_id_save[c] = cur->hist_id;
-	}
 }
 
 void crossrelease_hist_end(enum xhlock_context_t c)
@@ -4769,7 +4767,7 @@ void crossrelease_hist_end(enum xhlock_context_t c)
 		cur->xhlock_idx = idx;
 
 		/* Check if the ring was overwritten. */
-		if (h->hist_id != cur->hist_id_save[c])
+		if (h->hist_id != idx)
 			invalidate_xhlock(h);
 	}
 }
@@ -4849,7 +4847,7 @@ static void add_xhlock(struct held_lock *hlock)
 
 	/* Initialize hist_lock's members */
 	xhlock->hlock = *hlock;
-	xhlock->hist_id = current->hist_id++;
+	xhlock->hist_id = idx;
 
 	xhlock->trace.nr_entries = 0;
 	xhlock->trace.max_entries = MAX_XHLOCK_TRACE_ENTRIES;
@@ -5005,7 +5003,7 @@ static int commit_xhlock(struct cross_lock *xlock, struct hist_lock *xhlock)
 static void commit_xhlocks(struct cross_lock *xlock)
 {
 	unsigned int cur = current->xhlock_idx;
-	unsigned int prev_hist_id = xhlock(cur).hist_id;
+	unsigned int prev_hist_id = cur + 1;
 	unsigned int i;
 
 	if (!graph_lock())
@@ -5030,7 +5028,7 @@ static void commit_xhlocks(struct cross_lock *xlock)
 			 * hist_id than the following one, which is impossible
 			 * otherwise.
 			 */
-			if (unlikely(before(xhlock->hist_id, prev_hist_id)))
+			if (unlikely(before(prev_hist_id, xhlock->hist_id)))
 				break;
 
 			prev_hist_id = xhlock->hist_id;
@@ -5120,12 +5118,9 @@ void lockdep_init_task(struct task_struct *task)
 	int i;
 
 	task->xhlock_idx = UINT_MAX;
-	task->hist_id = 0;
 
-	for (i = 0; i < XHLOCK_NR; i++) {
+	for (i = 0; i < XHLOCK_NR; i++)
 		task->xhlock_idx_hist[i] = UINT_MAX;
-		task->hist_id_save[i] = 0;
-	}
 
 	task->xhlocks = kzalloc(sizeof(struct hist_lock) * MAX_XHLOCKS_NR,
 				GFP_KERNEL);