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Date:	Sun, 23 Feb 2014 15:50:12 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Stefan Richter <stefanr@...6.in-berlin.de>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Tejun Heo <tj@...nel.org>, laijs@...fujitsu.com,
	linux-kernel@...r.kernel.org,
	linux1394-devel@...ts.sourceforge.net,
	Chris Boot <bootc@...tc.net>, linux-scsi@...r.kernel.org,
	target-devel@...r.kernel.org
Subject: Re: memory-barriers.txt again (was Re: [PATCH 4/9] firewire: don't
 use PREPARE_DELAYED_WORK)

On Sun, Feb 23, 2014 at 03:35:31PM -0500, Peter Hurley wrote:
> Hi Paul,
> 
> On 02/23/2014 11:37 AM, Paul E. McKenney wrote:
> >commit aba6b0e82c9de53eb032844f1932599f148ff68d
> >Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> >Date:   Sun Feb 23 08:34:24 2014 -0800
> >
> >     Documentation/memory-barriers.txt: Clarify release/acquire ordering
> >
> >     This commit fixes a couple of typos and clarifies what happens when
> >     the CPU chooses to execute a later lock acquisition before a prior
> >     lock release, in particular, why deadlock is avoided.
> >
> >     Reported-by: Peter Hurley <peter@...leysoftware.com>
> >     Reported-by: James Bottomley <James.Bottomley@...senPartnership.com>
> >     Reported-by: Stefan Richter <stefanr@...6.in-berlin.de>
> >     Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> >
> >diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> >index 9dde54c55b24..c8932e06edf1 100644
> >--- a/Documentation/memory-barriers.txt
> >+++ b/Documentation/memory-barriers.txt
> >@@ -1674,12 +1674,12 @@ for each construct.  These operations all imply certain barriers:
> >       Memory operations issued after the ACQUIRE will be completed after the
> >       ACQUIRE operation has completed.
> >
> >-     Memory operations issued before the ACQUIRE may be completed after the
> >-     ACQUIRE operation has completed.  An smp_mb__before_spinlock(), combined
> >-     with a following ACQUIRE, orders prior loads against subsequent stores and
> >-     stores and prior stores against subsequent stores.  Note that this is
> >-     weaker than smp_mb()!  The smp_mb__before_spinlock() primitive is free on
> >-     many architectures.
> >+     Memory operations issued before the ACQUIRE may be completed after
> >+     the ACQUIRE operation has completed.  An smp_mb__before_spinlock(),
> >+     combined with a following ACQUIRE, orders prior loads against
> >+     subsequent loads and stores and also orders prior stores against
> >+     subsequent stores.  Note that this is weaker than smp_mb()!  The
> >+     smp_mb__before_spinlock() primitive is free on many architectures.
> >
> >   (2) RELEASE operation implication:
> >
> >@@ -1717,23 +1717,47 @@ the two accesses can themselves then cross:
> >
> >  	*A = a;
> >  	ACQUIRE M
> >-	RELEASE M
> >+	RELEASE N
> >  	*B = b;
> >
> >  may occur as:
> >
> >-	ACQUIRE M, STORE *B, STORE *A, RELEASE M
> 
> This example should remain as is; it refers to the porosity of a critical
> section to loads and stores occurring outside that critical section, and
> importantly that LOCK + UNLOCK is not a full barrier. It documents that
> memory operations from either side of the critical section may cross
> (in the absence of other specific memory barriers). IOW, it is the example
> to implication #1 above.

Good point, I needed to apply the changes further down.  How does the
following updated patch look?

							Thanx, Paul

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

commit 528c2771288df7f98f9224a56b93bdb2db27ec70
Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Date:   Sun Feb 23 08:34:24 2014 -0800

    Documentation/memory-barriers.txt: Clarify release/acquire ordering
    
    This commit fixes a couple of typos and clarifies what happens when
    the CPU chooses to execute a later lock acquisition before a prior
    lock release, in particular, why deadlock is avoided.
    
    Reported-by: Peter Hurley <peter@...leysoftware.com>
    Reported-by: James Bottomley <James.Bottomley@...senPartnership.com>
    Reported-by: Stefan Richter <stefanr@...6.in-berlin.de>
    Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 9dde54c55b24..9ea6de4eb252 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1674,12 +1674,12 @@ for each construct.  These operations all imply certain barriers:
      Memory operations issued after the ACQUIRE will be completed after the
      ACQUIRE operation has completed.
 
-     Memory operations issued before the ACQUIRE may be completed after the
-     ACQUIRE operation has completed.  An smp_mb__before_spinlock(), combined
-     with a following ACQUIRE, orders prior loads against subsequent stores and
-     stores and prior stores against subsequent stores.  Note that this is
-     weaker than smp_mb()!  The smp_mb__before_spinlock() primitive is free on
-     many architectures.
+     Memory operations issued before the ACQUIRE may be completed after
+     the ACQUIRE operation has completed.  An smp_mb__before_spinlock(),
+     combined with a following ACQUIRE, orders prior loads against
+     subsequent loads and stores and also orders prior stores against
+     subsequent stores.  Note that this is weaker than smp_mb()!  The
+     smp_mb__before_spinlock() primitive is free on many architectures.
 
  (2) RELEASE operation implication:
 
@@ -1724,24 +1724,20 @@ may occur as:
 
 	ACQUIRE M, STORE *B, STORE *A, RELEASE M
 
-This same reordering can of course occur if the lock's ACQUIRE and RELEASE are
-to the same lock variable, but only from the perspective of another CPU not
-holding that lock.
-
-In short, a RELEASE followed by an ACQUIRE may -not- be assumed to be a full
-memory barrier because it is possible for a preceding RELEASE to pass a
-later ACQUIRE from the viewpoint of the CPU, but not from the viewpoint
-of the compiler.  Note that deadlocks cannot be introduced by this
-interchange because if such a deadlock threatened, the RELEASE would
-simply complete.
+When the ACQUIRE and RELEASE are a lock acquisition and release,
+respectively, this same reordering can of course occur if the lock's
+ACQUIRE and RELEASE are to the same lock variable, but only from the
+perspective of another CPU not holding that lock.  In short, a RELEASE
+followed by an ACQUIRE may -not- be assumed to be a full memory barrier.
 
 If it is necessary for a RELEASE-ACQUIRE pair to produce a full barrier, the
 ACQUIRE can be followed by an smp_mb__after_unlock_lock() invocation.  This
 will produce a full barrier if either (a) the RELEASE and the ACQUIRE are
 executed by the same CPU or task, or (b) the RELEASE and ACQUIRE act on the
 same variable.  The smp_mb__after_unlock_lock() primitive is free on many
-architectures.  Without smp_mb__after_unlock_lock(), the critical sections
-corresponding to the RELEASE and the ACQUIRE can cross:
+architectures.  Without smp_mb__after_unlock_lock(), the CPU's execution of
+the critical sections corresponding to the RELEASE and the ACQUIRE can cross,
+so that:
 
 	*A = a;
 	RELEASE M
@@ -1752,7 +1748,36 @@ could occur as:
 
 	ACQUIRE N, STORE *B, STORE *A, RELEASE M
 
-With smp_mb__after_unlock_lock(), they cannot, so that:
+It might appear that this rearrangement could introduce a deadlock.
+However, this cannot happen because if such a deadlock threatened,
+the RELEASE would simply complete, thereby avoiding the deadlock.
+
+	Why does this work?
+
+	One key point is that we are only talking about the CPU doing
+	the interchanging, not the compiler.  If the compiler (or, for
+	that matter, the developer) switched the operations, deadlock
+	-could- occur.
+
+	But suppose the CPU interchanged the operations.  In this case,
+	the unlock precedes the lock in the assembly code.  The CPU simply
+	elected to try executing the later lock operation first.  If there
+	is a deadlock, this lock operation will simply spin (or try to
+	sleep, but more on that later).  The CPU will eventually execute
+	the unlock operation (which again preceded the lock operation
+	in the assembly code), which will unravel the potential deadlock,
+	allowing the lock operation to succeed.
+
+	But what if the lock is a sleeplock?  In that case, the code will
+	try to enter the scheduler, where it will eventually encounter
+	a memory barrier, which will force the earlier unlock operation
+	to complete, again unraveling the deadlock.  There might be
+	a sleep-unlock race, but the locking primitive needs to resolve
+	such races properly in any case.
+
+With smp_mb__after_unlock_lock(), the two critical sections cannot overlap.
+For example, with the following code, the store to *A will always be
+seen by other CPUs before the store to *B:
 
 	*A = a;
 	RELEASE M
@@ -1760,13 +1785,18 @@ With smp_mb__after_unlock_lock(), they cannot, so that:
 	smp_mb__after_unlock_lock();
 	*B = b;
 
-will always occur as either of the following:
+The operations will always occur in one of the following orders:
 
-	STORE *A, RELEASE, ACQUIRE, STORE *B
-	STORE *A, ACQUIRE, RELEASE, STORE *B
+	STORE *A, RELEASE, ACQUIRE, smp_mb__after_unlock_lock(), STORE *B
+	STORE *A, ACQUIRE, RELEASE, smp_mb__after_unlock_lock(), STORE *B
+	ACQUIRE, STORE *A, RELEASE, smp_mb__after_unlock_lock(), STORE *B
 
-If the RELEASE and ACQUIRE were instead both operating on the same lock
-variable, only the first of these two alternatives can occur.
+If the RELEASE and ACQUIRE were instead both operating on the
+same lock variable, only the first of these two alternatives can
+occur.  In addition, the more strongly ordered systems may rule out
+some of the above orders.  But in any case, as noted earlier, the
+smp_mb__after_unlock_lock() ensures that the store to *A will always be
+seen as happening before the store to *B.
 
 Locks and semaphores may not provide any guarantee of ordering on UP compiled
 systems, and so cannot be counted on in such a situation to actually achieve

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