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-rw-r--r--Documentation/locking/ww-mutex-design.txt57
1 files changed, 46 insertions, 11 deletions
diff --git a/Documentation/locking/ww-mutex-design.txt b/Documentation/locking/ww-mutex-design.txt
index 2fd7f2a2af21..f0ed7c30e695 100644
--- a/Documentation/locking/ww-mutex-design.txt
+++ b/Documentation/locking/ww-mutex-design.txt
@@ -1,4 +1,4 @@
-Wait/Wound Deadlock-Proof Mutex Design
+Wound/Wait Deadlock-Proof Mutex Design
======================================
Please read mutex-design.txt first, as it applies to wait/wound mutexes too.
@@ -32,10 +32,26 @@ the oldest task) wins, and the one with the higher reservation id (i.e. the
younger task) unlocks all of the buffers that it has already locked, and then
tries again.
-In the RDBMS literature this deadlock handling approach is called wait/die:
-The older tasks waits until it can acquire the contended lock. The younger tasks
-needs to back off and drop all the locks it is currently holding, i.e. the
-younger task dies.
+In the RDBMS literature, a reservation ticket is associated with a transaction.
+and the deadlock handling approach is called Wait-Die. The name is based on
+the actions of a locking thread when it encounters an already locked mutex.
+If the transaction holding the lock is younger, the locking transaction waits.
+If the transaction holding the lock is older, the locking transaction backs off
+and dies. Hence Wait-Die.
+There is also another algorithm called Wound-Wait:
+If the transaction holding the lock is younger, the locking transaction
+wounds the transaction holding the lock, requesting it to die.
+If the transaction holding the lock is older, it waits for the other
+transaction. Hence Wound-Wait.
+The two algorithms are both fair in that a transaction will eventually succeed.
+However, the Wound-Wait algorithm is typically stated to generate fewer backoffs
+compared to Wait-Die, but is, on the other hand, associated with more work than
+Wait-Die when recovering from a backoff. Wound-Wait is also a preemptive
+algorithm in that transactions are wounded by other transactions, and that
+requires a reliable way to pick up up the wounded condition and preempt the
+running transaction. Note that this is not the same as process preemption. A
+Wound-Wait transaction is considered preempted when it dies (returning
+-EDEADLK) following a wound.
Concepts
--------
@@ -47,10 +63,12 @@ Acquire context: To ensure eventual forward progress it is important the a task
trying to acquire locks doesn't grab a new reservation id, but keeps the one it
acquired when starting the lock acquisition. This ticket is stored in the
acquire context. Furthermore the acquire context keeps track of debugging state
-to catch w/w mutex interface abuse.
+to catch w/w mutex interface abuse. An acquire context is representing a
+transaction.
W/w class: In contrast to normal mutexes the lock class needs to be explicit for
-w/w mutexes, since it is required to initialize the acquire context.
+w/w mutexes, since it is required to initialize the acquire context. The lock
+class also specifies what algorithm to use, Wound-Wait or Wait-Die.
Furthermore there are three different class of w/w lock acquire functions:
@@ -90,6 +108,12 @@ provided.
Usage
-----
+The algorithm (Wait-Die vs Wound-Wait) is chosen by using either
+DEFINE_WW_CLASS() (Wound-Wait) or DEFINE_WD_CLASS() (Wait-Die)
+As a rough rule of thumb, use Wound-Wait iff you
+expect the number of simultaneous competing transactions to be typically small,
+and you want to reduce the number of rollbacks.
+
Three different ways to acquire locks within the same w/w class. Common
definitions for methods #1 and #2:
@@ -312,12 +336,23 @@ Design:
We maintain the following invariants for the wait list:
(1) Waiters with an acquire context are sorted by stamp order; waiters
without an acquire context are interspersed in FIFO order.
- (2) Among waiters with contexts, only the first one can have other locks
- acquired already (ctx->acquired > 0). Note that this waiter may come
- after other waiters without contexts in the list.
+ (2) For Wait-Die, among waiters with contexts, only the first one can have
+ other locks acquired already (ctx->acquired > 0). Note that this waiter
+ may come after other waiters without contexts in the list.
+
+ The Wound-Wait preemption is implemented with a lazy-preemption scheme:
+ The wounded status of the transaction is checked only when there is
+ contention for a new lock and hence a true chance of deadlock. In that
+ situation, if the transaction is wounded, it backs off, clears the
+ wounded status and retries. A great benefit of implementing preemption in
+ this way is that the wounded transaction can identify a contending lock to
+ wait for before restarting the transaction. Just blindly restarting the
+ transaction would likely make the transaction end up in a situation where
+ it would have to back off again.
In general, not much contention is expected. The locks are typically used to
- serialize access to resources for devices.
+ serialize access to resources for devices, and optimization focus should
+ therefore be directed towards the uncontended cases.
Lockdep:
Special care has been taken to warn for as many cases of api abuse