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Subject[PATCH] Documentation/locking/mutex-design: update to reflect latest changes
Date
Commit 3ca0ff571b09 ("locking/mutex: Rework mutex::owner") reworked the
basic mutex implementation to deal with several problems. Documentation
was however left unchanged and became stale.

Update mutex-design.txt to reflect changes introduced by the above commit.

Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: linux-kernel@vger.kernel.org
Cc: linux-doc@vger.kernel.org
---
Documentation/locking/mutex-design.txt | 49 ++++++++++++----------------------
1 file changed, 17 insertions(+), 32 deletions(-)

diff --git a/Documentation/locking/mutex-design.txt b/Documentation/locking/mutex-design.txt
index 60c482df1a38..42de80cb2449 100644
--- a/Documentation/locking/mutex-design.txt
+++ b/Documentation/locking/mutex-design.txt
@@ -21,37 +21,23 @@ Implementation
--------------

Mutexes are represented by 'struct mutex', defined in include/linux/mutex.h
-and implemented in kernel/locking/mutex.c. These locks use a three
-state atomic counter (->count) to represent the different possible
-transitions that can occur during the lifetime of a lock:
-
- 1: unlocked
- 0: locked, no waiters
- negative: locked, with potential waiters
-
-In its most basic form it also includes a wait-queue and a spinlock
-that serializes access to it. CONFIG_SMP systems can also include
-a pointer to the lock task owner (->owner) as well as a spinner MCS
-lock (->osq), both described below in (ii).
+and implemented in kernel/locking/mutex.c. These locks use an atomic variable
+(->owner) to keep track of the lock state during its lifetime. Field owner
+actually contains 'struct task_struct *' to the current lock owner and it is
+therefore NULL if not currently owned. Since task_struct pointers are aligned
+at at least L1_CACHE_BYTES, low bits (3) are used to store extra state (e.g.,
+if waiter list is non-empty). In its most basic form it also includes a
+wait-queue and a spinlock that serializes access to it. Furthermore,
+CONFIG_MUTEX_SPIN_ON_OWNER systems use a spinner MCS lock (->osq), described
+below in (ii).

When acquiring a mutex, there are three possible paths that can be
taken, depending on the state of the lock:

-(i) fastpath: tries to atomically acquire the lock by decrementing the
- counter. If it was already taken by another task it goes to the next
- possible path. This logic is architecture specific. On x86-64, the
- locking fastpath is 2 instructions:
-
- 0000000000000e10 <mutex_lock>:
- e21: f0 ff 0b lock decl (%rbx)
- e24: 79 08 jns e2e <mutex_lock+0x1e>
-
- the unlocking fastpath is equally tight:
-
- 0000000000000bc0 <mutex_unlock>:
- bc8: f0 ff 07 lock incl (%rdi)
- bcb: 7f 0a jg bd7 <mutex_unlock+0x17>
-
+(i) fastpath: tries to atomically acquire the lock by cmpxchg the owner with
+ current task. This only works in the uncontended case (cmpxchg checks
+ against 0UL, so all 3 state bits above have to be 0). If the lock is
+ contended it goes to the next possible path.

(ii) midpath: aka optimistic spinning, tries to spin for acquisition
while the lock owner is running and there are no other tasks ready
@@ -143,11 +129,10 @@ Test if the mutex is taken:
Disadvantages
-------------

-Unlike its original design and purpose, 'struct mutex' is larger than
-most locks in the kernel. E.g: on x86-64 it is 40 bytes, almost twice
-as large as 'struct semaphore' (24 bytes) and tied, along with rwsems,
-for the largest lock in the kernel. Larger structure sizes mean more
-CPU cache and memory footprint.
+Unlike its original design and purpose, 'struct mutex' is among the largest
+locks in the kernel. E.g: on x86-64 it is 32 bytes, where 'struct semaphore'
+is 24 bytes and rw_semaphore is 40 bytes. Larger structure sizes mean more CPU
+cache and memory footprint.

When to use mutexes
-------------------
--
2.14.3
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