Commit 79e90238 authored by Juri Lelli's avatar Juri Lelli Committed by Ingo Molnar
Browse files

Documentation/locking/mutex-design: Update to reflect latest changes

Commit 3ca0ff57

 ("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: default avatarJuri Lelli <juri.lelli@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-doc@vger.kernel.org
Link: http://lkml.kernel.org/r/20180209160114.19980-1-juri.lelli@redhat.com


[ Small readability tweaks to the text. ]
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent f1517df8
......@@ -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=y 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()ing the owner with
the 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
-------------------
......
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