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Diffstat (limited to 'Documentation/DocBook/kernel-locking.tmpl')
| -rw-r--r-- | Documentation/DocBook/kernel-locking.tmpl | 120 |
1 files changed, 72 insertions, 48 deletions
diff --git a/Documentation/DocBook/kernel-locking.tmpl b/Documentation/DocBook/kernel-locking.tmpl index 2e9d6b41f03..e584ee12a1e 100644 --- a/Documentation/DocBook/kernel-locking.tmpl +++ b/Documentation/DocBook/kernel-locking.tmpl @@ -219,10 +219,10 @@ </para> <sect1 id="lock-intro"> - <title>Three Main Types of Kernel Locks: Spinlocks, Mutexes and Semaphores</title> + <title>Two Main Types of Kernel Locks: Spinlocks and Mutexes</title> <para> - There are three main types of kernel locks. The fundamental type + There are two main types of kernel locks. The fundamental type is the spinlock (<filename class="headerfile">include/asm/spinlock.h</filename>), which is a very simple single-holder lock: if you can't get the @@ -240,14 +240,6 @@ use a spinlock instead. </para> <para> - The third type is a semaphore - (<filename class="headerfile">include/asm/semaphore.h</filename>): it - can have more than one holder at any time (the number decided at - initialization time), although it is most commonly used as a - single-holder lock (a mutex). If you can't get a semaphore, your - task will be suspended and later on woken up - just like for mutexes. - </para> - <para> Neither type of lock is recursive: see <xref linkend="deadlock"/>. </para> @@ -278,7 +270,7 @@ </para> <para> - Semaphores still exist, because they are required for + Mutexes still exist, because they are required for synchronization between <firstterm linkend="gloss-usercontext">user contexts</firstterm>, as we will see below. </para> @@ -289,18 +281,17 @@ <para> If you have a data structure which is only ever accessed from - user context, then you can use a simple semaphore - (<filename>linux/asm/semaphore.h</filename>) to protect it. This - is the most trivial case: you initialize the semaphore to the number - of resources available (usually 1), and call - <function>down_interruptible()</function> to grab the semaphore, and - <function>up()</function> to release it. There is also a - <function>down()</function>, which should be avoided, because it + user context, then you can use a simple mutex + (<filename>include/linux/mutex.h</filename>) to protect it. This + is the most trivial case: you initialize the mutex. Then you can + call <function>mutex_lock_interruptible()</function> to grab the mutex, + and <function>mutex_unlock()</function> to release it. There is also a + <function>mutex_lock()</function>, which should be avoided, because it will not return if a signal is received. </para> <para> - Example: <filename>linux/net/core/netfilter.c</filename> allows + Example: <filename>net/netfilter/nf_sockopt.c</filename> allows registration of new <function>setsockopt()</function> and <function>getsockopt()</function> calls, with <function>nf_register_sockopt()</function>. Registration and @@ -515,7 +506,7 @@ <listitem> <para> If you are in a process context (any syscall) and want to - lock other process out, use a semaphore. You can take a semaphore + lock other process out, use a mutex. You can take a mutex and sleep (<function>copy_from_user*(</function> or <function>kmalloc(x,GFP_KERNEL)</function>). </para> @@ -662,7 +653,7 @@ <entry>SLBH</entry> <entry>SLBH</entry> <entry>SLBH</entry> -<entry>DI</entry> +<entry>MLI</entry> <entry>None</entry> </row> @@ -692,8 +683,8 @@ <entry>spin_lock_bh</entry> </row> <row> -<entry>DI</entry> -<entry>down_interruptible</entry> +<entry>MLI</entry> +<entry>mutex_lock_interruptible</entry> </row> </tbody> @@ -703,6 +694,31 @@ </sect1> </chapter> +<chapter id="trylock-functions"> + <title>The trylock Functions</title> + <para> + There are functions that try to acquire a lock only once and immediately + return a value telling about success or failure to acquire the lock. + They can be used if you need no access to the data protected with the lock + when some other thread is holding the lock. You should acquire the lock + later if you then need access to the data protected with the lock. + </para> + + <para> + <function>spin_trylock()</function> does not spin but returns non-zero if + it acquires the spinlock on the first try or 0 if not. This function can + be used in all contexts like <function>spin_lock</function>: you must have + disabled the contexts that might interrupt you and acquire the spin lock. + </para> + + <para> + <function>mutex_trylock()</function> does not suspend your task + but returns non-zero if it could lock the mutex on the first try + or 0 if not. This function cannot be safely used in hardware or software + interrupt contexts despite not sleeping. + </para> +</chapter> + <chapter id="Examples"> <title>Common Examples</title> <para> @@ -854,7 +870,7 @@ The change is shown below, in standard patch format: the }; -static DEFINE_MUTEX(cache_lock); -+static spinlock_t cache_lock = SPIN_LOCK_UNLOCKED; ++static DEFINE_SPINLOCK(cache_lock); static LIST_HEAD(cache); static unsigned int cache_num = 0; #define MAX_CACHE_SIZE 10 @@ -1238,7 +1254,7 @@ Here is the "lock-per-object" implementation: - int popularity; }; - static spinlock_t cache_lock = SPIN_LOCK_UNLOCKED; + static DEFINE_SPINLOCK(cache_lock); @@ -77,6 +84,7 @@ obj->id = id; obj->popularity = 0; @@ -1285,7 +1301,7 @@ as Alan Cox says, <quote>Lock data, not code</quote>. <para> There is a coding bug where a piece of code tries to grab a spinlock twice: it will spin forever, waiting for the lock to - be released (spinlocks, rwlocks and semaphores are not + be released (spinlocks, rwlocks and mutexes are not recursive in Linux). This is trivial to diagnose: not a stay-up-five-nights-talk-to-fluffy-code-bunnies kind of problem. @@ -1310,7 +1326,7 @@ as Alan Cox says, <quote>Lock data, not code</quote>. <para> This complete lockup is easy to diagnose: on SMP boxes the - watchdog timer or compiling with <symbol>DEBUG_SPINLOCKS</symbol> set + watchdog timer or compiling with <symbol>DEBUG_SPINLOCK</symbol> set (<filename>include/linux/spinlock.h</filename>) will show this up immediately when it happens. </para> @@ -1533,7 +1549,7 @@ the amount of locking which needs to be done. <title>Read/Write Lock Variants</title> <para> - Both spinlocks and semaphores have read/write variants: + Both spinlocks and mutexes have read/write variants: <type>rwlock_t</type> and <structname>struct rw_semaphore</structname>. These divide users into two classes: the readers and the writers. If you are only reading the data, you can get a read lock, but to write to @@ -1629,7 +1645,9 @@ the amount of locking which needs to be done. all the readers who were traversing the list when we deleted the element are finished. We use <function>call_rcu()</function> to register a callback which will actually destroy the object once - the readers are finished. + all pre-existing readers are finished. Alternatively, + <function>synchronize_rcu()</function> may be used to block until + all pre-existing are finished. </para> <para> But how does Read Copy Update know when the readers are @@ -1656,7 +1674,7 @@ the amount of locking which needs to be done. #include <linux/slab.h> #include <linux/string.h> +#include <linux/rcupdate.h> - #include <asm/semaphore.h> + #include <linux/mutex.h> #include <asm/errno.h> struct object @@ -1698,7 +1716,7 @@ the amount of locking which needs to be done. - object_put(obj); + list_del_rcu(&obj->list); cache_num--; -+ call_rcu(&obj->rcu, cache_delete_rcu, obj); ++ call_rcu(&obj->rcu, cache_delete_rcu); } /* Must be holding cache_lock */ @@ -1709,14 +1727,6 @@ the amount of locking which needs to be done. if (++cache_num > MAX_CACHE_SIZE) { struct object *i, *outcast = NULL; list_for_each_entry(i, &cache, list) { -@@ -85,6 +94,7 @@ - obj->popularity = 0; - atomic_set(&obj->refcnt, 1); /* The cache holds a reference */ - spin_lock_init(&obj->lock); -+ INIT_RCU_HEAD(&obj->rcu); - - spin_lock_irqsave(&cache_lock, flags); - __cache_add(obj); @@ -104,12 +114,11 @@ struct object *cache_find(int id) { @@ -1750,10 +1760,10 @@ as it would be on UP. </para> <para> -There is a furthur optimization possible here: remember our original +There is a further optimization possible here: remember our original cache code, where there were no reference counts and the caller simply held the lock whenever using the object? This is still possible: if -you hold the lock, noone can delete the object, so you don't need to +you hold the lock, no one can delete the object, so you don't need to get and put the reference count. </para> @@ -1888,7 +1898,7 @@ machines due to caching. </listitem> <listitem> <para> - <function> put_user()</function> + <function>put_user()</function> </para> </listitem> </itemizedlist> @@ -1902,13 +1912,16 @@ machines due to caching. <listitem> <para> - <function>down_interruptible()</function> and - <function>down()</function> + <function>mutex_lock_interruptible()</function> and + <function>mutex_lock()</function> </para> <para> - There is a <function>down_trylock()</function> which can be - used inside interrupt context, as it will not sleep. - <function>up()</function> will also never sleep. + There is a <function>mutex_trylock()</function> which does not + sleep. Still, it must not be used inside interrupt context since + its implementation is not safe for that. + <function>mutex_unlock()</function> will also never sleep. + It cannot be used in interrupt context either since a mutex + must be released by the same task that acquired it. </para> </listitem> </itemizedlist> @@ -1942,6 +1955,17 @@ machines due to caching. </sect1> </chapter> + <chapter id="apiref-mutex"> + <title>Mutex API reference</title> +!Iinclude/linux/mutex.h +!Ekernel/locking/mutex.c + </chapter> + + <chapter id="apiref-futex"> + <title>Futex API reference</title> +!Ikernel/futex.c + </chapter> + <chapter id="references"> <title>Further reading</title> @@ -1998,7 +2022,7 @@ machines due to caching. <para> Prior to 2.5, or when <symbol>CONFIG_PREEMPT</symbol> is unset, processes in user context inside the kernel would not - preempt each other (ie. you had that CPU until you have it up, + preempt each other (ie. you had that CPU until you gave it up, except for interrupts). With the addition of <symbol>CONFIG_PREEMPT</symbol> in 2.5.4, this changed: when in user context, higher priority tasks can "cut in": spinlocks |