diff options
Diffstat (limited to 'Documentation')
-rw-r--r-- | Documentation/DocBook/kernel-locking.tmpl | 57 |
1 files changed, 24 insertions, 33 deletions
diff --git a/Documentation/DocBook/kernel-locking.tmpl b/Documentation/DocBook/kernel-locking.tmpl index 2510763295d0..084f6ad7b7a0 100644 --- a/Documentation/DocBook/kernel-locking.tmpl +++ b/Documentation/DocBook/kernel-locking.tmpl | |||
@@ -219,10 +219,10 @@ | |||
219 | </para> | 219 | </para> |
220 | 220 | ||
221 | <sect1 id="lock-intro"> | 221 | <sect1 id="lock-intro"> |
222 | <title>Three Main Types of Kernel Locks: Spinlocks, Mutexes and Semaphores</title> | 222 | <title>Two Main Types of Kernel Locks: Spinlocks and Mutexes</title> |
223 | 223 | ||
224 | <para> | 224 | <para> |
225 | There are three main types of kernel locks. The fundamental type | 225 | There are two main types of kernel locks. The fundamental type |
226 | is the spinlock | 226 | is the spinlock |
227 | (<filename class="headerfile">include/asm/spinlock.h</filename>), | 227 | (<filename class="headerfile">include/asm/spinlock.h</filename>), |
228 | which is a very simple single-holder lock: if you can't get the | 228 | which is a very simple single-holder lock: if you can't get the |
@@ -240,14 +240,6 @@ | |||
240 | use a spinlock instead. | 240 | use a spinlock instead. |
241 | </para> | 241 | </para> |
242 | <para> | 242 | <para> |
243 | The third type is a semaphore | ||
244 | (<filename class="headerfile">include/linux/semaphore.h</filename>): it | ||
245 | can have more than one holder at any time (the number decided at | ||
246 | initialization time), although it is most commonly used as a | ||
247 | single-holder lock (a mutex). If you can't get a semaphore, your | ||
248 | task will be suspended and later on woken up - just like for mutexes. | ||
249 | </para> | ||
250 | <para> | ||
251 | Neither type of lock is recursive: see | 243 | Neither type of lock is recursive: see |
252 | <xref linkend="deadlock"/>. | 244 | <xref linkend="deadlock"/>. |
253 | </para> | 245 | </para> |
@@ -278,7 +270,7 @@ | |||
278 | </para> | 270 | </para> |
279 | 271 | ||
280 | <para> | 272 | <para> |
281 | Semaphores still exist, because they are required for | 273 | Mutexes still exist, because they are required for |
282 | synchronization between <firstterm linkend="gloss-usercontext">user | 274 | synchronization between <firstterm linkend="gloss-usercontext">user |
283 | contexts</firstterm>, as we will see below. | 275 | contexts</firstterm>, as we will see below. |
284 | </para> | 276 | </para> |
@@ -289,18 +281,17 @@ | |||
289 | 281 | ||
290 | <para> | 282 | <para> |
291 | If you have a data structure which is only ever accessed from | 283 | If you have a data structure which is only ever accessed from |
292 | user context, then you can use a simple semaphore | 284 | user context, then you can use a simple mutex |
293 | (<filename>linux/linux/semaphore.h</filename>) to protect it. This | 285 | (<filename>include/linux/mutex.h</filename>) to protect it. This |
294 | is the most trivial case: you initialize the semaphore to the number | 286 | is the most trivial case: you initialize the mutex. Then you can |
295 | of resources available (usually 1), and call | 287 | call <function>mutex_lock_interruptible()</function> to grab the mutex, |
296 | <function>down_interruptible()</function> to grab the semaphore, and | 288 | and <function>mutex_unlock()</function> to release it. There is also a |
297 | <function>up()</function> to release it. There is also a | 289 | <function>mutex_lock()</function>, which should be avoided, because it |
298 | <function>down()</function>, which should be avoided, because it | ||
299 | will not return if a signal is received. | 290 | will not return if a signal is received. |
300 | </para> | 291 | </para> |
301 | 292 | ||
302 | <para> | 293 | <para> |
303 | Example: <filename>linux/net/core/netfilter.c</filename> allows | 294 | Example: <filename>net/netfilter/nf_sockopt.c</filename> allows |
304 | registration of new <function>setsockopt()</function> and | 295 | registration of new <function>setsockopt()</function> and |
305 | <function>getsockopt()</function> calls, with | 296 | <function>getsockopt()</function> calls, with |
306 | <function>nf_register_sockopt()</function>. Registration and | 297 | <function>nf_register_sockopt()</function>. Registration and |
@@ -515,7 +506,7 @@ | |||
515 | <listitem> | 506 | <listitem> |
516 | <para> | 507 | <para> |
517 | If you are in a process context (any syscall) and want to | 508 | If you are in a process context (any syscall) and want to |
518 | lock other process out, use a semaphore. You can take a semaphore | 509 | lock other process out, use a mutex. You can take a mutex |
519 | and sleep (<function>copy_from_user*(</function> or | 510 | and sleep (<function>copy_from_user*(</function> or |
520 | <function>kmalloc(x,GFP_KERNEL)</function>). | 511 | <function>kmalloc(x,GFP_KERNEL)</function>). |
521 | </para> | 512 | </para> |
@@ -662,7 +653,7 @@ | |||
662 | <entry>SLBH</entry> | 653 | <entry>SLBH</entry> |
663 | <entry>SLBH</entry> | 654 | <entry>SLBH</entry> |
664 | <entry>SLBH</entry> | 655 | <entry>SLBH</entry> |
665 | <entry>DI</entry> | 656 | <entry>MLI</entry> |
666 | <entry>None</entry> | 657 | <entry>None</entry> |
667 | </row> | 658 | </row> |
668 | 659 | ||
@@ -692,8 +683,8 @@ | |||
692 | <entry>spin_lock_bh</entry> | 683 | <entry>spin_lock_bh</entry> |
693 | </row> | 684 | </row> |
694 | <row> | 685 | <row> |
695 | <entry>DI</entry> | 686 | <entry>MLI</entry> |
696 | <entry>down_interruptible</entry> | 687 | <entry>mutex_lock_interruptible</entry> |
697 | </row> | 688 | </row> |
698 | 689 | ||
699 | </tbody> | 690 | </tbody> |
@@ -1310,7 +1301,7 @@ as Alan Cox says, <quote>Lock data, not code</quote>. | |||
1310 | <para> | 1301 | <para> |
1311 | There is a coding bug where a piece of code tries to grab a | 1302 | There is a coding bug where a piece of code tries to grab a |
1312 | spinlock twice: it will spin forever, waiting for the lock to | 1303 | spinlock twice: it will spin forever, waiting for the lock to |
1313 | be released (spinlocks, rwlocks and semaphores are not | 1304 | be released (spinlocks, rwlocks and mutexes are not |
1314 | recursive in Linux). This is trivial to diagnose: not a | 1305 | recursive in Linux). This is trivial to diagnose: not a |
1315 | stay-up-five-nights-talk-to-fluffy-code-bunnies kind of | 1306 | stay-up-five-nights-talk-to-fluffy-code-bunnies kind of |
1316 | problem. | 1307 | problem. |
@@ -1335,7 +1326,7 @@ as Alan Cox says, <quote>Lock data, not code</quote>. | |||
1335 | 1326 | ||
1336 | <para> | 1327 | <para> |
1337 | This complete lockup is easy to diagnose: on SMP boxes the | 1328 | This complete lockup is easy to diagnose: on SMP boxes the |
1338 | watchdog timer or compiling with <symbol>DEBUG_SPINLOCKS</symbol> set | 1329 | watchdog timer or compiling with <symbol>DEBUG_SPINLOCK</symbol> set |
1339 | (<filename>include/linux/spinlock.h</filename>) will show this up | 1330 | (<filename>include/linux/spinlock.h</filename>) will show this up |
1340 | immediately when it happens. | 1331 | immediately when it happens. |
1341 | </para> | 1332 | </para> |
@@ -1558,7 +1549,7 @@ the amount of locking which needs to be done. | |||
1558 | <title>Read/Write Lock Variants</title> | 1549 | <title>Read/Write Lock Variants</title> |
1559 | 1550 | ||
1560 | <para> | 1551 | <para> |
1561 | Both spinlocks and semaphores have read/write variants: | 1552 | Both spinlocks and mutexes have read/write variants: |
1562 | <type>rwlock_t</type> and <structname>struct rw_semaphore</structname>. | 1553 | <type>rwlock_t</type> and <structname>struct rw_semaphore</structname>. |
1563 | These divide users into two classes: the readers and the writers. If | 1554 | These divide users into two classes: the readers and the writers. If |
1564 | you are only reading the data, you can get a read lock, but to write to | 1555 | you are only reading the data, you can get a read lock, but to write to |
@@ -1681,7 +1672,7 @@ the amount of locking which needs to be done. | |||
1681 | #include <linux/slab.h> | 1672 | #include <linux/slab.h> |
1682 | #include <linux/string.h> | 1673 | #include <linux/string.h> |
1683 | +#include <linux/rcupdate.h> | 1674 | +#include <linux/rcupdate.h> |
1684 | #include <linux/semaphore.h> | 1675 | #include <linux/mutex.h> |
1685 | #include <asm/errno.h> | 1676 | #include <asm/errno.h> |
1686 | 1677 | ||
1687 | struct object | 1678 | struct object |
@@ -1913,7 +1904,7 @@ machines due to caching. | |||
1913 | </listitem> | 1904 | </listitem> |
1914 | <listitem> | 1905 | <listitem> |
1915 | <para> | 1906 | <para> |
1916 | <function> put_user()</function> | 1907 | <function>put_user()</function> |
1917 | </para> | 1908 | </para> |
1918 | </listitem> | 1909 | </listitem> |
1919 | </itemizedlist> | 1910 | </itemizedlist> |
@@ -1927,13 +1918,13 @@ machines due to caching. | |||
1927 | 1918 | ||
1928 | <listitem> | 1919 | <listitem> |
1929 | <para> | 1920 | <para> |
1930 | <function>down_interruptible()</function> and | 1921 | <function>mutex_lock_interruptible()</function> and |
1931 | <function>down()</function> | 1922 | <function>mutex_lock()</function> |
1932 | </para> | 1923 | </para> |
1933 | <para> | 1924 | <para> |
1934 | There is a <function>down_trylock()</function> which can be | 1925 | There is a <function>mutex_trylock()</function> which can be |
1935 | used inside interrupt context, as it will not sleep. | 1926 | used inside interrupt context, as it will not sleep. |
1936 | <function>up()</function> will also never sleep. | 1927 | <function>mutex_unlock()</function> will also never sleep. |
1937 | </para> | 1928 | </para> |
1938 | </listitem> | 1929 | </listitem> |
1939 | </itemizedlist> | 1930 | </itemizedlist> |
@@ -2023,7 +2014,7 @@ machines due to caching. | |||
2023 | <para> | 2014 | <para> |
2024 | Prior to 2.5, or when <symbol>CONFIG_PREEMPT</symbol> is | 2015 | Prior to 2.5, or when <symbol>CONFIG_PREEMPT</symbol> is |
2025 | unset, processes in user context inside the kernel would not | 2016 | unset, processes in user context inside the kernel would not |
2026 | preempt each other (ie. you had that CPU until you have it up, | 2017 | preempt each other (ie. you had that CPU until you gave it up, |
2027 | except for interrupts). With the addition of | 2018 | except for interrupts). With the addition of |
2028 | <symbol>CONFIG_PREEMPT</symbol> in 2.5.4, this changed: when | 2019 | <symbol>CONFIG_PREEMPT</symbol> in 2.5.4, this changed: when |
2029 | in user context, higher priority tasks can "cut in": spinlocks | 2020 | in user context, higher priority tasks can "cut in": spinlocks |