Add description of bounded sleep vs unbounded sleep (aka blocking). Move
rules into their own section.
This commit is contained in:
Edward Tomasz Napierala
parent
fbbd6fb28c
commit
561205dff6
Notes:
svn2git
2020-12-20 02:59:44 +00:00
svn path=/head/; revision=203127
@ -73,38 +73,15 @@ code.
|
||||
Mutexes are the most commonly used synchronization primitive in the kernel.
|
||||
Thread acquires (locks) a mutex before accessing data shared with other
|
||||
threads (including interrupt threads), and releases (unlocks) it afterwards.
|
||||
If the mutex cannot be acquired, the thread requesting it will block.
|
||||
.Pp
|
||||
Sleeping while holding mutex is generally prohibited.
|
||||
You may only call the
|
||||
.Xr sleep 9
|
||||
call via
|
||||
.Fn msleep
|
||||
or the new
|
||||
.Fn mtx_sleep
|
||||
variant.
|
||||
These will atomically drop the mutex and reacquire it
|
||||
as part of waking up.
|
||||
This is often however a
|
||||
.Em BAD
|
||||
idea because it generally relies on you having
|
||||
such a good knowledge of all the call graph above you
|
||||
and what assumptions it is making that there are a lot
|
||||
of ways to make hard-to-find mistakes.
|
||||
For example you MUST re-test all the assumptions you made before,
|
||||
all the way up the call graph to where you got the lock.
|
||||
You can not just assume that mtx_sleep can be inserted anywhere.
|
||||
If any caller above you has any mutex or
|
||||
rwlock, your sleep, will cause a panic.
|
||||
If the sleep only happens rarely it may be years before the
|
||||
bad code path is found.
|
||||
If the mutex cannot be acquired, the thread requesting it will sleep.
|
||||
Mutexes fully support priority propagation.
|
||||
.Pp
|
||||
See the
|
||||
.Xr mutex 9
|
||||
page for more information.
|
||||
.Ss Spin mutexes
|
||||
Spin mutexes are variation of basic mutexes; the main difference between
|
||||
the two is that spin mutexes never block - instead, they spin, waiting
|
||||
the two is that spin mutexes never sleep - instead, they spin, waiting
|
||||
for the thread holding the lock, which runs on another CPU, to release it.
|
||||
Differently from ordinary mutex, spin mutexes disable interrupts when acquired.
|
||||
Since disabling interrupts is expensive, they are also generally slower.
|
||||
@ -136,23 +113,12 @@ A thread with exclusive access is known as a
|
||||
.Em writer
|
||||
since it may modify protected data.
|
||||
.Pp
|
||||
Although reader/writer locks look very similar to
|
||||
.Xr sx 9
|
||||
(see below) locks, their usage pattern is different.
|
||||
Reader/writer locks can be treated as mutexes (see above and
|
||||
.Xr mutex 9 )
|
||||
with shared/exclusive semantics.
|
||||
More specifically, regular mutexes can be
|
||||
considered to be equivalent to a write-lock on an
|
||||
.Em rw_lock.
|
||||
In the future this may in fact
|
||||
become literally the fact.
|
||||
An
|
||||
.Em rw_lock
|
||||
can be locked while holding a regular mutex, but
|
||||
can
|
||||
.Em not
|
||||
be held while sleeping.
|
||||
The
|
||||
.Em rw_lock
|
||||
locks have priority propagation like mutexes, but priority
|
||||
@ -164,19 +130,15 @@ Another important property is that shared holders of
|
||||
can recurse, but exclusive locks are not allowed to recurse.
|
||||
This ability should not be used lightly and
|
||||
.Em may go away.
|
||||
Users of recursion in any locks should be prepared to
|
||||
defend their decision against vigorous criticism.
|
||||
.Pp
|
||||
See the
|
||||
.Xr rwlock 9
|
||||
page for more information.
|
||||
.Ss Read-mostly locks
|
||||
Mostly reader locks are similar to
|
||||
.Em Reader/write
|
||||
locks but optimized for very infrequent
|
||||
.Em writer
|
||||
locking.
|
||||
.Em rm_lock
|
||||
.Em reader/writer
|
||||
locks but optimized for very infrequent write locking.
|
||||
.Em Read-mostly
|
||||
locks implement full priority propagation by tracking shared owners
|
||||
using a lock user supplied
|
||||
.Em tracker
|
||||
@ -186,27 +148,12 @@ See the
|
||||
.Xr rmlock 9
|
||||
page for more information.
|
||||
.Ss Shared/exclusive locks
|
||||
Shared/exclusive locks are used to protect data that are read far more often
|
||||
than they are written.
|
||||
Mutexes are inherently more efficient than shared/exclusive locks, so
|
||||
shared/exclusive locks should be used prudently.
|
||||
The main reason for using an
|
||||
.Em sx_lock
|
||||
is that a thread may hold a shared or exclusive lock on an
|
||||
.Em sx_lock
|
||||
lock while sleeping.
|
||||
As a consequence of this however, an
|
||||
.Em sx_lock
|
||||
lock may not be acquired while holding a mutex.
|
||||
The reason for this is that, if one thread slept while holding an
|
||||
.Em sx_lock
|
||||
lock while another thread blocked on the same
|
||||
.Em sx_lock
|
||||
lock after acquiring a mutex, then the second thread would effectively
|
||||
end up sleeping while holding a mutex, which is not allowed.
|
||||
The
|
||||
.Em sx_lock
|
||||
should be considered to be closely related to
|
||||
Shared/exclusive locks are similar to reader/writer locks; the main difference
|
||||
between them is that shared/exclusive locks may be held during unbounded sleep
|
||||
(and may thus perform an unbounded sleep).
|
||||
They are inherently less efficient than mutexes, reader/writer locks
|
||||
and read-mostly locks. They don't support priority propagation.
|
||||
They should be considered to be closely related to
|
||||
.Xr sleep 9 .
|
||||
In fact it could in some cases be
|
||||
considered a conditional sleep.
|
||||
@ -239,8 +186,7 @@ See the
|
||||
.Xr condvar 9
|
||||
page for more information.
|
||||
.Ss Giant
|
||||
Giant is a special instance of a sleep lock.
|
||||
It has several special characteristics.
|
||||
Giant is an instance of a mutex, with some special characteristics:
|
||||
.Bl -enum
|
||||
.It
|
||||
It is recursive.
|
||||
@ -255,7 +201,7 @@ Giant must be locked first before other locks.
|
||||
There are places in the kernel that drop Giant and pick it back up
|
||||
again.
|
||||
Sleep locks will do this before sleeping.
|
||||
Parts of the Network or VM code may do this as well, depending on the
|
||||
Parts of the network or VM code may do this as well, depending on the
|
||||
setting of a sysctl.
|
||||
This means that you cannot count on Giant keeping other code from
|
||||
running if your code sleeps, even if you want it to.
|
||||
@ -321,7 +267,7 @@ See the
|
||||
page for more information.
|
||||
.Pp
|
||||
.Ss Lockmanager locks
|
||||
Shared/exclusive sleep locks, used mostly in
|
||||
Shared/exclusive locks, used mostly in
|
||||
.Xr VFS 9 ,
|
||||
in particular as a
|
||||
.Xr vnode 9
|
||||
@ -334,19 +280,54 @@ See the
|
||||
.Xr lock 9
|
||||
page for more information.
|
||||
.Sh INTERACTIONS
|
||||
.Ss Interaction table.
|
||||
.Ss Bounded vs. unbounded sleep
|
||||
The following primitives perform bounded sleep: mutexes, pool mutexes,
|
||||
reader/writer locks and read-mostly locks.
|
||||
.Pp
|
||||
The following primitives block (perform unbounded sleep): shared/exclusive locks,
|
||||
counting semaphores, condition variables, sleep/wakeup and lockmanager locks.
|
||||
.Pp
|
||||
It is an error to do any operation that could result in any kind of sleep while
|
||||
holding spin mutex.
|
||||
.Pp
|
||||
As a general rule, it is an error to do any operation that could result
|
||||
in unbounded sleep while holding any primitive from the 'bounded sleep' group.
|
||||
For example, it is an error to try to acquire shared/exclusive lock while
|
||||
holding mutex, or to try to allocate memory with M_WAITOK while holding
|
||||
read-write lock.
|
||||
.Pp
|
||||
As a special case, it is possible to call
|
||||
.Fn sleep 9
|
||||
or
|
||||
.Fn mtx_sleep 9
|
||||
while holding a mutex.
|
||||
It will atomically drop the mutex and reacquire it
|
||||
as part of waking up.
|
||||
This is often however a bad
|
||||
idea because it generally relies on you having
|
||||
such a good knowledge of all the call graph above you
|
||||
and what assumptions it is making that there are a lot
|
||||
of ways to make hard-to-find mistakes.
|
||||
For example you must re-test all the assumptions you made before,
|
||||
all the way up the call graph to where you got the lock.
|
||||
You can not just assume that mtx_sleep can be inserted anywhere.
|
||||
If any caller above you has any mutex or
|
||||
rwlock, your sleep, will cause a panic.
|
||||
If the sleep only happens rarely it may be years before the
|
||||
bad code path is found.
|
||||
.Ss Interaction table
|
||||
The following table shows what you can and can not do if you hold
|
||||
one of the synchronization primitives discussed here:
|
||||
(someone who knows what they are talking about should write this table)
|
||||
.Bl -column ".Ic xxxxxxxxxxxxxxxxxxxx" ".Xr XXXXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXXX" ".Xr XXXXX" -offset indent
|
||||
.Bl -column ".Ic xxxxxxxxxxxxxxxxxxx" ".Xr XXXXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXX" -offset indent
|
||||
.It Xo
|
||||
.Em "You have: You want:" Ta Spin_mtx Ta Slp_mtx Ta sx_lock Ta rw_lock Ta rm_lock Ta sleep
|
||||
.Em "You have: You want:" Ta spin mtx Ta mutex Ta sx Ta rwlock Ta rmlock Ta sleep
|
||||
.Xc
|
||||
.It Ic SPIN mutex Ta \&ok-1 Ta \&no Ta \&no Ta \&no Ta \&no Ta \&no-3
|
||||
.It Ic Sleep mutex Ta \&ok Ta \&ok-1 Ta \&no Ta \&ok Ta \&ok Ta \&no-3
|
||||
.It Ic sx_lock Ta \&ok Ta \&ok Ta \&ok-2 Ta \&ok Ta \&ok Ta \&ok-4
|
||||
.It Ic rw_lock Ta \&ok Ta \&ok Ta \&no Ta \&ok-2 Ta \&ok Ta \&no-3
|
||||
.It Ic rm_lock Ta \&ok Ta \&ok Ta \&no Ta \&ok Ta \&ok-2 Ta \&no
|
||||
.It Ic spin mtx Ta \&ok-1 Ta \&no Ta \&no Ta \&no Ta \&no Ta \&no-3
|
||||
.It Ic mutex Ta \&ok Ta \&ok-1 Ta \&no Ta \&ok Ta \&ok Ta \&no-3
|
||||
.It Ic sxlock Ta \&ok Ta \&ok Ta \&ok-2 Ta \&ok Ta \&ok Ta \&ok-4
|
||||
.It Ic rwlock Ta \&ok Ta \&ok Ta \&no Ta \&ok-2 Ta \&ok Ta \&no-3
|
||||
.It Ic rmlock Ta \&ok Ta \&ok Ta \&no Ta \&ok Ta \&ok-2 Ta \&no
|
||||
.El
|
||||
.Pp
|
||||
.Em *1
|
||||
@ -371,12 +352,12 @@ Though one can sleep holding an sx lock, one can also use
|
||||
.Fn sx_sleep
|
||||
which atomically release this primitive when going to sleep and
|
||||
reacquire it on wakeup.
|
||||
.Ss Context mode table.
|
||||
.Ss Context mode table
|
||||
The next table shows what can be used in different contexts.
|
||||
At this time this is a rather easy to remember table.
|
||||
.Bl -column ".Ic Xxxxxxxxxxxxxxxxxxxx" ".Xr XXXXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXXX" ".Xr XXXXX" -offset indent
|
||||
.Bl -column ".Ic Xxxxxxxxxxxxxxxxxxx" ".Xr XXXXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXX" -offset indent
|
||||
.It Xo
|
||||
.Em "Context:" Ta Spin_mtx Ta Slp_mtx Ta sx_lock Ta rw_lock Ta rm_lock Ta sleep
|
||||
.Em "Context:" Ta spin mtx Ta mutex Ta sx Ta rwlock Ta rmlock Ta sleep
|
||||
.Xc
|
||||
.It interrupt: Ta \&ok Ta \&no Ta \&no Ta \&no Ta \&no Ta \&no
|
||||
.It idle: Ta \&ok Ta \&no Ta \&no Ta \&no Ta \&no Ta \&no
|
||||
|
||||
Loading…
Reference in New Issue
Block a user