diff --git a/share/man/man9/rmlock.9 b/share/man/man9/rmlock.9 index 893eb2534f05..78083458072b 100644 --- a/share/man/man9/rmlock.9 +++ b/share/man/man9/rmlock.9 @@ -26,7 +26,7 @@ .\" $FreeBSD$ .\" .\" Based on rwlock.9 man page -.Dd November 16, 2011 +.Dd June 8, 2012 .Dt RMLOCK 9 .Os .Sh NAME @@ -41,7 +41,7 @@ .Nm rm_wunlock , .Nm rm_wowned , .Nm RM_SYSINIT -.Nd kernel reader/writer lock optimized for mostly read access patterns +.Nd kernel reader/writer lock optimized for read-mostly access patterns .Sh SYNOPSIS .In sys/param.h .In sys/lock.h @@ -67,7 +67,7 @@ .In sys/kernel.h .Fn RM_SYSINIT "name" "struct rmlock *rm" "const char *desc" "int opts" .Sh DESCRIPTION -Mostly reader locks allow shared access to protected data by multiple threads, +Read-mostly locks allow shared access to protected data by multiple threads, or exclusive access by a single thread. The threads with shared access are known as .Em readers @@ -76,83 +76,82 @@ A thread with exclusive access is known as a .Em writer since it can modify protected data. .Pp -Read mostly locks are designed to be efficient for locks almost exclusively +Read-mostly locks are designed to be efficient for locks almost exclusively used as reader locks and as such should be used for protecting data that rarely changes. -Acquiring an exclusive lock after the lock had been locked for shared access +Acquiring an exclusive lock after the lock has been locked for shared access is an expensive operation. .Pp -Although reader/writer locks look very similar to -.Xr sx 9 -locks, their usage pattern is different. -Reader/writer locks can be treated as mutexes (see -.Xr mutex 9 ) -with shared/exclusive semantics unless initialized with -.Dv RM_SLEEPABLE . +Normal read-mostly locks are similar to +.Xr rwlock 9 +locks and follow the same lock ordering rules as +.Xr rwlock 9 +locks. +Read-mostly locks have full priority propagation like mutexes. Unlike -.Xr sx 9 , -an -.Nm -can be locked while holding a non-spin mutex, and an -.Nm -cannot be held while sleeping, again unless initialized with -.Dv RM_SLEEPABLE . -The -.Nm -locks have full priority propagation like mutexes. -The +.Xr rwlock 9 , +read-mostly locks propagate priority to both readers and writers. +This is implemented via the .Va rm_priotracker -structure argument supplied in +structure argument supplied to .Fn rm_rlock and -.Fn rm_runlock -is used to keep track of the read owner(s). -Another important property is that shared holders of -.Nm -can recurse if the lock has been initialized with the -.Dv LO_RECURSABLE -option, however exclusive locks are not allowed to recurse. +.Fn rm_runlock . +Readers can recurse if the lock is initialized with the +.Dv RM_RECURSE +option; +however, writers are never allowed to recurse. +.Pp +Sleepable read-mostly locks are created by passing +.Dv RM_SLEEPABLE +to +.Fn rm_init_flags . +Unlike normal read-mostly locks, +sleepable read-mostly locks follow the same lock ordering rules as +.Xr sx 9 +locks. +Sleepable read-mostly locks do not propagate priority to writers, +but they do propagate priority to readers. +Writers are permitted to sleep while holding a read-mostly lock, +but readers are not. +Unlike other sleepable locks such as +.Xr sx 9 +locks, +readers must use try operations on other sleepable locks to avoid sleeping. .Ss Macros and Functions .Bl -tag -width indent .It Fn rm_init "struct rmlock *rm" "const char *name" -Initialize structure located at -.Fa rm -as mostly reader lock, described by -.Fa name . -The name description is used solely for debugging purposes. +Initialize the read-mostly lock +.Fa rm . +The +.Fa name +description is used solely for debugging purposes. This function must be called before any other operations on the lock. .It Fn rm_init_flags "struct rmlock *rm" "const char *name" "int opts" -Initialize the rm lock just like the -.Fn rm_init -function, but specifying a set of optional flags to alter the -behaviour of -.Fa rm , -through the +Similar to +.Fn rm_init , +initialize the read-mostly lock +.Fa rm +with a set of optional flags. +The .Fa opts -argument. -It contains one or more of the following flags: +arguments contains one or more of the following flags: .Bl -tag -width ".Dv RM_NOWITNESS" .It Dv RM_NOWITNESS Instruct .Xr witness 4 to ignore this lock. .It Dv RM_RECURSE -Allow threads to recursively acquire exclusive locks for +Allow threads to recursively acquire shared locks for .Fa rm . .It Dv RM_SLEEPABLE -Allow writers to sleep while holding the lock. -Readers must not sleep while holding the lock and can avoid to sleep on -taking the lock by using -.Fn rm_try_rlock -instead of -.Fn rm_rlock . +Create a sleepable read-mostly lock. .El .It Fn rm_rlock "struct rmlock *rm" "struct rm_priotracker* tracker" Lock .Fa rm -as a reader. -Using +as a reader using .Fa tracker to track read owners of a lock for priority propagation. This data structure is only used internally by @@ -161,28 +160,32 @@ and must persist until .Fn rm_runlock has been called. This data structure can be allocated on the stack since -rmlocks cannot be held while sleeping. +readers cannot sleep. If any thread holds this lock exclusively, the current thread blocks, and its priority is propagated to the exclusive holder. If the lock was initialized with the -.Dv LO_RECURSABLE +.Dv RM_RECURSE option the .Fn rm_rlock -function can be called when the thread has already acquired reader +function can be called when the current thread has already acquired reader access on .Fa rm . -This is called -.Dq "recursing on a lock" . .It Fn rm_try_rlock "struct rmlock *rm" "struct rm_priotracker* tracker" Try to lock .Fa rm as a reader. .Fn rm_try_rlock will return 0 if the lock cannot be acquired immediately; -otherwise the lock will be acquired and a non-zero value will be returned. +otherwise, +the lock will be acquired and a non-zero value will be returned. Note that .Fn rm_try_rlock may fail even while the lock is not currently held by a writer. +If the lock was initialized with the +.Dv RM_RECURSE +option, +.Fn rm_try_rlock +will succeed if the current thread has already acquired reader access. .It Fn rm_wlock "struct rmlock *rm" Lock .Fa rm