manual: Document __libc_single_threaded

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
Reviewed-by: DJ Delorie <dj@redhat.com>

manual/threads.texi

@@ -628,6 +628,7 @@ the standard.
 * Initial Thread Signal Mask::            Setting the initial mask of threads.
 * Waiting with Explicit Clocks::          Functions for waiting with an
                                           explicit clock specification.
+* Single-Threaded::                       Detecting single-threaded execution.
 * Restartable Sequences::                 Linux-specific Restartable Sequences
                                           integration.
 @end menu
@@ -845,6 +846,118 @@ Behaves like @code{pthread_timedjoin_np} except that the absolute time in
 @var{abstime} is measured against the clock specified by @var{clockid}.
 @end deftypefun
 
+@node Single-Threaded
+@subsubsection Detecting Single-Threaded Execution
+
+Multi-threaded programs require synchronization among threads.  This
+synchronization can be costly even if there is just a single thread
+and no data is shared between multiple processors.  @Theglibc{} offers
+an interface to detect whether the process is in single-threaded mode.
+Applications can use this information to avoid synchronization, for
+example by using regular instructions to load and store memory instead
+of atomic instructions, or using relaxed memory ordering instead of
+stronger memory ordering.
+
+@deftypevar char __libc_single_threaded
+@standards{GNU, sys/single_threaded.h}
+This variable is non-zero if the current process is definitely
+single-threaded.  If it is zero, the process may be multi-threaded,
+or @theglibc{} cannot determine at this point of the program execution
+whether the process is single-threaded or not.
+
+Applications must never write to this variable.
+@end deftypevar
+
+Most applications should perform the same actions whether or not
+@code{__libc_single_threaded} is true, except with less
+synchronization.  If this rule is followed, a process that
+subsequently becomes multi-threaded is already in a consistent state.
+For example, in order to increment a reference count, the following
+code can be used:
+
+@smallexample
+if (__libc_single_threaded)
+  atomic_fetch_add (&reference_count, 1, memory_order_relaxed);
+else
+  atomic_fetch_add (&reference_count, 1, memory_order_acq_rel);
+@end smallexample
+
+@c Note: No memory order on __libc_single_threaded.  The
+@c implementation must ensure that exit of the critical
+@c (second-to-last) thread happens-before setting
+@c __libc_single_threaded to true.  Otherwise, acquire MO might be
+@c needed for reading the variable in some scenarios, and that would
+@c completely defeat its purpose.
+
+This still requires some form of synchronization on the
+single-threaded branch, so it can be beneficial not to declare the
+reference count as @code{_Atomic}, and use the GCC @code{__atomic}
+built-ins.  @xref{__atomic Builtins,, Built-in Functions for Memory
+Model Aware Atomic Operations, gcc, Using the GNU Compiler Collection
+(GCC)}.  Then the code to increment a reference count looks like this:
+
+@smallexample
+if (__libc_single_threaded)
+  ++refeference_count;
+else
+  __atomic_fetch_add (&reference_count, 1, __ATOMIC_ACQ_REL);
+@end smallexample
+
+(Depending on the data associated with the reference count, it may be
+possible to use the weaker @code{__ATOMIC_RELAXED} memory ordering on
+the multi-threaded branch.)
+
+Several functions in @theglibc{} can change the value of the
+@code{__libc_single_threaded} variable.  For example, creating new
+threads using the @code{pthread_create} or @code{thrd_create} function
+sets the variable to false.  This can also happen indirectly, say via
+a call to @code{dlopen}.  Therefore, applications need to make a copy
+of the value of @code{__libc_single_threaded} if after such a function
+call, behavior must match the value as it was before the call, like
+this:
+
+@smallexample
+bool single_threaded = __libc_single_threaded;
+if (single_threaded)
+  prepare_single_threaded ();
+else
+  prepare_multi_thread ();
+
+void *handle = dlopen (shared_library_name, RTLD_NOW);
+lookup_symbols (handle);
+
+if (single_threaded)
+  cleanup_single_threaded ();
+else
+  cleanup_multi_thread ();
+@end smallexample
+
+Since the value of @code{__libc_single_threaded} can change from true
+to false during the execution of the program, it is not useful for
+selecting optimized function implementations in IFUNC resolvers.
+
+Atomic operations can also be used on mappings shared among
+single-threaded processes.  This means that a compiler must not use
+@code{__libc_single_threaded} to optimize atomic operations, unless it
+is able to prove that the memory is not shared.
+
+@strong{Implementation Note:} The @code{__libc_single_threaded}
+variable is not declared as @code{volatile} because it is expected
+that compilers optimize a sequence of single-threaded checks into one
+check, for example if several reference counts are updated.  The
+current implementation in @theglibc{} does not set the
+@code{__libc_single_threaded} variable to a true value if a process
+turns single-threaded again.  Future versions of @theglibc{} may do
+this, but only as the result of function calls which imply an acquire
+(compiler) barrier.  (Some compilers assume that well-known functions
+such as @code{malloc} do not write to global variables, and setting
+@code{__libc_single_threaded} would introduce a data race and
+undefined behavior.)  In any case, an application must not write to
+@code{__libc_single_threaded} even if it has joined the last
+application-created thread because future versions of @theglibc{} may
+create background threads after the first thread has been created, and
+the application has no way of knowning that these threads are present.
+
 @node Restartable Sequences
 @subsubsection Restartable Sequences