[PATCH 2/2] manual: Document __libc_single_threaded

[Michael Kerrisk (man-pages)]

On 5/20/20 8:12 PM, Florian Weimer wrote:
> ---
>  manual/threads.texi | 89 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 89 insertions(+)
> 
> diff --git a/manual/threads.texi b/manual/threads.texi
> index a425635179..d4c261a0e9 100644
> --- a/manual/threads.texi
> +++ b/manual/threads.texi
> @@ -627,6 +627,7 @@ the standard.
>  					  threads in a process.
>  * Waiting with Explicit Clocks::          Functions for waiting with an
>                                            explicit clock specification.
> +* Single-Threaded::     Detecting single-threaded execution.
>  @end menu
>  
>  @node Default Thread Attributes
> @@ -771,6 +772,94 @@ 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 can be multi-threaded,

s/can be/may be/

> +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
> +
> +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;inf
> +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 directly, say via a

s/directly/indirectly/ ?

> +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 cannot use
> +@code{__libc_single_threaded} to optimize atomic operations, unless it
> +is able to prove that the memory is not shared.
> +
>  @c FIXME these are undocumented:
>  @c pthread_atfork
>  @c pthread_attr_destroy

Good info!

Thanks,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/