[PATCH v2] Single threaded stdio optimization

[Szabolcs Nagy]

On 21/06/17 17:52, Joseph Myers wrote:
> On Wed, 21 Jun 2017, Szabolcs Nagy wrote:
> 
>> created.  A new libc abi symbol, _IO_enable_locks, does this.
>> (The abilist files will have to be updated in a separate patch).
> 
> I don't see any declarations or uses of this symbol in public headers, so 
> why does it need to be at a public symbol version instead of 
> GLIBC_PRIVATE?
> 

PATCH v3: use GLIBC_PRIVATE for _IO_enable_locks.
(should i use __libc_* naming convention for private symbols?)

Locking overhead can be significant in some stdio operations
that are common in single threaded applications.

I'd like to address it in this release if possible as it
causes performance problems to aarch64 users.  I prefer this
high-level approach to be reviewed, if that does not work then
the aarch64 specific low-level approach will be taken.

This patch adds the _IO_FLAGS2_NEED_LOCK flag to indicate if
an _IO_FILE object needs to be locked and some of the stdio
functions just jump to their _unlocked variant when not.  The
flag is set on all _IO_FILE objects when the first thread is
created.  A new GLIBC_PRIVATE libc symbol, _IO_enable_locks,
was added to do this from libpthread.

The optimization can be applied to more stdio functions,
currently it is only applied to single flag check or single
non-wide-char standard operations.  The flag should probably
be never set for files with _IO_USER_LOCK, but that's just a
further optimization, not a correctness requirement.

The optimization is valid in a single thread because stdio
operations are non-as-safe (so lock state is not observable
from a signal handler) and stdio locks are recursive (so lock
state is not observable via deadlock).  The optimization is not
valid if a thread may be created while an stdio lock is taken
and thus it should be disabled if any user code may run during
an stdio operation (interposed malloc, printf hooks, etc).
This makes the optimization more complicated for some stdio
operations (e.g. printf), but those are bigger and thus less
important to optimize so this patch does not try to do that.

2017-06-22  Szabolcs Nagy  <szabolcs.nagy@arm.com>

	* libio/libio.h (_IO_FLAGS2_NEED_LOCK, _IO_need_lock): Define.
	* libio/libioP.h (_IO_enable_locks): Declare.
	* libio/Versions (_IO_enable_locks): New symbol.
	* libio/genops.c (_IO_enable_locks): Define.
	(_IO_old_init): Initialize flags2.
	* libio/feof.c.c (_IO_feof): Avoid locking when not needed.
	* libio/ferror.c (_IO_ferror): Likewise.
	* libio/fputc.c (fputc): Likewise.
	* libio/putc.c (_IO_putc): Likewise.
	* libio/getc.c (_IO_getc): Likewise.
	* libio/getchar.c (getchar): Likewise.
	* libio/ioungetc.c (_IO_ungetc): Likewise.
	* nptl/pthread_create.c (__pthread_create_2_1): Enable stdio locks.
	* libio/iofopncook.c (_IO_fopencookie): Enable locking for the file.
	* sysdeps/pthread/flockfile.c (__flockfile): Likewise.
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