eventfd: make eventfd_signal{_mask}() void

No caller care about the return value.

Link: https://lore.kernel.org/r/[email protected]
Reviewed-by: Jan Kara <jac

eventfd: make eventfd_signal{_mask}() void

No caller care about the return value.

Link: https://lore.kernel.org/r/[email protected]
Reviewed-by: Jan Kara <[email protected]>
Reviewed-by: Jens Axboe <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

show more ...

eventfd: simplify eventfd_signal_mask()

The eventfd_signal_mask() helper was introduced for io_uring and similar
to eventfd_signal() it always passed 1 for @n. So don't bother with that
argument at

eventfd: simplify eventfd_signal_mask()

The eventfd_signal_mask() helper was introduced for io_uring and similar
to eventfd_signal() it always passed 1 for @n. So don't bother with that
argument at all.

Link: https://lore.kernel.org/r/[email protected]
Reviewed-by: Jan Kara <[email protected]>
Reviewed-by: Jens Axboe <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

show more ...

eventfd: simplify eventfd_signal()

Ever since the eventfd type was introduced back in 2007 in commit
e1ad7468c77d ("signal/timer/event: eventfd core") the eventfd_signal()
function only ever passed

eventfd: simplify eventfd_signal()

Ever since the eventfd type was introduced back in 2007 in commit
e1ad7468c77d ("signal/timer/event: eventfd core") the eventfd_signal()
function only ever passed 1 as a value for @n. There's no point in
keeping that additional argument.

Link: https://lore.kernel.org/r/[email protected]
Acked-by: Xu Yilun <[email protected]>
Acked-by: Andrew Donnellan <[email protected]> # ocxl
Acked-by: Eric Farman <[email protected]> # s390
Reviewed-by: Jan Kara <[email protected]>
Reviewed-by: Jens Axboe <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

show more ...

eventfd: add a uapi header for eventfd userspace APIs

Create a uapi header include/uapi/linux/eventfd.h, move the associated
flags to the uapi header, and include it from linux/eventfd.h.

Suggested

eventfd: add a uapi header for eventfd userspace APIs

Create a uapi header include/uapi/linux/eventfd.h, move the associated
flags to the uapi header, and include it from linux/eventfd.h.

Suggested-by: Christian Brauner <[email protected]>
Signed-off-by: Wen Yang <[email protected]>
Reviewed-by: Matthew Wilcox (Oracle) <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Dylan Yudaken <[email protected]>
Cc: David Woodhouse <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Eric Biggers <[email protected]>
Cc: [email protected]
Cc: [email protected]
Message-Id: <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

show more ...

fs: use correct __poll_t type

Fix the following sparse warnings by using __poll_t instead
of unsigned type.

fs/eventpoll.c:541:9: sparse: warning: restricted __poll_t degrades to integer
fs/eventfd

fs: use correct __poll_t type

Fix the following sparse warnings by using __poll_t instead
of unsigned type.

fs/eventpoll.c:541:9: sparse: warning: restricted __poll_t degrades to integer
fs/eventfd.c:67:17: sparse: warning: restricted __poll_t degrades to integer

Signed-off-by: Min-Hua Chen <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

show more ...

eventfd: change int to __u64 in eventfd_signal() ifndef CONFIG_EVENTFD

Commit ee62c6b2dc93 ("eventfd: change int to __u64 in eventfd_signal()")
forgot to change int to __u64 in the CONFIG_EVENTFD=n

eventfd: change int to __u64 in eventfd_signal() ifndef CONFIG_EVENTFD

Commit ee62c6b2dc93 ("eventfd: change int to __u64 in eventfd_signal()")
forgot to change int to __u64 in the CONFIG_EVENTFD=n stub function.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: ee62c6b2dc93 ("eventfd: change int to __u64 in eventfd_signal()")
Signed-off-by: Zhang Qilong <[email protected]>
Cc: Dylan Yudaken <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Sha Zhengju <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

show more ...

eventfd: provide a eventfd_signal_mask() helper

This is identical to eventfd_signal(), but it allows the caller to pass
in a mask to be used for the poll wakeup key. The use case is avoiding
repeate

eventfd: provide a eventfd_signal_mask() helper

This is identical to eventfd_signal(), but it allows the caller to pass
in a mask to be used for the poll wakeup key. The use case is avoiding
repeated multishot triggers if we have a dependency between eventfd and
io_uring.

If we setup an eventfd context and register that as the io_uring eventfd,
and at the same time queue a multishot poll request for the eventfd
context, then any CQE posted will repeatedly trigger the multishot request
until it terminates when the CQ ring overflows.

In preparation for io_uring detecting this circular dependency, add the
mentioned helper so that io_uring can pass in EPOLL_URING as part of the
poll wakeup key.

Cc: [email protected] # 6.0
[axboe: fold in !CONFIG_EVENTFD fix from Zhang Qilong]
Signed-off-by: Jens Axboe <[email protected]>

show more ...

eventfd: guard wake_up in eventfd fs calls as well

Guard wakeups that the user can trigger, and that may end up triggering a
call back into eventfd_signal. This is in addition to the current approac

eventfd: guard wake_up in eventfd fs calls as well

Guard wakeups that the user can trigger, and that may end up triggering a
call back into eventfd_signal. This is in addition to the current approach
that only guards in eventfd_signal.

Rename in_eventfd_signal -> in_eventfd at the same time to reflect this.

Without this there would be a deadlock in the following code using libaio:

int main()
{
struct io_context *ctx = NULL;
struct iocb iocb;
struct iocb *iocbs[] = { &iocb };
int evfd;
uint64_t val = 1;

evfd = eventfd(0, EFD_CLOEXEC);
assert(!io_setup(2, &ctx));
io_prep_poll(&iocb, evfd, POLLIN);
io_set_eventfd(&iocb, evfd);
assert(1 == io_submit(ctx, 1, iocbs));
write(evfd, &val, 8);
}

Signed-off-by: Dylan Yudaken <[email protected]>
Reviewed-by: Jens Axboe <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

show more ...

eventfd: Make signal recursion protection a task bit

The recursion protection for eventfd_signal() is based on a per CPU
variable and relies on the !RT semantics of spin_lock_irqsave() for
protectin

eventfd: Make signal recursion protection a task bit

The recursion protection for eventfd_signal() is based on a per CPU
variable and relies on the !RT semantics of spin_lock_irqsave() for
protecting this per CPU variable. On RT kernels spin_lock_irqsave() neither
disables preemption nor interrupts which allows the spin lock held section
to be preempted. If the preempting task invokes eventfd_signal() as well,
then the recursion warning triggers.

Paolo suggested to protect the per CPU variable with a local lock, but
that's heavyweight and actually not necessary. The goal of this protection
is to prevent the task stack from overflowing, which can be achieved with a
per task recursion protection as well.

Replace the per CPU variable with a per task bit similar to other recursion
protection bits like task_struct::in_page_owner. This works on both !RT and
RT kernels and removes as a side effect the extra per CPU storage.

No functional change for !RT kernels.

Reported-by: Daniel Bristot de Oliveira <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Tested-by: Daniel Bristot de Oliveira <[email protected]>
Acked-by: Jason Wang <[email protected]>
Cc: Al Viro <[email protected]>
Link: https://lore.kernel.org/r/87wnp9idso.ffs@tglx

show more ...

eventfd: Export eventfd_ctx_do_read()

Where events are consumed in the kernel, for example by KVM's
irqfd_wakeup() and VFIO's virqfd_wakeup(), they currently lack a
mechanism to drain the eventfd's

eventfd: Export eventfd_ctx_do_read()

Where events are consumed in the kernel, for example by KVM's
irqfd_wakeup() and VFIO's virqfd_wakeup(), they currently lack a
mechanism to drain the eventfd's counter.

Since the wait queue is already locked while the wakeup functions are
invoked, all they really need to do is call eventfd_ctx_do_read().

Add a check for the lock, and export it for them.

Signed-off-by: David Woodhouse <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

show more ...

eventfd: track eventfd_signal() recursion depth

eventfd use cases from aio and io_uring can deadlock due to circular
or resursive calling, when eventfd_signal() tries to grab the waitqueue
lock. On

eventfd: track eventfd_signal() recursion depth

eventfd use cases from aio and io_uring can deadlock due to circular
or resursive calling, when eventfd_signal() tries to grab the waitqueue
lock. On top of that, it's also possible to construct notification
chains that are deep enough that we could blow the stack.

Add a percpu counter that tracks the percpu recursion depth, warn if we
exceed it. The counter is also exposed so that users of eventfd_signal()
can do the right thing if it's non-zero in the context where it is
called.

Cc: [email protected] # 4.19+
Signed-off-by: Jens Axboe <[email protected]>

show more ...

include/linux/eventfd.h: include linux/errno.h

The new gasket staging driver ran into a randconfig build failure when
CONFIG_EVENTFD is disabled:

In file included from drivers/staging/gasket/gask

include/linux/eventfd.h: include linux/errno.h

The new gasket staging driver ran into a randconfig build failure when
CONFIG_EVENTFD is disabled:

In file included from drivers/staging/gasket/gasket_interrupt.h:11,
from drivers/staging/gasket/gasket_interrupt.c:4:
include/linux/eventfd.h: In function 'eventfd_ctx_fdget':
include/linux/eventfd.h:51:9: error: implicit declaration of function 'ERR_PTR' [-Werror=implicit-function-declaration]

I can't see anything wrong with including eventfd.h before err.h, so the
easiest fix is to make it possible to do this by including the file
where it is needed.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: 9a69f5087ccc ("drivers/staging: Gasket driver framework + Apex driver")
Signed-off-by: Arnd Bergmann <[email protected]>
Cc: Eric Biggers <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

show more ...

eventfd: fold eventfd_ctx_get() into eventfd_ctx_fileget()

eventfd_ctx_get() is not used outside of eventfd.c, so unexport it and
fold it into eventfd_ctx_fileget().

(eventfd_ctx_get() was apparent

eventfd: fold eventfd_ctx_get() into eventfd_ctx_fileget()

eventfd_ctx_get() is not used outside of eventfd.c, so unexport it and
fold it into eventfd_ctx_fileget().

(eventfd_ctx_get() was apparently added years ago for KVM irqfd's, but
was never used.)

Signed-off-by: Eric Biggers <[email protected]>
Signed-off-by: Al Viro <[email protected]>

show more ...

eventfd: fold eventfd_ctx_read() into eventfd_read()

eventfd_ctx_read() is not used outside of eventfd.c, so unexport it and
fold it into eventfd_read(). This slightly simplifies the code and
makes

eventfd: fold eventfd_ctx_read() into eventfd_read()

eventfd_ctx_read() is not used outside of eventfd.c, so unexport it and
fold it into eventfd_read(). This slightly simplifies the code and
makes it more analogous to eventfd_write().

(eventfd_ctx_read() was apparently added years ago for KVM irqfd's, but
was never used.)

Signed-off-by: Eric Biggers <[email protected]>
Signed-off-by: Al Viro <[email protected]>

show more ...

eventfd: convert to use anon_inode_getfd()

Nothing actually calls eventfd_file_create() besides the eventfd2()
system call itself. So simplify things by folding it into the system
call and using an

eventfd: convert to use anon_inode_getfd()

Nothing actually calls eventfd_file_create() besides the eventfd2()
system call itself. So simplify things by folding it into the system
call and using anon_inode_getfd() instead of anon_inode_getfile(). This
removes over 40 lines with no change in functionality.

(eventfd_file_create() was apparently added years ago for KVM irqfd's,
but was never used.)

Signed-off-by: Eric Biggers <[email protected]>
Signed-off-by: Al Viro <[email protected]>

show more ...

License cleanup: add SPDX GPL-2.0 license identifier to files with no license

Many source files in the tree are missing licensing information, which
makes it harder for compliance tools to determine

License cleanup: add SPDX GPL-2.0 license identifier to files with no license

Many source files in the tree are missing licensing information, which
makes it harder for compliance tools to determine the correct license.

By default all files without license information are under the default
license of the kernel, which is GPL version 2.

Update the files which contain no license information with the 'GPL-2.0'
SPDX license identifier. The SPDX identifier is a legally binding
shorthand, which can be used instead of the full boiler plate text.

This patch is based on work done by Thomas Gleixner and Kate Stewart and
Philippe Ombredanne.

How this work was done:

Patches were generated and checked against linux-4.14-rc6 for a subset of
the use cases:
- file had no licensing information it it.
- file was a */uapi/* one with no licensing information in it,
- file was a */uapi/* one with existing licensing information,

Further patches will be generated in subsequent months to fix up cases
where non-standard license headers were used, and references to license
had to be inferred by heuristics based on keywords.

The analysis to determine which SPDX License Identifier to be applied to
a file was done in a spreadsheet of side by side results from of the
output of two independent scanners (ScanCode & Windriver) producing SPDX
tag:value files created by Philippe Ombredanne. Philippe prepared the
base worksheet, and did an initial spot review of a few 1000 files.

The 4.13 kernel was the starting point of the analysis with 60,537 files
assessed. Kate Stewart did a file by file comparison of the scanner
results in the spreadsheet to determine which SPDX license identifier(s)
to be applied to the file. She confirmed any determination that was not
immediately clear with lawyers working with the Linux Foundation.

Criteria used to select files for SPDX license identifier tagging was:
- Files considered eligible had to be source code files.
- Make and config files were included as candidates if they contained >5
lines of source
- File already had some variant of a license header in it (even if <5
lines).

All documentation files were explicitly excluded.

The following heuristics were used to determine which SPDX license
identifiers to apply.

- when both scanners couldn't find any license traces, file was
considered to have no license information in it, and the top level
COPYING file license applied.

For non */uapi/* files that summary was:

SPDX license identifier # files
---------------------------------------------------|-------
GPL-2.0 11139

and resulted in the first patch in this series.

If that file was a */uapi/* path one, it was "GPL-2.0 WITH
Linux-syscall-note" otherwise it was "GPL-2.0". Results of that was:

SPDX license identifier # files
---------------------------------------------------|-------
GPL-2.0 WITH Linux-syscall-note 930

and resulted in the second patch in this series.

- if a file had some form of licensing information in it, and was one
of the */uapi/* ones, it was denoted with the Linux-syscall-note if
any GPL family license was found in the file or had no licensing in
it (per prior point). Results summary:

SPDX license identifier # files
---------------------------------------------------|------
GPL-2.0 WITH Linux-syscall-note 270
GPL-2.0+ WITH Linux-syscall-note 169
((GPL-2.0 WITH Linux-syscall-note) OR BSD-2-Clause) 21
((GPL-2.0 WITH Linux-syscall-note) OR BSD-3-Clause) 17
LGPL-2.1+ WITH Linux-syscall-note 15
GPL-1.0+ WITH Linux-syscall-note 14
((GPL-2.0+ WITH Linux-syscall-note) OR BSD-3-Clause) 5
LGPL-2.0+ WITH Linux-syscall-note 4
LGPL-2.1 WITH Linux-syscall-note 3
((GPL-2.0 WITH Linux-syscall-note) OR MIT) 3
((GPL-2.0 WITH Linux-syscall-note) AND MIT) 1

and that resulted in the third patch in this series.

- when the two scanners agreed on the detected license(s), that became
the concluded license(s).

- when there was disagreement between the two scanners (one detected a
license but the other didn't, or they both detected different
licenses) a manual inspection of the file occurred.

- In most cases a manual inspection of the information in the file
resulted in a clear resolution of the license that should apply (and
which scanner probably needed to revisit its heuristics).

- When it was not immediately clear, the license identifier was
confirmed with lawyers working with the Linux Foundation.

- If there was any question as to the appropriate license identifier,
the file was flagged for further research and to be revisited later
in time.

In total, over 70 hours of logged manual review was done on the
spreadsheet to determine the SPDX license identifiers to apply to the
source files by Kate, Philippe, Thomas and, in some cases, confirmation
by lawyers working with the Linux Foundation.

Kate also obtained a third independent scan of the 4.13 code base from
FOSSology, and compared selected files where the other two scanners
disagreed against that SPDX file, to see if there was new insights. The
Windriver scanner is based on an older version of FOSSology in part, so
they are related.

Thomas did random spot checks in about 500 files from the spreadsheets
for the uapi headers and agreed with SPDX license identifier in the
files he inspected. For the non-uapi files Thomas did random spot checks
in about 15000 files.

In initial set of patches against 4.14-rc6, 3 files were found to have
copy/paste license identifier errors, and have been fixed to reflect the
correct identifier.

Additionally Philippe spent 10 hours this week doing a detailed manual
inspection and review of the 12,461 patched files from the initial patch
version early this week with:
- a full scancode scan run, collecting the matched texts, detected
license ids and scores
- reviewing anything where there was a license detected (about 500+
files) to ensure that the applied SPDX license was correct
- reviewing anything where there was no detection but the patch license
was not GPL-2.0 WITH Linux-syscall-note to ensure that the applied
SPDX license was correct

This produced a worksheet with 20 files needing minor correction. This
worksheet was then exported into 3 different .csv files for the
different types of files to be modified.

These .csv files were then reviewed by Greg. Thomas wrote a script to
parse the csv files and add the proper SPDX tag to the file, in the
format that the file expected. This script was further refined by Greg
based on the output to detect more types of files automatically and to
distinguish between header and source .c files (which need different
comment types.) Finally Greg ran the script using the .csv files to
generate the patches.

Reviewed-by: Kate Stewart <[email protected]>
Reviewed-by: Philippe Ombredanne <[email protected]>
Reviewed-by: Thomas Gleixner <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

show more ...

sched/wait: Rename wait_queue_t => wait_queue_entry_t

Rename:

wait_queue_t => wait_queue_entry_t

'wait_queue_t' was always a slight misnomer: its name implies that it's a "queue",
but in reality

sched/wait: Rename wait_queue_t => wait_queue_entry_t

Rename:

wait_queue_t => wait_queue_entry_t

'wait_queue_t' was always a slight misnomer: its name implies that it's a "queue",
but in reality it's a queue *entry*. The 'real' queue is the wait queue head,
which had to carry the name.

Start sorting this out by renaming it to 'wait_queue_entry_t'.

This also allows the real structure name 'struct __wait_queue' to
lose its double underscore and become 'struct wait_queue_entry',
which is the more canonical nomenclature for such data types.

Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Signed-off-by: Ingo Molnar <[email protected]>

show more ...

Kill indirect include of file.h from eventfd.h, use fdget() in cgroup.c

kernel/cgroup.c is the only place in the tree that relies on eventfd.h
pulling file.h; move that include there. Switch from e

Kill indirect include of file.h from eventfd.h, use fdget() in cgroup.c

kernel/cgroup.c is the only place in the tree that relies on eventfd.h
pulling file.h; move that include there. Switch from eventfd_fget()/fput()
to fdget()/fdput(), while we are at it - eventfd_ctx_fileget() will fail
on non-eventfd descriptors just fine, no need to do that check twice...

Signed-off-by: Al Viro <[email protected]>

show more ...

include/linux/eventfd.h: fix incorrect filename is a comment

Comment in eventfd.h referred to 'include/asm-generic/fcntl.h'
while the correct path is 'include/uapi/asm-generic/fcntl.h'.

Signed-off-

include/linux/eventfd.h: fix incorrect filename is a comment

Comment in eventfd.h referred to 'include/asm-generic/fcntl.h'
while the correct path is 'include/uapi/asm-generic/fcntl.h'.

Signed-off-by: Martin Sustrik <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

show more ...

eventfd: change int to __u64 in eventfd_signal()

eventfd_ctx->count is an __u64 counter which is allowed to reach
ULLONG_MAX. eventfd_write() adds a __u64 value to "count", but the kernel
side even

eventfd: change int to __u64 in eventfd_signal()

eventfd_ctx->count is an __u64 counter which is allowed to reach
ULLONG_MAX. eventfd_write() adds a __u64 value to "count", but the kernel
side eventfd_signal() only adds an int value to it. Make them consistent.

[[email protected]: update interface documentation]
Signed-off-by: Sha Zhengju <[email protected]>
Cc: Davide Libenzi <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

show more ...

eventfd - allow atomic read and waitqueue remove

KVM needs a wait to atomically remove themselves from the eventfd ->poll()
wait queue head, in order to handle correctly their IRQfd deassign
operati

eventfd - allow atomic read and waitqueue remove

KVM needs a wait to atomically remove themselves from the eventfd ->poll()
wait queue head, in order to handle correctly their IRQfd deassign
operation.

This patch introduces such API, plus a way to read an eventfd from its
context.

Signed-off-by: Davide Libenzi <[email protected]>
Signed-off-by: Avi Kivity <[email protected]>

show more ...

anonfd: split interface into file creation and install

Split the anonfd interface into a bare file pointer creation one, and a
file pointer creation plus install one.

There are cases, like the usag

anonfd: split interface into file creation and install

Split the anonfd interface into a bare file pointer creation one, and a
file pointer creation plus install one.

There are cases, like the usage of eventfds inside other kernel
interfaces, where the file pointer created by anonfd needs to be used
inside the initialization of other structures.

As it is right now, as soon as anon_inode_getfd() returns, the kenrle can
race with userspace closing the newly installed file descriptor.

This patch, while keeping the old anon_inode_getfd(), introduces a new
anon_inode_getfile() (whose services are reused in anon_inode_getfd())
that allows to split the file creation phase and the fd install one.

Once all the kernel structures are initialized, the code can call the
proper fd_install().

Gregory manifested the need for something like this inside KVM.

Signed-off-by: Davide Libenzi <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: James Morris <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Gregory Haskins <[email protected]>
Acked-by: Serge Hallyn <[email protected]>
Acked-by: Roland Dreier <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

show more ...

eventfd: revised interface and cleanups

Change the eventfd interface to de-couple the eventfd memory context, from
the file pointer instance.

Without such change, there is no clean way to racely fr

eventfd: revised interface and cleanups

Change the eventfd interface to de-couple the eventfd memory context, from
the file pointer instance.

Without such change, there is no clean way to racely free handle the
POLLHUP event sent when the last instance of the file* goes away. Also,
now the internal eventfd APIs are using the eventfd context instead of the
file*.

This patch is required by KVM's IRQfd code, which is still under
development.

Signed-off-by: Davide Libenzi <[email protected]>
Cc: Gregory Haskins <[email protected]>
Cc: Rusty Russell <[email protected]>
Cc: Benjamin LaHaise <[email protected]>
Cc: Avi Kivity <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

show more ...

eventfd: improve support for semaphore-like behavior

People started using eventfd in a semaphore-like way where before they
were using pipes.

That is, counter-based resource access. Where a "wait(

eventfd: improve support for semaphore-like behavior

People started using eventfd in a semaphore-like way where before they
were using pipes.

That is, counter-based resource access. Where a "wait()" returns
immediately by decrementing the counter by one, if counter is greater than
zero. Otherwise will wait. And where a "post(count)" will add count to
the counter releasing the appropriate amount of waiters. If eventfd the
"post" (write) part is fine, while the "wait" (read) does not dequeue 1,
but the whole counter value.

The problem with eventfd is that a read() on the fd returns and wipes the
whole counter, making the use of it as semaphore a little bit more
cumbersome. You can do a read() followed by a write() of COUNTER-1, but
IMO it's pretty easy and cheap to make this work w/out extra steps. This
patch introduces a new eventfd flag that tells eventfd to only dequeue 1
from the counter, allowing simple read/write to make it behave like a
semaphore. Simple test here:

http://www.xmailserver.org/eventfd-sem.c

To be back-compatible with earlier kernels, userspace applications should
probe for the availability of this feature via

#ifdef EFD_SEMAPHORE
fd = eventfd2 (CNT, EFD_SEMAPHORE);
if (fd == -1 && errno == EINVAL)
<fallback>
#else
<fallback>
#endif

Signed-off-by: Davide Libenzi <[email protected]>
Cc: <[email protected]>
Tested-by: Michael Kerrisk <[email protected]>
Cc: Ulrich Drepper <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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flag parameters: NONBLOCK in eventfd

This patch adds support for the EFD_NONBLOCK flag to eventfd2. The
additional changes needed are minimal.

The following test must be adjusted for architectures

flag parameters: NONBLOCK in eventfd

This patch adds support for the EFD_NONBLOCK flag to eventfd2. The
additional changes needed are minimal.

The following test must be adjusted for architectures other than x86 and
x86-64 and in case the syscall numbers changed.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/syscall.h>

#ifndef __NR_eventfd2
# ifdef __x86_64__
# define __NR_eventfd2 290
# elif defined __i386__
# define __NR_eventfd2 328
# else
# error "need __NR_eventfd2"
# endif
#endif

#define EFD_NONBLOCK O_NONBLOCK

int
main (void)
{
int fd = syscall (__NR_eventfd2, 1, 0);
if (fd == -1)
{
puts ("eventfd2(0) failed");
return 1;
}
int fl = fcntl (fd, F_GETFL);
if (fl == -1)
{
puts ("fcntl failed");
return 1;
}
if (fl & O_NONBLOCK)
{
puts ("eventfd2(0) sets non-blocking mode");
return 1;
}
close (fd);

fd = syscall (__NR_eventfd2, 1, EFD_NONBLOCK);
if (fd == -1)
{
puts ("eventfd2(EFD_NONBLOCK) failed");
return 1;
}
fl = fcntl (fd, F_GETFL);
if (fl == -1)
{
puts ("fcntl failed");
return 1;
}
if ((fl & O_NONBLOCK) == 0)
{
puts ("eventfd2(EFD_NONBLOCK) does not set non-blocking mode");
return 1;
}
close (fd);

puts ("OK");

return 0;
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Ulrich Drepper <[email protected]>
Acked-by: Davide Libenzi <[email protected]>
Cc: Michael Kerrisk <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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