[PATCH] eventfd: show flags in fdinfo

[PATCH] eventfd: show flags in fdinfo

[wenyang.linux]

From: Wen Yang <wenyang.linux@foxmail.com>

The flags should be displayed in fdinfo, as different flags
could affect the behavior of eventfd.

Signed-off-by: Wen Yang <wenyang.linux@foxmail.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dylan Yudaken <dylany@fb.com>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Eric Biggers <ebiggers@google.com>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
 fs/eventfd.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/eventfd.c b/fs/eventfd.c
index 6c06a527747f..5b5448e65f6f 100644
--- a/fs/eventfd.c
+++ b/fs/eventfd.c
@@ -33,10 +33,10 @@ struct eventfd_ctx {
 	/*
 	 * Every time that a write(2) is performed on an eventfd, the
 	 * value of the __u64 being written is added to "count" and a
-	 * wakeup is performed on "wqh". A read(2) will return the "count"
-	 * value to userspace, and will reset "count" to zero. The kernel
-	 * side eventfd_signal() also, adds to the "count" counter and
-	 * issue a wakeup.
+	 * wakeup is performed on "wqh". If EFD_SEMAPHORE flag was not
+	 * specified, a read(2) will return the "count" value to userspace,
+	 * and will reset "count" to zero. The kernel side eventfd_signal()
+	 * also, adds to the "count" counter and issue a wakeup.
 	 */
 	__u64 count;
 	unsigned int flags;
@@ -301,6 +301,7 @@ static void eventfd_show_fdinfo(struct seq_file *m, struct file *f)
 		   (unsigned long long)ctx->count);
 	spin_unlock_irq(&ctx->wqh.lock);
 	seq_printf(m, "eventfd-id: %d\n", ctx->id);
+	seq_printf(m, "eventfd-flags: 0%o\n", ctx->flags);
 }
 #endif
 
-- 
2.34.1

Re: [PATCH] eventfd: show flags in fdinfo

[Christian Brauner]

On Mon, Jun 12, 2023 at 02:59:47AM +0800, wenyang.linux@foxmail.com wrote:
> From: Wen Yang <wenyang.linux@foxmail.com>
> 
> The flags should be displayed in fdinfo, as different flags
> could affect the behavior of eventfd.
> 
> Signed-off-by: Wen Yang <wenyang.linux@foxmail.com>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Dylan Yudaken <dylany@fb.com>
> Cc: David Woodhouse <dwmw@amazon.co.uk>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Eric Biggers <ebiggers@google.com>
> Cc: linux-fsdevel@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> ---
>  fs/eventfd.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/eventfd.c b/fs/eventfd.c
> index 6c06a527747f..5b5448e65f6f 100644
> --- a/fs/eventfd.c
> +++ b/fs/eventfd.c
> @@ -33,10 +33,10 @@ struct eventfd_ctx {
>  	/*
>  	 * Every time that a write(2) is performed on an eventfd, the
>  	 * value of the __u64 being written is added to "count" and a
> -	 * wakeup is performed on "wqh". A read(2) will return the "count"
> -	 * value to userspace, and will reset "count" to zero. The kernel
> -	 * side eventfd_signal() also, adds to the "count" counter and
> -	 * issue a wakeup.
> +	 * wakeup is performed on "wqh". If EFD_SEMAPHORE flag was not
> +	 * specified, a read(2) will return the "count" value to userspace,
> +	 * and will reset "count" to zero. The kernel side eventfd_signal()
> +	 * also, adds to the "count" counter and issue a wakeup.
>  	 */
>  	__u64 count;
>  	unsigned int flags;
> @@ -301,6 +301,7 @@ static void eventfd_show_fdinfo(struct seq_file *m, struct file *f)
>  		   (unsigned long long)ctx->count);
>  	spin_unlock_irq(&ctx->wqh.lock);
>  	seq_printf(m, "eventfd-id: %d\n", ctx->id);
> +	seq_printf(m, "eventfd-flags: 0%o\n", ctx->flags);

EFD_CLOEXEC and EFD_NONBLOCK are mapped to generic O_* flags and are
included in fdinfo output already:

pos:    0
flags:  02000002 -> O_CLOEXEC/EFD_CLOEXEC | O_RDWR
mnt_id: 15
ino:    12497
eventfd-count:                0
eventfd-id: 156

So the only thing you really care about is EFD_SEMAPHORE. Since this
changes the type of the eventfd I would just do either:

eventfd-semaphore: {0,1}

or - if we can reasonably expect a third type:

eventfd-type: {0, ..., n}

though I suspect since it hasn't changed since 2.6-something that
eventfd-semaphore: {0,1} is probably fine.





so you only really care about EFD_SEMAPHORE.