linux-kernel - Re: [PATCH] pipe_read: don't wake up the writer if the pipe is still full

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Message-ID: <20250304221138.GG5756@redhat.com>
Date: Tue, 4 Mar 2025 23:11:39 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mateusz Guzik <mjguzik@...il.com>,
	K Prateek Nayak <kprateek.nayak@....com>,
	"Sapkal, Swapnil" <swapnil.sapkal@....com>,
	Manfred Spraul <manfred@...orfullife.com>,
	Christian Brauner <brauner@...nel.org>,
	David Howells <dhowells@...hat.com>,
	WangYuli <wangyuli@...ontech.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"Shenoy, Gautham Ranjal" <gautham.shenoy@....com>,
	Neeraj.Upadhyay@....com, Ananth.narayan@....com
Subject: Re: [PATCH] pipe_read: don't wake up the writer if the pipe is still
 full

On 03/04, Linus Torvalds wrote:
>
> On Tue, 4 Mar 2025 at 02:55, Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > > + * Really only alpha needs 32-bit fields, but
> > > + * might as well do it for 64-bit architectures
> > > + * since that's what we've historically done,
> > > + * and it makes 'head_tail' always be a simple
> > > + * 'unsigned long'.
> > > + */
> > > +#ifdef CONFIG_64BIT
> > > +  typedef unsigned int pipe_index_t;
> > > +#else
> > > +  typedef unsigned short pipe_index_t;
> > > +#endif
> >
> > I am just curious, why we can't use "unsigned short" unconditionally
> > and avoid #ifdef ?
> >
> > Is "unsigned int" more efficient on 64-bit?
>
> The main reason is that a "unsigned short" write on alpha isn't atomic

Yes, I have already realized this when I tried to actually read the
comment, see my next email in reply to myself.

But thanks for your detailed explanation!

Oleg.