linux-kernel - Re: [RFC PATCH v3] cleanup: Add cond

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Date: Thu, 01 Feb 2024 16:32:25 +0100
From: "Fabio M. De Francesco" <fabio.maria.de.francesco@...ux.intel.com>
To: Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc: Peter Zijlstra <peterz@...radead.org>, dan.j.williams@...el.com,
 linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
 linux-cxl@...r.kernel.org, Ira Weiny <ira.weiny@...el.com>
Subject: Re: [RFC PATCH v3] cleanup: Add cond_guard() to conditional guards

On Thursday, 1 February 2024 16:13:34 CET Fabio M. De Francesco wrote:
> On Thursday, 1 February 2024 12:36:12 CET Jonathan Cameron wrote:
> > On Thu, 01 Feb 2024 09:16:59 +0100
> > 
> > "Fabio M. De Francesco" <fabio.maria.de.francesco@...ux.intel.com> wrote:
> > > [snip]
> > > 
> > > Actually, I'm doing this:
> > > 	cond_guard(..., rc, 0, -EINTR, ...);
> > 
> > Can we not works some magic to do.
> > 
> > 	cond_guard(..., return -EINTR, ...)
> > 
> > and not have an rc at all if we don't want to.
> > 
> > Something like
> > 
> > #define cond_guard(_name, _fail, args...) \
> > 
> > 	CLASS(_name, scope)(args); \
> > 	if (!__guard_ptr(_name)(&scope)) _fail
> > 
> > Completely untested so I'm probably missing some subtleties.
> > 
> > Jonathan
> 
> Jonathan,
> 
> Can you please comment on the v5 of this RFC?
> It is at
> https://lore.kernel.org/all/20240201131033.9850-1-fabio.maria.de.francesco@
> linux.intel.com/
> 
> The macro introduced in v5 has the following, more general, use case:
> 
> * * 	int ret;
> + * 	// down_read_trylock() returns 1 on success, 0 on contention
> + * 	cond_guard(rwsem_read_try, ret, 1, 0, &sem);
> + * 	if (!ret) {
> + * 		dev_dbg("down_read_trylock() failed to down 'sem')\n");
> + * 		return ret;
> + * 	}
> 
> The text above has been copy-pasted from the RFC Patch v5.
> 
> Please notice that we need to provide both the success and the failure code
> to make it work also with the _trylock() variants (more details in the
> patch).

The next three lines have been messed up by a copy-paste.
They are:

If we simply do something like:

	cond_guard(..., ret = 0, ...)

We won't store the success (that is 1) in ret and it would still contain 0, 
that is the code of the contended case.

> If we simply do something like:
> 
> 	cond_guard(..., ret = 0, ...)
> 
> to be able store in 'ret' the code of the contended case, that is 0.
> 
> Since down_read_trylock() returns 1 on down semaphore, when we later check
> 'ret' with "if (!ret) <failure path>;" we always enter in that failure path
> even if the semaphore is down because we didn't store the success code in
> ret (and ret is still probably 0).
> 
> This is why, I think, we need a five arguments cond_guard(). This can manage
> also the _interruptible() and _killable() cases as:
> 
> 	cond_guard(..., ret, 0, -EINTR, ...)
> 
> In this case we don't need 5 arguments, but we have a general use case, one
> only macro, that can work with all the three variants of locks.
> 
> Fabio